Merge branch 'feature-new-units-def' into develop

17 files changed, 2626 insertions, 651 deletions

diff --git a/src/org/unitConverter/unit/AbstractUnit.java b/src/org/unitConverter/unit/AbstractUnit.java
deleted file mode 100644
index 6045127..0000000
--- a/src/org/unitConverter/unit/AbstractUnit.java
+++ /dev/null
@@ -1,124 +0,0 @@
-/**
- * Copyright (C) 2019 Adrien Hopkins
- *
- * This program is free software: you can redistribute it and/or modify
- * it under the terms of the GNU Affero General Public License as published by
- * the Free Software Foundation, either version 3 of the License, or
- * (at your option) any later version.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU Affero General Public License for more details.
- *
- * You should have received a copy of the GNU Affero General Public License
- * along with this program.  If not, see <https://www.gnu.org/licenses/>.
- */
-package org.unitConverter.unit;
-
-import java.util.Objects;
-
-import org.unitConverter.dimension.UnitDimension;
-
-/**
- * The default abstract implementation of the {@code Unit} interface.
- * 
- * <p>
- * With the addition of {@link Unit#fromConversionFunctions}, there is no longer any reason to use {@code AbstractUnit}
- * for any purpose other than making subclasses. Units should never be declared as {@code AbstractUnit}, they should be
- * declared as {@code Unit}. Now that {@code Unit.fromConversionFunctions} exists, it is preferred to creating anonymous
- * inner types of {@code AbstractUnit}.
- * </p>
- * 
- * @author Adrien Hopkins
- * @since 2018-12-22
- * @since v0.1.0
- */
-public abstract class AbstractUnit implements Unit {
-	/**
-	 * The dimension, or what the unit measures.
-	 * 
-	 * @since 2018-12-22
-	 * @since v0.1.0
-	 */
-	private final UnitDimension dimension;
-
-	/**
-	 * The unit's base unit. Values converted by {@code convertFromBase} and {@code convertToBase} are expressed in this
-	 * unit.
-	 * 
-	 * @since 2018-12-22
-	 * @since v0.1.0
-	 */
-	private final BaseUnit base;
-
-	/**
-	 * The system that this unit is a part of.
-	 * 
-	 * @since 2018-12-23
-	 * @since v0.1.0
-	 */
-	private final UnitSystem system;
-
-	/**
-	 * Creates the {@code AbstractUnit}.
-	 * 
-	 * @param base
-	 *            unit's base
-	 * @throws NullPointerException
-	 *             if name, symbol or base is null
-	 * @since 2018-12-22
-	 * @since v0.1.0
-	 */
-	public AbstractUnit(final BaseUnit base) {
-		this.base = Objects.requireNonNull(base, "base must not be null.");
-		this.dimension = this.base.getDimension();
-		this.system = this.base.getSystem();
-	}
-
-	/**
-	 * Creates the {@code AbstractUnit} using a unique dimension. This constructor is for making base units and should
-	 * only be used by {@code BaseUnit}.
-	 * 
-	 * @param dimension
-	 *            dimension measured by unit
-	 * @param system
-	 *            system that unit is a part of
-	 * @throws AssertionError
-	 *             if this constructor is not run by {@code BaseUnit} or a subclass
-	 * @throws NullPointerException
-	 *             if name, symbol or dimension is null
-	 * @since 2018-12-23
-	 * @since v0.1.0
-	 */
-	AbstractUnit(final UnitDimension dimension, final UnitSystem system) {
-		// try to set this as a base unit
-		if (this instanceof BaseUnit) {
-			this.base = (BaseUnit) this;
-		} else
-			throw new AssertionError();
-
-		this.dimension = Objects.requireNonNull(dimension, "dimension must not be null.");
-		this.system = Objects.requireNonNull(system, "system must not be null.");
-	}
-
-	@Override
-	public final BaseUnit getBase() {
-		return this.base;
-	}
-
-	@Override
-	public final UnitDimension getDimension() {
-		return this.dimension;
-	}
-
-	@Override
-	public final UnitSystem getSystem() {
-		return this.system;
-	}
-
-	@Override
-	public String toString() {
-		return String.format("%s-derived unit of dimension %s", this.getSystem(), this.getDimension());
-	}
-}
diff --git a/src/org/unitConverter/unit/BaseDimension.java b/src/org/unitConverter/unit/BaseDimension.java
new file mode 100644
index 0000000..35acd18
--- /dev/null
+++ b/src/org/unitConverter/unit/BaseDimension.java
@@ -0,0 +1,81 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.unit;
+
+import java.util.Objects;
+
+/**
+ * A dimension that defines a {@code BaseUnit}
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-10-16
+ */
+public final class BaseDimension {
+	/**
+	 * Gets a {@code BaseDimension} with the provided name and symbol.
+	 * 
+	 * @param name
+	 *            name of dimension
+	 * @param symbol
+	 *            symbol used for dimension
+	 * @return dimension
+	 * @since 2019-10-16
+	 */
+	public static BaseDimension valueOf(final String name, final String symbol) {
+		return new BaseDimension(name, symbol);
+	}
+
+	private final String name;
+	private final String symbol;
+
+	/**
+	 * Creates the {@code BaseDimension}.
+	 * 
+	 * @param name
+	 *            name of unit
+	 * @param symbol
+	 *            symbol of unit
+	 * @throws NullPointerException
+	 *             if any argument is null
+	 * @since 2019-10-16
+	 */
+	private BaseDimension(final String name, final String symbol) {
+		this.name = Objects.requireNonNull(name, "name must not be null.");
+		this.symbol = Objects.requireNonNull(symbol, "symbol must not be null.");
+	}
+
+	/**
+	 * @return name
+	 * @since 2019-10-16
+	 */
+	public final String getName() {
+		return this.name;
+	}
+
+	/**
+	 * @return symbol
+	 * @since 2019-10-16
+	 */
+	public final String getSymbol() {
+		return this.symbol;
+	}
+
+	@Override
+	public String toString() {
+		return String.format("%s (%s)", this.getName(), this.getSymbol());
+	}
+}
diff --git a/src/org/unitConverter/unit/BaseUnit.java b/src/org/unitConverter/unit/BaseUnit.java
index 67309cf..8f44861 100644
--- a/src/org/unitConverter/unit/BaseUnit.java
+++ b/src/org/unitConverter/unit/BaseUnit.java
@@ -1,5 +1,5 @@
 /**
- * Copyright (C) 2018 Adrien Hopkins
+ * Copyright (C) 2019 Adrien Hopkins
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as published by
@@ -18,151 +18,100 @@ package org.unitConverter.unit;
 
 import java.util.Objects;
 
-import org.unitConverter.dimension.StandardDimensions;
-import org.unitConverter.dimension.UnitDimension;
-
 /**
- * A unit that is the base for its dimension. It does not have to be for a base dimension, so units like the Newton and
- * Joule are still base units.
- * <p>
- * {@code BaseUnit} does not have any public constructors or static factories. There are two ways to obtain
- * {@code BaseUnit} instances.
- * <ol>
- * <li>The class {@link SI} in this package has constants for all of the SI base units. You can use these constants and
- * multiply or divide them to get other units. For example:
- * 
- * <pre>
- * BaseUnit JOULE = SI.KILOGRAM.times(SI.METRE.toExponent(2)).dividedBy(SI.SECOND.toExponent(2));
- * </pre>
- * 
- * </li>
- * <li>You can also query a unit system for a base unit using a unit dimension. The previously mentioned {@link SI}
- * class can do this for SI and SI-derived units (including imperial and USC), but if you want to use another system,
- * this is the way to do it. {@link StandardDimensions} contains common unit dimensions that you can use for this. Here
- * is an example:
- * 
- * <pre>
- * BaseUnit JOULE = SI.SI.getBaseUnit(StandardDimensions.ENERGY);
- * </pre>
- * 
- * </li>
- * </ol>
+ * A unit that other units are defined by.
  * 
  * @author Adrien Hopkins
- * @since 2018-12-23
- * @since v0.1.0
+ * @since 2019-10-16
  */
-public final class BaseUnit extends LinearUnit {
+public final class BaseUnit extends Unit {
 	/**
-	 * Is this unit a full base (i.e. m, s, ... but not N, J, ...)
+	 * Gets a base unit from the dimension it measures, its name and its symbol.
 	 * 
-	 * @since 2019-01-15
-	 * @since v0.1.0
+	 * @param dimension
+	 *            dimension measured by this unit
+	 * @param name
+	 *            name of unit
+	 * @param symbol
+	 *            symbol of unit
+	 * @return base unit
+	 * @since 2019-10-16
 	 */
-	private final boolean isFullBase;
+	public static BaseUnit valueOf(final BaseDimension dimension, final String name, final String symbol) {
+		return new BaseUnit(dimension, name, symbol);
+	}
+
+	private final BaseDimension dimension;
+	private final String name;
+	private final String symbol;
 
 	/**
 	 * Creates the {@code BaseUnit}.
 	 * 
 	 * @param dimension
-	 *            dimension measured by unit
-	 * @param system
-	 *            system that unit is a part of
+	 *            dimension of unit
 	 * @param name
 	 *            name of unit
 	 * @param symbol
 	 *            symbol of unit
-	 * @since 2018-12-23
-	 * @since v0.1.0
+	 * @throws NullPointerException
+	 *             if any argument is null
+	 * @since 2019-10-16
 	 */
-	BaseUnit(final UnitDimension dimension, final UnitSystem system) {
-		super(dimension, system, 1);
-		this.isFullBase = dimension.isBase();
+	private BaseUnit(final BaseDimension dimension, final String name, final String symbol) {
+		super();
+		this.dimension = Objects.requireNonNull(dimension, "dimension must not be null.");
+		this.name = Objects.requireNonNull(name, "name must not be null.");
+		this.symbol = Objects.requireNonNull(symbol, "symbol must not be null.");
 	}
 
 	/**
-	 * Returns the quotient of this unit and another.
-	 * <p>
-	 * Two units can be divided if they are part of the same unit system. If {@code divisor} does not meet this
-	 * condition, an {@code IllegalArgumentException} should be thrown.
-	 * </p>
+	 * Returns a {@code LinearUnit} with this unit as a base and a conversion factor of 1. This operation must be done
+	 * in order to allow units to be created with operations.
 	 * 
-	 * @param divisor
-	 *            unit to divide by
-	 * @return quotient of two units
-	 * @throws IllegalArgumentException
-	 *             if {@code divisor} is not compatible for division as described above
-	 * @throws NullPointerException
-	 *             if {@code divisor} is null
-	 * @since 2018-12-22
-	 * @since v0.1.0
+	 * @return this unit as a {@code LinearUnit}
+	 * @since 2019-10-16
 	 */
-	public BaseUnit dividedBy(final BaseUnit divisor) {
-		Objects.requireNonNull(divisor, "other must not be null.");
+	public LinearUnit asLinearUnit() {
+		return LinearUnit.valueOf(this.getBase(), 1);
+	}
 
-		// check that these units can be multiplied
-		if (!this.getSystem().equals(divisor.getSystem()))
-			throw new IllegalArgumentException(
-					String.format("Incompatible units for division \"%s\" and \"%s\".", this, divisor));
+	@Override
+	public double convertFromBase(final double value) {
+		return value;
+	}
 
-		return new BaseUnit(this.getDimension().dividedBy(divisor.getDimension()), this.getSystem());
+	@Override
+	public double convertToBase(final double value) {
+		return value;
 	}
 
 	/**
-	 * @return true if the unit is a "full base" unit like the metre or second.
-	 * @since 2019-04-10
-	 * @since v0.2.0
+	 * @return dimension
+	 * @since 2019-10-16
 	 */
-	public final boolean isFullBase() {
-		return this.isFullBase;
+	public final BaseDimension getBaseDimension() {
+		return this.dimension;
 	}
 
 	/**
-	 * Returns the product of this unit and another.
-	 * <p>
-	 * Two units can be multiplied if they are part of the same unit system. If {@code multiplier} does not meet this
-	 * condition, an {@code IllegalArgumentException} should be thrown.
-	 * </p>
-	 * 
-	 * @param multiplier
-	 *            unit to multiply by
-	 * @return product of two units
-	 * @throws IllegalArgumentException
-	 *             if {@code multiplier} is not compatible for multiplication as described above
-	 * @throws NullPointerException
-	 *             if {@code multiplier} is null
-	 * @since 2018-12-22
-	 * @since v0.1.0
+	 * @return name
+	 * @since 2019-10-16
 	 */
-	public BaseUnit times(final BaseUnit multiplier) {
-		Objects.requireNonNull(multiplier, "other must not be null");
-
-		// check that these units can be multiplied
-		if (!this.getSystem().equals(multiplier.getSystem()))
-			throw new IllegalArgumentException(
-					String.format("Incompatible units for multiplication \"%s\" and \"%s\".", this, multiplier));
-
-		// multiply the units
-		return new BaseUnit(this.getDimension().times(multiplier.getDimension()), this.getSystem());
+	public final String getName() {
+		return this.name;
 	}
 
 	/**
-	 * Returns this unit, but to an exponent.
-	 * 
-	 * @param exponent
-	 *            exponent
-	 * @return result of exponentiation
-	 * @since 2019-01-15
-	 * @since v0.1.0
+	 * @return symbol
+	 * @since 2019-10-16
 	 */
-	@Override
-	public BaseUnit toExponent(final int exponent) {
-		return this.getSystem().getBaseUnit(this.getDimension().toExponent(exponent));
+	public final String getSymbol() {
+		return this.symbol;
 	}
 
 	@Override
 	public String toString() {
-		return String.format("%s base unit of%s dimension %s", this.getSystem(), this.isFullBase ? " base" : "",
-				this.getDimension());
+		return String.format("%s (%s)", this.getName(), this.getSymbol());
 	}
 }
diff --git a/src/org/unitConverter/unit/NonlinearUnits.java b/src/org/unitConverter/unit/BritishImperial.java
index eda4a74..c9316f3 100644
--- a/src/org/unitConverter/unit/NonlinearUnits.java
+++ b/src/org/unitConverter/unit/BritishImperial.java
@@ -1,5 +1,5 @@
 /**
- * Copyright (C) 2018 Adrien Hopkins
+ * Copyright (C) 2019 Adrien Hopkins
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as published by
@@ -17,21 +17,12 @@
 package org.unitConverter.unit;
 
 /**
- * Some major nonlinear units.
+ * A static utility class that contains units in the British Imperial system.
  * 
  * @author Adrien Hopkins
- * @since 2019-01-14
- * @since v0.1.0
+ * @since 2019-10-21
  */
-public final class NonlinearUnits {
-	public static final Unit CELSIUS = Unit.fromConversionFunctions(SI.KELVIN, tempK -> tempK - 273.15,
-			tempC -> tempC + 273.15);
-
-	public static final Unit FAHRENHEIT = Unit.fromConversionFunctions(SI.KELVIN, tempK -> tempK * 1.8 - 459.67,
-			tempF -> (tempF + 459.67) / 1.8);
-
-	// You may NOT get a NonlinearUnits instance.
-	private NonlinearUnits() {
-		throw new AssertionError();
-	}
+public final class BritishImperial {
+	public static final Unit FAHRENHEIT = Unit.fromConversionFunctions(SI.KELVIN.getBase(),
+			tempK -> tempK * 1.8 - 459.67, tempF -> (tempF + 459.67) / 1.8);
 }
diff --git a/src/org/unitConverter/unit/DefaultUnitPrefix.java b/src/org/unitConverter/unit/DefaultUnitPrefix.java
deleted file mode 100644
index 4a9e487..0000000
--- a/src/org/unitConverter/unit/DefaultUnitPrefix.java
+++ /dev/null
@@ -1,68 +0,0 @@
-/**
- * Copyright (C) 2018 Adrien Hopkins
- *
- * This program is free software: you can redistribute it and/or modify
- * it under the terms of the GNU Affero General Public License as published by
- * the Free Software Foundation, either version 3 of the License, or
- * (at your option) any later version.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU Affero General Public License for more details.
- *
- * You should have received a copy of the GNU Affero General Public License
- * along with this program.  If not, see <https://www.gnu.org/licenses/>.
- */
-package org.unitConverter.unit;
-
-import java.util.Objects;
-
-/**
- * The default implementation of {@code UnitPrefix}, which contains a multiplier and nothing else.
- * 
- * @author Adrien Hopkins
- * @since 2019-01-14
- * @since v0.1.0
- */
-public final class DefaultUnitPrefix implements UnitPrefix {
-	private final double multiplier;
-
-	/**
-	 * Creates the {@code DefaultUnitPrefix}.
-	 * 
-	 * @param multiplier
-	 * @since 2019-01-14
-	 * @since v0.2.0
-	 */
-	public DefaultUnitPrefix(final double multiplier) {
-		this.multiplier = multiplier;
-	}
-
-	@Override
-	public boolean equals(final Object obj) {
-		if (this == obj)
-			return true;
-		if (obj == null)
-			return false;
-		if (!(obj instanceof DefaultUnitPrefix))
-			return false;
-		final DefaultUnitPrefix other = (DefaultUnitPrefix) obj;
-		return Double.doubleToLongBits(this.multiplier) == Double.doubleToLongBits(other.multiplier);
-	}
-
-	@Override
-	public double getMultiplier() {
-		return this.multiplier;
-	}
-
-	@Override
-	public int hashCode() {
-		return Objects.hash(this.multiplier);
-	}
-
-	@Override
-	public String toString() {
-		return String.format("Unit prefix equal to %s", this.multiplier);
-	}
-}
diff --git a/src/org/unitConverter/unit/FunctionalUnit.java b/src/org/unitConverter/unit/FunctionalUnit.java
index e3db43a..7ddd876 100644
--- a/src/org/unitConverter/unit/FunctionalUnit.java
+++ b/src/org/unitConverter/unit/FunctionalUnit.java
@@ -19,13 +19,15 @@ package org.unitConverter.unit;
 import java.util.Objects;
 import java.util.function.DoubleUnaryOperator;
 
+import org.unitConverter.math.ObjectProduct;
+
 /**
  * A unit that uses functional objects to convert to and from its base.
  * 
  * @author Adrien Hopkins
  * @since 2019-05-22
  */
-final class FunctionalUnit extends AbstractUnit {
+final class FunctionalUnit extends Unit {
 	/**
 	 * Returns a unit from its base and the functions it uses to convert to and from its base.
 	 * 
@@ -42,7 +44,7 @@ final class FunctionalUnit extends AbstractUnit {
 	 * @throws NullPointerException
 	 *             if any argument is null
 	 */
-	public static FunctionalUnit valueOf(final BaseUnit base, final DoubleUnaryOperator converterFrom,
+	public static FunctionalUnit valueOf(final ObjectProduct<BaseUnit> base, final DoubleUnaryOperator converterFrom,
 			final DoubleUnaryOperator converterTo) {
 		return new FunctionalUnit(base, converterFrom, converterTo);
 	}
@@ -76,7 +78,7 @@ final class FunctionalUnit extends AbstractUnit {
 	 *             if any argument is null
 	 * @since 2019-05-22
 	 */
-	private FunctionalUnit(final BaseUnit base, final DoubleUnaryOperator converterFrom,
+	private FunctionalUnit(final ObjectProduct<BaseUnit> base, final DoubleUnaryOperator converterFrom,
 			final DoubleUnaryOperator converterTo) {
 		super(base);
 		this.converterFrom = Objects.requireNonNull(converterFrom, "converterFrom must not be null.");
diff --git a/src/org/unitConverter/unit/LinearUnit.java b/src/org/unitConverter/unit/LinearUnit.java
index 1b1ac97..1918d6b 100644
--- a/src/org/unitConverter/unit/LinearUnit.java
+++ b/src/org/unitConverter/unit/LinearUnit.java
@@ -18,73 +18,78 @@ package org.unitConverter.unit;
 
 import java.util.Objects;
 
-import org.unitConverter.dimension.UnitDimension;
 import org.unitConverter.math.DecimalComparison;
+import org.unitConverter.math.ObjectProduct;
 
 /**
- * A unit that is equal to a certain number multiplied by its base.
- * <p>
- * {@code LinearUnit} does not have any public constructors or static factories. In order to obtain a {@code LinearUnit}
- * instance, multiply its base by the conversion factor. Example:
- * 
- * <pre>
- * LinearUnit foot = METRE.times(0.3048);
- * </pre>
- * 
- * (where {@code METRE} is a {@code BaseUnit} instance)
- * </p>
+ * A unit that can be expressed as a product of its base and a number. For example, kilometres, inches and pounds.
  * 
  * @author Adrien Hopkins
- * @since 2018-12-22
- * @since v0.1.0
+ * @since 2019-10-16
  */
-public class LinearUnit extends AbstractUnit {
+public final class LinearUnit extends Unit {
 	/**
-	 * The value of one of this unit in this unit's base unit
+	 * Gets a {@code LinearUnit} from a unit and a value. For example, converts '59 °F' to a linear unit with the value
+	 * of '288.15 K'
 	 * 
-	 * @since 2018-12-22
-	 * @since v0.1.0
+	 * @param unit
+	 *            unit to convert
+	 * @param value
+	 *            value to convert
+	 * @return value expressed as a {@code LinearUnit}
+	 * @since 2019-10-16
 	 */
-	private final double conversionFactor;
+	public static LinearUnit fromUnitValue(final Unit unit, final double value) {
+		return new LinearUnit(unit.getBase(), unit.convertToBase(value));
+	}
 
 	/**
+	 * Gets a {@code LinearUnit} from a unit base and a conversion factor. In other words, gets the product of
+	 * {@code unitBase} and {@code conversionFactor}, expressed as a {@code LinearUnit}.
 	 * 
-	 * Creates the {@code LinearUnit}.
-	 * 
-	 * @param base
-	 *            unit's base
+	 * @param unitBase
+	 *            unit base to multiply by
 	 * @param conversionFactor
-	 *            value of one of this unit in its base
-	 * @since 2018-12-23
-	 * @since v0.1.0
+	 *            number to multiply base by
+	 * @return product of base and conversion factor
+	 * @since 2019-10-16
 	 */
-	LinearUnit(final BaseUnit base, final double conversionFactor) {
-		super(base);
-		this.conversionFactor = conversionFactor;
+	public static LinearUnit valueOf(final ObjectProduct<BaseUnit> unitBase, final double conversionFactor) {
+		return new LinearUnit(unitBase, conversionFactor);
 	}
 
 	/**
-	 * Creates the {@code LinearUnit} as a base unit.
+	 * The value of this unit as represented in its base form. Mathematically,
 	 * 
-	 * @param dimension
-	 *            dimension measured by unit
-	 * @param system
-	 *            system unit is part of
-	 * @since 2019-01-25
-	 * @since v0.1.0
+	 * <pre>
+	 * this = conversionFactor * getBase()
+	 * </pre>
+	 * 
+	 * @since 2019-10-16
+	 */
+	private final double conversionFactor;
+
+	/**
+	 * Creates the {@code LinearUnit}.
+	 * 
+	 * @param unitBase
+	 *            base of linear unit
+	 * @param conversionFactor
+	 *            conversion factor between base and unit
+	 * @since 2019-10-16
 	 */
-	LinearUnit(final UnitDimension dimension, final UnitSystem system, final double conversionFactor) {
-		super(dimension, system);
+	private LinearUnit(final ObjectProduct<BaseUnit> unitBase, final double conversionFactor) {
+		super(unitBase);
 		this.conversionFactor = conversionFactor;
 	}
 
 	@Override
-	public double convertFromBase(final double value) {
+	protected double convertFromBase(final double value) {
 		return value / this.getConversionFactor();
 	}
 
 	@Override
-	public double convertToBase(final double value) {
+	protected double convertToBase(final double value) {
 		return value * this.getConversionFactor();
 	}
 
@@ -98,21 +103,15 @@ public class LinearUnit extends AbstractUnit {
 	 * @since v0.1.0
 	 */
 	public LinearUnit dividedBy(final double divisor) {
-		return new LinearUnit(this.getBase(), this.getConversionFactor() / divisor);
+		return valueOf(this.getBase(), this.getConversionFactor() / divisor);
 	}
 
 	/**
 	 * Returns the quotient of this unit and another.
-	 * <p>
-	 * Two units can be divided if they are part of the same unit system. If {@code divisor} does not meet this
-	 * condition, an {@code IllegalArgumentException} should be thrown.
-	 * </p>
 	 * 
 	 * @param divisor
 	 *            unit to divide by
 	 * @return quotient of two units
-	 * @throws IllegalArgumentException
-	 *             if {@code divisor} is not compatible for division as described above
 	 * @throws NullPointerException
 	 *             if {@code divisor} is null
 	 * @since 2018-12-22
@@ -121,14 +120,9 @@ public class LinearUnit extends AbstractUnit {
 	public LinearUnit dividedBy(final LinearUnit divisor) {
 		Objects.requireNonNull(divisor, "other must not be null");
 
-		// check that these units can be multiplied
-		if (!this.getSystem().equals(divisor.getSystem()))
-			throw new IllegalArgumentException(
-					String.format("Incompatible units for division \"%s\" and \"%s\".", this, divisor));
-
 		// divide the units
-		final BaseUnit base = this.getBase().dividedBy(divisor.getBase());
-		return new LinearUnit(base, this.getConversionFactor() / divisor.getConversionFactor());
+		final ObjectProduct<BaseUnit> base = this.getBase().dividedBy(divisor.getBase());
+		return valueOf(base, this.getConversionFactor() / divisor.getConversionFactor());
 	}
 
 	@Override
@@ -136,35 +130,45 @@ public class LinearUnit extends AbstractUnit {
 		if (!(obj instanceof LinearUnit))
 			return false;
 		final LinearUnit other = (LinearUnit) obj;
-		return Objects.equals(this.getSystem(), other.getSystem())
-				&& Objects.equals(this.getDimension(), other.getDimension())
+		return Objects.equals(this.getBase(), other.getBase())
 				&& DecimalComparison.equals(this.getConversionFactor(), other.getConversionFactor());
 	}
 
 	/**
-	 * @return conversion factor between this unit and its base
-	 * @since 2018-12-22
-	 * @since v0.1.0
+	 * @return conversion factor
+	 * @since 2019-10-16
 	 */
-	public final double getConversionFactor() {
+	public double getConversionFactor() {
 		return this.conversionFactor;
 	}
 
 	@Override
 	public int hashCode() {
-		final int prime = 31;
-		int result = 1;
-		result = result * prime + this.getSystem().hashCode();
-		result = result * prime + this.getDimension().hashCode();
-		result = result * prime + Double.hashCode(this.getConversionFactor());
-		return result;
+		return 31 * this.getBase().hashCode() + DecimalComparison.hash(this.getConversionFactor());
+	}
+
+	/**
+	 * @return whether this unit is equivalent to a {@code BaseUnit} (i.e. there is a {@code BaseUnit b} where
+	 *         {@code b.asLinearUnit().equals(this)} returns {@code true}.)
+	 * @since 2019-10-16
+	 */
+	public boolean isBase() {
+		return this.isCoherent() && this.getBase().isSingleObject();
+	}
+
+	/**
+	 * @return whether this unit is coherent (i.e. has conversion factor 1)
+	 * @since 2019-10-16
+	 */
+	public boolean isCoherent() {
+		return this.getConversionFactor() == 1;
 	}
 
 	/**
 	 * Returns the difference of this unit and another.
 	 * <p>
-	 * Two units can be subtracted if they have the same base. If {@code subtrahend} does not meet this condition, an
-	 * {@code IllegalArgumentException} will be thrown.
+	 * Two units can be subtracted if they have the same base. Note that {@link #canConvertTo} can be used to determine
+	 * this. If {@code subtrahend} does not meet this condition, an {@code IllegalArgumentException} will be thrown.
 	 * </p>
 	 * 
 	 * @param subtrahend
@@ -186,14 +190,14 @@ public class LinearUnit extends AbstractUnit {
 					String.format("Incompatible units for subtraction \"%s\" and \"%s\".", this, subtrahendend));
 
 		// add the units
-		return new LinearUnit(this.getBase(), this.getConversionFactor() - subtrahendend.getConversionFactor());
+		return valueOf(this.getBase(), this.getConversionFactor() - subtrahendend.getConversionFactor());
 	}
 
 	/**
 	 * Returns the sum of this unit and another.
 	 * <p>
-	 * Two units can be added if they have the same base. If {@code addend} does not meet this condition, an
-	 * {@code IllegalArgumentException} will be thrown.
+	 * Two units can be added if they have the same base. Note that {@link #canConvertTo} can be used to determine this.
+	 * If {@code addend} does not meet this condition, an {@code IllegalArgumentException} will be thrown.
 	 * </p>
 	 * 
 	 * @param addend
@@ -215,7 +219,7 @@ public class LinearUnit extends AbstractUnit {
 					String.format("Incompatible units for addition \"%s\" and \"%s\".", this, addend));
 
 		// add the units
-		return new LinearUnit(this.getBase(), this.getConversionFactor() + addend.getConversionFactor());
+		return valueOf(this.getBase(), this.getConversionFactor() + addend.getConversionFactor());
 	}
 
 	/**
@@ -228,21 +232,15 @@ public class LinearUnit extends AbstractUnit {
 	 * @since v0.1.0
 	 */
 	public LinearUnit times(final double multiplier) {
-		return new LinearUnit(this.getBase(), this.getConversionFactor() * multiplier);
+		return valueOf(this.getBase(), this.getConversionFactor() * multiplier);
 	}
 
 	/**
 	 * Returns the product of this unit and another.
-	 * <p>
-	 * Two units can be multiplied if they are part of the same unit system. If {@code multiplier} does not meet this
-	 * condition, an {@code IllegalArgumentException} should be thrown.
-	 * </p>
 	 * 
 	 * @param multiplier
 	 *            unit to multiply by
 	 * @return product of two units
-	 * @throws IllegalArgumentException
-	 *             if {@code multiplier} is not compatible for multiplication as described above
 	 * @throws NullPointerException
 	 *             if {@code multiplier} is null
 	 * @since 2018-12-22
@@ -251,32 +249,28 @@ public class LinearUnit extends AbstractUnit {
 	public LinearUnit times(final LinearUnit multiplier) {
 		Objects.requireNonNull(multiplier, "other must not be null");
 
-		// check that these units can be multiplied
-		if (!this.getSystem().equals(multiplier.getSystem()))
-			throw new IllegalArgumentException(
-					String.format("Incompatible units for multiplication \"%s\" and \"%s\".", this, multiplier));
-
 		// multiply the units
-		final BaseUnit base = this.getBase().times(multiplier.getBase());
-		return new LinearUnit(base, this.getConversionFactor() * multiplier.getConversionFactor());
+		final ObjectProduct<BaseUnit> base = this.getBase().times(multiplier.getBase());
+		return valueOf(base, this.getConversionFactor() * multiplier.getConversionFactor());
 	}
 
 	/**
 	 * Returns this unit but to an exponent.
 	 * 
 	 * @param exponent
-	 *            exponent to exponientate unit to
-	 * @return exponientated unit
+	 *            exponent to exponentiate unit to
+	 * @return exponentiated unit
 	 * @since 2019-01-15
 	 * @since v0.1.0
 	 */
 	public LinearUnit toExponent(final int exponent) {
-		return new LinearUnit(this.getBase().toExponent(exponent), Math.pow(this.conversionFactor, exponent));
+		return valueOf(this.getBase().toExponent(exponent), Math.pow(this.conversionFactor, exponent));
 	}
 
+	// returns a definition of the unit
 	@Override
 	public String toString() {
-		return super.toString() + String.format(" (equal to %s * base)", this.getConversionFactor());
+		return Double.toString(this.conversionFactor) + " * " + this.getBase().toString(BaseUnit::getSymbol);
 	}
 
 	/**
diff --git a/src/org/unitConverter/unit/SI.java b/src/org/unitConverter/unit/SI.java
index 46e6ff1..f623179 100644
--- a/src/org/unitConverter/unit/SI.java
+++ b/src/org/unitConverter/unit/SI.java
@@ -1,74 +1,228 @@
-/**
- * Copyright (C) 2018 Adrien Hopkins
- *
- * This program is free software: you can redistribute it and/or modify
- * it under the terms of the GNU Affero General Public License as published by
- * the Free Software Foundation, either version 3 of the License, or
- * (at your option) any later version.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU Affero General Public License for more details.
- *
- * You should have received a copy of the GNU Affero General Public License
- * along with this program.  If not, see <https://www.gnu.org/licenses/>.
- */
-package org.unitConverter.unit;
-
-import java.util.HashSet;
-import java.util.Objects;
-import java.util.Set;
-
-import org.unitConverter.dimension.StandardDimensions;
-import org.unitConverter.dimension.UnitDimension;
-
-/**
- * The SI, which holds all SI units
- * 
- * @author Adrien Hopkins
- * @since 2018-12-23
- * @since v0.1.0
- */
-public enum SI implements UnitSystem {
-	SI;
-
-	/**
-	 * This system's base units.
-	 * 
-	 * @since 2019-01-25
-	 * @since v0.1.0
-	 */
-	private static final Set<BaseUnit> baseUnits = new HashSet<>();
-
-	// base units
-	public static final BaseUnit METRE = SI.getBaseUnit(StandardDimensions.LENGTH);
-	public static final BaseUnit KILOGRAM = SI.getBaseUnit(StandardDimensions.MASS);
-	public static final BaseUnit SECOND = SI.getBaseUnit(StandardDimensions.TIME);
-	public static final BaseUnit AMPERE = SI.getBaseUnit(StandardDimensions.ELECTRIC_CURRENT);
-	public static final BaseUnit KELVIN = SI.getBaseUnit(StandardDimensions.TEMPERATURE);
-	public static final BaseUnit MOLE = SI.getBaseUnit(StandardDimensions.QUANTITY);
-	public static final BaseUnit CANDELA = SI.getBaseUnit(StandardDimensions.LUMINOUS_INTENSITY);
-
-	@Override
-	public BaseUnit getBaseUnit(final UnitDimension dimension) {
-		// try to find an existing unit before creating a new one
-
-		Objects.requireNonNull(dimension, "dimension must not be null.");
-		for (final BaseUnit unit : baseUnits) {
-			// it will be equal since the conditions for equality are dimension and system,
-			// and system is always SI.
-			if (unit.getDimension().equals(dimension))
-				return unit;
-		}
-		// could not find an existing base unit
-		final BaseUnit unit = new BaseUnit(dimension, this);
-		baseUnits.add(unit);
-		return unit;
-	}
-
-	@Override
-	public String getName() {
-		return "SI";
-	}
-}
+/**

+ * Copyright (C) 2018 Adrien Hopkins

+ *

+ * This program is free software: you can redistribute it and/or modify

+ * it under the terms of the GNU Affero General Public License as published by

+ * the Free Software Foundation, either version 3 of the License, or

+ * (at your option) any later version.

+ *

+ * This program is distributed in the hope that it will be useful,

+ * but WITHOUT ANY WARRANTY; without even the implied warranty of

+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

+ * GNU Affero General Public License for more details.

+ *

+ * You should have received a copy of the GNU Affero General Public License

+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.

+ */

+package org.unitConverter.unit;

+

+import org.unitConverter.math.ObjectProduct;

+

+/**

+ * All of the units, prefixes and dimensions that are used by the SI, as well as some outside the SI.

+ * 

+ * <p>

+ * This class does not include prefixed units. To obtain prefixed units, use {@link LinearUnit#withPrefix}:

+ * 

+ * <pre>

+ * LinearUnit KILOMETRE = SI.METRE.withPrefix(SI.KILO);

+ * </pre>

+ * 

+ * 

+ * @author Adrien Hopkins

+ * @since 2019-10-16

+ */

+public final class SI {

+	/// dimensions used by SI units

+	// base dimensions, as BaseDimensions

+	public static final class BaseDimensions {

+		public static final BaseDimension LENGTH = BaseDimension.valueOf("Length", "L");

+		public static final BaseDimension MASS = BaseDimension.valueOf("Mass", "M");

+		public static final BaseDimension TIME = BaseDimension.valueOf("Time", "T");

+		public static final BaseDimension ELECTRIC_CURRENT = BaseDimension.valueOf("Electric Current", "I");

+		public static final BaseDimension TEMPERATURE = BaseDimension.valueOf("Temperature", "\u0398"); // theta symbol

+		public static final BaseDimension QUANTITY = BaseDimension.valueOf("Quantity", "N");

+		public static final BaseDimension LUMINOUS_INTENSITY = BaseDimension.valueOf("Luminous Intensity", "J");

+		public static final BaseDimension INFORMATION = BaseDimension.valueOf("Information", "Info"); // non-SI

+		public static final BaseDimension CURRENCY = BaseDimension.valueOf("Currency", "$$"); // non-SI

+

+		// You may NOT get SI.BaseDimensions instances!

+		private BaseDimensions() {

+			throw new AssertionError();

+		}

+	}

+

+	/// base units of the SI

+	// suppressing warnings since these are the same object, but in a different form (class)

+	@SuppressWarnings("hiding")

+	public static final class BaseUnits {

+		public static final BaseUnit METRE = BaseUnit.valueOf(BaseDimensions.LENGTH, "metre", "m");

+		public static final BaseUnit KILOGRAM = BaseUnit.valueOf(BaseDimensions.MASS, "kilogram", "kg");

+		public static final BaseUnit SECOND = BaseUnit.valueOf(BaseDimensions.TIME, "second", "s");

+		public static final BaseUnit AMPERE = BaseUnit.valueOf(BaseDimensions.ELECTRIC_CURRENT, "ampere", "A");

+		public static final BaseUnit KELVIN = BaseUnit.valueOf(BaseDimensions.TEMPERATURE, "kelvin", "K");

+		public static final BaseUnit MOLE = BaseUnit.valueOf(BaseDimensions.QUANTITY, "mole", "mol");

+		public static final BaseUnit CANDELA = BaseUnit.valueOf(BaseDimensions.LUMINOUS_INTENSITY, "candela", "cd");

+		public static final BaseUnit BIT = BaseUnit.valueOf(BaseDimensions.INFORMATION, "bit", "b");

+		public static final BaseUnit DOLLAR = BaseUnit.valueOf(BaseDimensions.CURRENCY, "dollar", "$");

+

+		// You may NOT get SI.BaseUnits instances!

+		private BaseUnits() {

+			throw new AssertionError();

+		}

+	}

+

+	// dimensions used in the SI, as ObjectProducts

+	public static final class Dimensions {

+		public static final ObjectProduct<BaseDimension> EMPTY = ObjectProduct.empty();

+		public static final ObjectProduct<BaseDimension> LENGTH = ObjectProduct.oneOf(BaseDimensions.LENGTH);

+		public static final ObjectProduct<BaseDimension> MASS = ObjectProduct.oneOf(BaseDimensions.MASS);

+		public static final ObjectProduct<BaseDimension> TIME = ObjectProduct.oneOf(BaseDimensions.TIME);

+		public static final ObjectProduct<BaseDimension> ELECTRIC_CURRENT = ObjectProduct

+				.oneOf(BaseDimensions.ELECTRIC_CURRENT);

+		public static final ObjectProduct<BaseDimension> TEMPERATURE = ObjectProduct.oneOf(BaseDimensions.TEMPERATURE);

+		public static final ObjectProduct<BaseDimension> QUANTITY = ObjectProduct.oneOf(BaseDimensions.QUANTITY);

+		public static final ObjectProduct<BaseDimension> LUMINOUS_INTENSITY = ObjectProduct

+				.oneOf(BaseDimensions.LUMINOUS_INTENSITY);

+		public static final ObjectProduct<BaseDimension> INFORMATION = ObjectProduct.oneOf(BaseDimensions.INFORMATION);

+		public static final ObjectProduct<BaseDimension> CURRENCY = ObjectProduct.oneOf(BaseDimensions.CURRENCY);

+		// derived dimensions without named SI units

+		public static final ObjectProduct<BaseDimension> AREA = LENGTH.times(LENGTH);

+

+		public static final ObjectProduct<BaseDimension> VOLUME = AREA.times(LENGTH);

+		public static final ObjectProduct<BaseDimension> VELOCITY = LENGTH.dividedBy(TIME);

+		public static final ObjectProduct<BaseDimension> ACCELERATION = VELOCITY.dividedBy(TIME);

+		public static final ObjectProduct<BaseDimension> WAVENUMBER = EMPTY.dividedBy(LENGTH);

+		public static final ObjectProduct<BaseDimension> MASS_DENSITY = MASS.dividedBy(VOLUME);

+		public static final ObjectProduct<BaseDimension> SURFACE_DENSITY = MASS.dividedBy(AREA);

+		public static final ObjectProduct<BaseDimension> SPECIFIC_VOLUME = VOLUME.dividedBy(MASS);

+		public static final ObjectProduct<BaseDimension> CURRENT_DENSITY = ELECTRIC_CURRENT.dividedBy(AREA);

+		public static final ObjectProduct<BaseDimension> MAGNETIC_FIELD_STRENGTH = ELECTRIC_CURRENT.dividedBy(LENGTH);

+		public static final ObjectProduct<BaseDimension> CONCENTRATION = QUANTITY.dividedBy(VOLUME);

+		public static final ObjectProduct<BaseDimension> MASS_CONCENTRATION = CONCENTRATION.times(MASS);

+		public static final ObjectProduct<BaseDimension> LUMINANCE = LUMINOUS_INTENSITY.dividedBy(AREA);

+		public static final ObjectProduct<BaseDimension> REFRACTIVE_INDEX = VELOCITY.dividedBy(VELOCITY);

+		public static final ObjectProduct<BaseDimension> REFLACTIVE_PERMEABILITY = EMPTY.times(EMPTY);

+		public static final ObjectProduct<BaseDimension> ANGLE = LENGTH.dividedBy(LENGTH);

+		public static final ObjectProduct<BaseDimension> SOLID_ANGLE = AREA.dividedBy(AREA);

+		// derived dimensions with named SI units

+		public static final ObjectProduct<BaseDimension> FREQUENCY = EMPTY.dividedBy(TIME);

+

+		public static final ObjectProduct<BaseDimension> FORCE = MASS.times(ACCELERATION);

+		public static final ObjectProduct<BaseDimension> ENERGY = FORCE.times(LENGTH);

+		public static final ObjectProduct<BaseDimension> POWER = ENERGY.dividedBy(TIME);

+		public static final ObjectProduct<BaseDimension> ELECTRIC_CHARGE = ELECTRIC_CURRENT.times(TIME);

+		public static final ObjectProduct<BaseDimension> VOLTAGE = ENERGY.dividedBy(ELECTRIC_CHARGE);

+		public static final ObjectProduct<BaseDimension> CAPACITANCE = ELECTRIC_CHARGE.dividedBy(VOLTAGE);

+		public static final ObjectProduct<BaseDimension> ELECTRIC_RESISTANCE = VOLTAGE.dividedBy(ELECTRIC_CURRENT);

+		public static final ObjectProduct<BaseDimension> ELECTRIC_CONDUCTANCE = ELECTRIC_CURRENT.dividedBy(VOLTAGE);

+		public static final ObjectProduct<BaseDimension> MAGNETIC_FLUX = VOLTAGE.times(TIME);

+		public static final ObjectProduct<BaseDimension> MAGNETIC_FLUX_DENSITY = MAGNETIC_FLUX.dividedBy(AREA);

+		public static final ObjectProduct<BaseDimension> INDUCTANCE = MAGNETIC_FLUX.dividedBy(ELECTRIC_CURRENT);

+		public static final ObjectProduct<BaseDimension> LUMINOUS_FLUX = LUMINOUS_INTENSITY.times(SOLID_ANGLE);

+		public static final ObjectProduct<BaseDimension> ILLUMINANCE = LUMINOUS_FLUX.dividedBy(AREA);

+		public static final ObjectProduct<BaseDimension> SPECIFIC_ENERGY = ENERGY.dividedBy(MASS);

+		public static final ObjectProduct<BaseDimension> CATALYTIC_ACTIVITY = QUANTITY.dividedBy(TIME);

+

+		// You may NOT get SI.Dimension instances!

+		private Dimensions() {

+			throw new AssertionError();

+		}

+	}

+

+	/// The units of the SI

+	public static final LinearUnit ONE = LinearUnit.valueOf(ObjectProduct.empty(), 1);

+	public static final LinearUnit METRE = BaseUnits.METRE.asLinearUnit();

+	public static final LinearUnit KILOGRAM = BaseUnits.KILOGRAM.asLinearUnit();

+	public static final LinearUnit SECOND = BaseUnits.SECOND.asLinearUnit();

+	public static final LinearUnit AMPERE = BaseUnits.AMPERE.asLinearUnit();

+	public static final LinearUnit KELVIN = BaseUnits.KELVIN.asLinearUnit();

+	public static final LinearUnit MOLE = BaseUnits.MOLE.asLinearUnit();

+	public static final LinearUnit CANDELA = BaseUnits.CANDELA.asLinearUnit();

+	public static final LinearUnit BIT = BaseUnits.BIT.asLinearUnit();

+	public static final LinearUnit DOLLAR = BaseUnits.DOLLAR.asLinearUnit();

+

+	// Non-base units

+	public static final LinearUnit RADIAN = METRE.dividedBy(METRE);

+	public static final LinearUnit STERADIAN = RADIAN.times(RADIAN);

+	public static final LinearUnit HERTZ = ONE.dividedBy(SECOND); // for periodic phenomena

+	public static final LinearUnit NEWTON = KILOGRAM.times(METRE).dividedBy(SECOND.times(SECOND));

+	public static final LinearUnit PASCAL = NEWTON.dividedBy(METRE.times(METRE));

+	public static final LinearUnit JOULE = NEWTON.times(METRE);

+	public static final LinearUnit WATT = JOULE.dividedBy(SECOND);

+	public static final LinearUnit COULOMB = AMPERE.times(SECOND);

+	public static final LinearUnit VOLT = JOULE.dividedBy(COULOMB);

+	public static final LinearUnit FARAD = COULOMB.dividedBy(VOLT);

+	public static final LinearUnit OHM = VOLT.dividedBy(AMPERE);

+	public static final LinearUnit SIEMENS = ONE.dividedBy(OHM);

+	public static final LinearUnit WEBER = VOLT.times(SECOND);

+	public static final LinearUnit TESLA = WEBER.dividedBy(METRE.times(METRE));

+	public static final LinearUnit HENRY = WEBER.dividedBy(AMPERE);

+	public static final LinearUnit LUMEN = CANDELA.times(STERADIAN);

+	public static final LinearUnit LUX = LUMEN.dividedBy(METRE.times(METRE));

+	public static final LinearUnit BEQUEREL = ONE.dividedBy(SECOND); // for activity referred to a nucleotide

+	public static final LinearUnit GRAY = JOULE.dividedBy(KILOGRAM); // for absorbed dose

+	public static final LinearUnit SIEVERT = JOULE.dividedBy(KILOGRAM); // for dose equivalent

+	public static final LinearUnit KATAL = MOLE.dividedBy(SECOND);

+

+	// Non-SI units included for convenience

+	public static final Unit CELSIUS = Unit.fromConversionFunctions(KELVIN.getBase(), tempK -> tempK - 273.15,

+			tempC -> tempC + 273.15);

+	public static final LinearUnit MINUTE = SECOND.times(60);

+	public static final LinearUnit HOUR = MINUTE.times(60);

+	public static final LinearUnit DAY = HOUR.times(60);

+	public static final LinearUnit DEGREE = RADIAN.times(360 / (2 * Math.PI));

+	public static final LinearUnit ARCMINUTE = DEGREE.dividedBy(60);

+	public static final LinearUnit ARCSECOND = ARCMINUTE.dividedBy(60);

+	public static final LinearUnit ASTRONOMICAL_UNIT = METRE.times(149597870700.0);

+	public static final LinearUnit PARSEC = ASTRONOMICAL_UNIT.times(ARCSECOND);

+	public static final LinearUnit HECTARE = METRE.times(METRE).times(10000.0);

+	public static final LinearUnit LITRE = METRE.times(METRE).times(METRE).dividedBy(1000.0);

+	public static final LinearUnit TONNE = KILOGRAM.times(1000.0);

+	public static final LinearUnit DALTON = KILOGRAM.times(1.660539040e-27); // approximate value

+	public static final LinearUnit ELECTRONVOLT = JOULE.times(1.602176634e-19);

+	public static final Unit NEPER = Unit.fromConversionFunctions(ONE.getBase(), pr -> 0.5 * Math.log(pr),

+			Np -> Math.exp(2 * Np));

+	public static final Unit BEL = Unit.fromConversionFunctions(ONE.getBase(), pr -> Math.log10(pr),

+			dB -> Math.pow(10, dB));

+	public static final Unit DECIBEL = Unit.fromConversionFunctions(ONE.getBase(), pr -> 10 * Math.log10(pr),

+			dB -> Math.pow(10, dB / 10));

+

+	/// The prefixes of the SI

+	// expanding decimal prefixes

+	public static final UnitPrefix KILO = UnitPrefix.valueOf(1e3);

+	public static final UnitPrefix MEGA = UnitPrefix.valueOf(1e6);

+	public static final UnitPrefix GIGA = UnitPrefix.valueOf(1e9);

+	public static final UnitPrefix TERA = UnitPrefix.valueOf(1e12);

+	public static final UnitPrefix PETA = UnitPrefix.valueOf(1e15);

+	public static final UnitPrefix EXA = UnitPrefix.valueOf(1e18);

+	public static final UnitPrefix ZETTA = UnitPrefix.valueOf(1e21);

+	public static final UnitPrefix YOTTA = UnitPrefix.valueOf(1e24);

+

+	// contracting decimal prefixes

+	public static final UnitPrefix MILLI = UnitPrefix.valueOf(1e-3);

+	public static final UnitPrefix MICRO = UnitPrefix.valueOf(1e-6);

+	public static final UnitPrefix NANO = UnitPrefix.valueOf(1e-9);

+	public static final UnitPrefix PICO = UnitPrefix.valueOf(1e-12);

+	public static final UnitPrefix FEMTO = UnitPrefix.valueOf(1e-15);

+	public static final UnitPrefix ATTO = UnitPrefix.valueOf(1e-18);

+	public static final UnitPrefix ZEPTO = UnitPrefix.valueOf(1e-21);

+	public static final UnitPrefix YOCTO = UnitPrefix.valueOf(1e-24);

+

+	// prefixes that don't match the pattern of thousands

+	public static final UnitPrefix DEKA = UnitPrefix.valueOf(1e1);

+	public static final UnitPrefix HECTO = UnitPrefix.valueOf(1e2);

+	public static final UnitPrefix DECI = UnitPrefix.valueOf(1e-1);

+	public static final UnitPrefix CENTI = UnitPrefix.valueOf(1e-2);

+	public static final UnitPrefix KIBI = UnitPrefix.valueOf(1024);

+	public static final UnitPrefix MEBI = KIBI.times(1024);

+	public static final UnitPrefix GIBI = MEBI.times(1024);

+	public static final UnitPrefix TEBI = GIBI.times(1024);

+	public static final UnitPrefix PEBI = TEBI.times(1024);

+	public static final UnitPrefix EXBI = PEBI.times(1024);

+

+	// You may NOT get SI instances!

+	private SI() {

+		throw new AssertionError();

+	}

+}

diff --git a/src/org/unitConverter/unit/SIPrefix.java b/src/org/unitConverter/unit/SIPrefix.java
deleted file mode 100644
index 31d7ff2..0000000
--- a/src/org/unitConverter/unit/SIPrefix.java
+++ /dev/null
@@ -1,54 +0,0 @@
-/**
- * Copyright (C) 2018 Adrien Hopkins
- *
- * This program is free software: you can redistribute it and/or modify
- * it under the terms of the GNU Affero General Public License as published by
- * the Free Software Foundation, either version 3 of the License, or
- * (at your option) any later version.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU Affero General Public License for more details.
- *
- * You should have received a copy of the GNU Affero General Public License
- * along with this program.  If not, see <https://www.gnu.org/licenses/>.
- */
-package org.unitConverter.unit;
-
-/**
- * The SI prefixes.
- * 
- * @author Adrien Hopkins
- * @since 2019-01-14
- * @since v0.1.0
- */
-public enum SIPrefix implements UnitPrefix {
-	DECA(10), HECTO(100), KILO(1e3), MEGA(1e6), GIGA(1e9), TERA(1e12), PETA(1e15), EXA(1e18), ZETTA(1e21), YOTTA(
-			1e24), DECI(0.1), CENTI(0.01), MILLI(
-					1e-3), MICRO(1e-6), NANO(1e-9), PICO(1e-12), FEMTO(1e-15), ATTO(1e-18), ZEPTO(1e-21), YOCTO(1e-24);
-
-	private final double multiplier;
-
-	/**
-	 * Creates the {@code SIPrefix}.
-	 * 
-	 * @param multiplier
-	 *            prefix's multiplier
-	 * @since 2019-01-14
-	 * @since v0.1.0
-	 */
-	private SIPrefix(final double multiplier) {
-		this.multiplier = multiplier;
-	}
-
-	/**
-	 * @return value
-	 * @since 2019-01-14
-	 * @since v0.1.0
-	 */
-	@Override
-	public final double getMultiplier() {
-		return this.multiplier;
-	}
-}
diff --git a/src/org/unitConverter/unit/USCustomary.java b/src/org/unitConverter/unit/USCustomary.java
new file mode 100644
index 0000000..f5f9a7f
--- /dev/null
+++ b/src/org/unitConverter/unit/USCustomary.java
@@ -0,0 +1,27 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.unit;
+
+/**
+ * A static utility class that contains units in the US Customary system.
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-10-21
+ */
+public final class USCustomary {
+	public static final Unit FAHRENHEIT = BritishImperial.FAHRENHEIT;
+}
diff --git a/src/org/unitConverter/unit/Unit.java b/src/org/unitConverter/unit/Unit.java
index 54f0ab5..7971a41 100644
--- a/src/org/unitConverter/unit/Unit.java
+++ b/src/org/unitConverter/unit/Unit.java
@@ -1,5 +1,5 @@
 /**
- * Copyright (C) 2018 Adrien Hopkins
+ * Copyright (C) 2019 Adrien Hopkins
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as published by
@@ -16,19 +16,20 @@
  */
 package org.unitConverter.unit;
 
+import java.util.HashMap;
+import java.util.Map;
 import java.util.Objects;
 import java.util.function.DoubleUnaryOperator;
 
-import org.unitConverter.dimension.UnitDimension;
+import org.unitConverter.math.ObjectProduct;
 
 /**
- * A unit that has an associated base unit, and can convert a value expressed in it to and from that base.
+ * A unit that is composed of base units.
  * 
  * @author Adrien Hopkins
- * @since 2018-12-22
- * @since v0.1.0
+ * @since 2019-10-16
  */
-public interface Unit {
+public abstract class Unit {
 	/**
 	 * Returns a unit from its base and the functions it uses to convert to and from its base.
 	 * 
@@ -51,12 +52,50 @@ public interface Unit {
 	 * @throws NullPointerException
 	 *             if any argument is null
 	 */
-	public static Unit fromConversionFunctions(final BaseUnit base, final DoubleUnaryOperator converterFrom,
-			final DoubleUnaryOperator converterTo) {
+	public static final Unit fromConversionFunctions(final ObjectProduct<BaseUnit> base,
+			final DoubleUnaryOperator converterFrom, final DoubleUnaryOperator converterTo) {
 		return FunctionalUnit.valueOf(base, converterFrom, converterTo);
 	}
 
 	/**
+	 * The combination of units that this unit is based on.
+	 * 
+	 * @since 2019-10-16
+	 */
+	private final ObjectProduct<BaseUnit> unitBase;
+
+	/**
+	 * Cache storing the result of getDimension()
+	 *
+	 * @since 2019-10-16
+	 */
+	private transient ObjectProduct<BaseDimension> dimension = null;
+
+	/**
+	 * A constructor that constructs {@code BaseUnit} instances.
+	 * 
+	 * @since 2019-10-16
+	 */
+	Unit() {
+		if (this instanceof BaseUnit) {
+			this.unitBase = ObjectProduct.oneOf((BaseUnit) this);
+		} else
+			throw new AssertionError();
+	}
+
+	/**
+	 * Creates the {@code AbstractUnit}.
+	 * 
+	 * @param unitBase
+	 * @since 2019-10-16
+	 * @throws NullPointerException
+	 *             if unitBase is null
+	 */
+	protected Unit(final ObjectProduct<BaseUnit> unitBase) {
+		this.unitBase = Objects.requireNonNull(unitBase, "unitBase must not be null.");
+	}
+
+	/**
 	 * Checks if a value expressed in this unit can be converted to a value expressed in {@code other}
 	 * 
 	 * @param other
@@ -67,7 +106,7 @@ public interface Unit {
 	 * @throws NullPointerException
 	 *             if other is null
 	 */
-	default boolean canConvertTo(final Unit other) {
+	public final boolean canConvertTo(final Unit other) {
 		Objects.requireNonNull(other, "other must not be null.");
 		return Objects.equals(this.getBase(), other.getBase());
 	}
@@ -82,17 +121,24 @@ public interface Unit {
 	 * If this unit <i>is</i> a base unit, this method should return {@code value}.
 	 * </p>
 	 * 
+	 * @implSpec This method is used by {@link #convertTo}, and its behaviour affects the behaviour of
+	 *           {@code convertTo}.
+	 * 
 	 * @param value
 	 *            value expressed in <b>base</b> unit
 	 * @return value expressed in <b>this</b> unit
 	 * @since 2018-12-22
 	 * @since v0.1.0
 	 */
-	double convertFromBase(double value);
+	protected abstract double convertFromBase(double value);
 
 	/**
 	 * Converts a value expressed in this unit to a value expressed in {@code other}.
 	 * 
+	 * @implSpec If unit conversion is possible, this implementation returns
+	 *           {@code other.convertFromBase(this.convertToBase(value))}. Therefore, overriding either of those methods
+	 *           will change the output of this method.
+	 * 
 	 * @param other
 	 *            unit to convert to
 	 * @param value
@@ -105,7 +151,7 @@ public interface Unit {
 	 * @throws NullPointerException
 	 *             if other is null
 	 */
-	default double convertTo(final Unit other, final double value) {
+	public final double convertTo(final Unit other, final double value) {
 		Objects.requireNonNull(other, "other must not be null.");
 		if (this.canConvertTo(other))
 			return other.convertFromBase(this.convertToBase(value));
@@ -123,42 +169,47 @@ public interface Unit {
 	 * If this unit <i>is</i> a base unit, this method should return {@code value}.
 	 * </p>
 	 * 
+	 * @implSpec This method is used by {@link #convertTo}, and its behaviour affects the behaviour of
+	 *           {@code convertTo}.
+	 * 
 	 * @param value
 	 *            value expressed in <b>this</b> unit
 	 * @return value expressed in <b>base</b> unit
 	 * @since 2018-12-22
 	 * @since v0.1.0
 	 */
-	double convertToBase(double value);
+	protected abstract double convertToBase(double value);
 
 	/**
-	 * <p>
-	 * Returns the base unit associated with this unit.
-	 * </p>
-	 * <p>
-	 * The dimension of this unit must be equal to the dimension of the returned unit.
-	 * </p>
-	 * <p>
-	 * If this unit <i>is</i> a base unit, this method should return this unit.\
-	 * </p>
-	 * 
-	 * @return base unit associated with this unit
+	 * @return combination of units that this unit is based on
 	 * @since 2018-12-22
 	 * @since v0.1.0
 	 */
-	BaseUnit getBase();
+	public final ObjectProduct<BaseUnit> getBase() {
+		return this.unitBase;
+	}
 
 	/**
 	 * @return dimension measured by this unit
 	 * @since 2018-12-22
 	 * @since v0.1.0
 	 */
-	UnitDimension getDimension();
+	public final ObjectProduct<BaseDimension> getDimension() {
+		if (this.dimension == null) {
+			final Map<BaseUnit, Integer> mapping = this.unitBase.exponentMap();
+			final Map<BaseDimension, Integer> dimensionMap = new HashMap<>();
 
-	/**
-	 * @return system that this unit is a part of
-	 * @since 2018-12-23
-	 * @since v0.1.0
-	 */
-	UnitSystem getSystem();
+			for (final BaseUnit key : mapping.keySet()) {
+				dimensionMap.put(key.getBaseDimension(), mapping.get(key));
+			}
+
+			this.dimension = ObjectProduct.fromExponentMapping(dimensionMap);
+		}
+		return this.dimension;
+	}
+
+	@Override
+	public String toString() {
+		return "Unit derived from base " + this.getBase().toString();
+	}
 }
diff --git a/src/org/unitConverter/unit/UnitDatabase.java b/src/org/unitConverter/unit/UnitDatabase.java
new file mode 100644
index 0000000..a2b11c3
--- /dev/null
+++ b/src/org/unitConverter/unit/UnitDatabase.java
@@ -0,0 +1,1653 @@
+/**
+ * Copyright (C) 2018 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.unit;
+
+import java.io.BufferedReader;
+import java.io.File;
+import java.io.FileNotFoundException;
+import java.io.FileReader;
+import java.io.IOException;
+import java.util.AbstractSet;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+import java.util.NoSuchElementException;
+import java.util.Objects;
+import java.util.Set;
+import java.util.function.BiFunction;
+import java.util.function.Function;
+import java.util.function.Predicate;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+import org.unitConverter.math.DecimalComparison;
+import org.unitConverter.math.ExpressionParser;
+import org.unitConverter.math.ObjectProduct;
+
+/**
+ * A database of units, prefixes and dimensions, and their names.
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-01-07
+ * @since v0.1.0
+ */
+public final class UnitDatabase {
+	/**
+	 * A map for units that allows the use of prefixes.
+	 * <p>
+	 * As this map implementation is intended to be used as a sort of "augmented view" of a unit and prefix map, it is
+	 * unmodifiable but instead reflects the changes to the maps passed into it. Do not edit this map, instead edit the
+	 * maps that were passed in during construction.
+	 * </p>
+	 * <p>
+	 * The rules for applying prefixes onto units are the following:
+	 * <ul>
+	 * <li>Prefixes can only be applied to linear units.</li>
+	 * <li>Before attempting to search for prefixes in a unit name, this map will first search for a unit name. So, if
+	 * there are two units, "B" and "AB", and a prefix "A", this map will favour the unit "AB" over the unit "B" with
+	 * the prefix "A", even though they have the same string.</li>
+	 * <li>Longer prefixes are preferred to shorter prefixes. So, if you have units "BC" and "C", and prefixes "AB" and
+	 * "A", inputting "ABC" will return the unit "C" with the prefix "AB", not "BC" with the prefix "A".</li>
+	 * </ul>
+	 * </p>
+	 * <p>
+	 * This map is infinite in size if there is at least one unit and at least one prefix. If it is infinite, some
+	 * operations that only work with finite collections, like converting name/entry sets to arrays, will throw an
+	 * {@code IllegalStateException}.
+	 * </p>
+	 * <p>
+	 * Because of ambiguities between prefixes (i.e. kilokilo = mega), {@link #containsValue} and {@link #values()}
+	 * currently ignore prefixes.
+	 * </p>
+	 * 
+	 * @author Adrien Hopkins
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	private static final class PrefixedUnitMap implements Map<String, Unit> {
+		/**
+		 * The class used for entry sets.
+		 * 
+		 * <p>
+		 * If the map that created this set is infinite in size (has at least one unit and at least one prefix), this
+		 * set is infinite as well. If this set is infinite in size, {@link #toArray} will fail with a
+		 * {@code IllegalStateException} instead of creating an infinite-sized array.
+		 * </p>
+		 * 
+		 * @author Adrien Hopkins
+		 * @since 2019-04-13
+		 * @since v0.2.0
+		 */
+		private static final class PrefixedUnitEntrySet extends AbstractSet<Map.Entry<String, Unit>> {
+			/**
+			 * The entry for this set.
+			 * 
+			 * @author Adrien Hopkins
+			 * @since 2019-04-14
+			 * @since v0.2.0
+			 */
+			private static final class PrefixedUnitEntry implements Entry<String, Unit> {
+				private final String key;
+				private final Unit value;
+
+				/**
+				 * Creates the {@code PrefixedUnitEntry}.
+				 * 
+				 * @param key
+				 *            key
+				 * @param value
+				 *            value
+				 * @since 2019-04-14
+				 * @since v0.2.0
+				 */
+				public PrefixedUnitEntry(final String key, final Unit value) {
+					this.key = key;
+					this.value = value;
+				}
+
+				/**
+				 * @since 2019-05-03
+				 */
+				@Override
+				public boolean equals(final Object o) {
+					if (!(o instanceof Map.Entry))
+						return false;
+					final Map.Entry<?, ?> other = (Map.Entry<?, ?>) o;
+					return Objects.equals(this.getKey(), other.getKey())
+							&& Objects.equals(this.getValue(), other.getValue());
+				}
+
+				@Override
+				public String getKey() {
+					return this.key;
+				}
+
+				@Override
+				public Unit getValue() {
+					return this.value;
+				}
+
+				/**
+				 * @since 2019-05-03
+				 */
+				@Override
+				public int hashCode() {
+					return (this.getKey() == null ? 0 : this.getKey().hashCode())
+							^ (this.getValue() == null ? 0 : this.getValue().hashCode());
+				}
+
+				@Override
+				public Unit setValue(final Unit value) {
+					throw new UnsupportedOperationException("Cannot set value in an immutable entry");
+				}
+
+				/**
+				 * Returns a string representation of the entry. The format of the string is the string representation
+				 * of the key, then the equals ({@code =}) character, then the string representation of the value.
+				 * 
+				 * @since 2019-05-03
+				 */
+				@Override
+				public String toString() {
+					return this.getKey() + "=" + this.getValue();
+				}
+			}
+
+			/**
+			 * An iterator that iterates over the units of a {@code PrefixedUnitNameSet}.
+			 * 
+			 * @author Adrien Hopkins
+			 * @since 2019-04-14
+			 * @since v0.2.0
+			 */
+			private static final class PrefixedUnitEntryIterator implements Iterator<Entry<String, Unit>> {
+				// position in the unit list
+				private int unitNamePosition = 0;
+				// the indices of the prefixes attached to the current unit
+				private final List<Integer> prefixCoordinates = new ArrayList<>();
+
+				// values from the unit entry set
+				private final Map<String, Unit> map;
+				private transient final List<String> unitNames;
+				private transient final List<String> prefixNames;
+
+				/**
+				 * Creates the {@code UnitsDatabase.PrefixedUnitMap.PrefixedUnitNameSet.PrefixedUnitNameIterator}.
+				 * 
+				 * @since 2019-04-14
+				 * @since v0.2.0
+				 */
+				public PrefixedUnitEntryIterator(final PrefixedUnitMap map) {
+					this.map = map;
+					this.unitNames = new ArrayList<>(map.units.keySet());
+					this.prefixNames = new ArrayList<>(map.prefixes.keySet());
+				}
+
+				/**
+				 * @return current unit name
+				 * @since 2019-04-14
+				 * @since v0.2.0
+				 */
+				private String getCurrentUnitName() {
+					final StringBuilder unitName = new StringBuilder();
+					for (final int i : this.prefixCoordinates) {
+						unitName.append(this.prefixNames.get(i));
+					}
+					unitName.append(this.unitNames.get(this.unitNamePosition));
+
+					return unitName.toString();
+				}
+
+				@Override
+				public boolean hasNext() {
+					if (this.unitNames.isEmpty())
+						return false;
+					else {
+						if (this.prefixNames.isEmpty())
+							return this.unitNamePosition >= this.unitNames.size() - 1;
+						else
+							return true;
+					}
+				}
+
+				/**
+				 * Changes this iterator's position to the next available one.
+				 * 
+				 * @since 2019-04-14
+				 * @since v0.2.0
+				 */
+				private void incrementPosition() {
+					this.unitNamePosition++;
+
+					if (this.unitNamePosition >= this.unitNames.size()) {
+						// we have used all of our units, go to a different prefix
+						this.unitNamePosition = 0;
+
+						// if the prefix coordinates are empty, then set it to [0]
+						if (this.prefixCoordinates.isEmpty()) {
+							this.prefixCoordinates.add(0, 0);
+						} else {
+							// get the prefix coordinate to increment, then increment
+							int i = this.prefixCoordinates.size() - 1;
+							this.prefixCoordinates.set(i, this.prefixCoordinates.get(i) + 1);
+
+							// fix any carrying errors
+							while (i >= 0 && this.prefixCoordinates.get(i) >= this.prefixNames.size()) {
+								// carry over
+								this.prefixCoordinates.set(i--, 0); // null and decrement at the same time
+
+								if (i < 0) { // we need to add a new coordinate
+									this.prefixCoordinates.add(0, 0);
+								} else { // increment an existing one
+									this.prefixCoordinates.set(i, this.prefixCoordinates.get(i) + 1);
+								}
+							}
+						}
+					}
+				}
+
+				@Override
+				public Entry<String, Unit> next() {
+					// get next element
+					final Entry<String, Unit> nextEntry = this.peek();
+
+					// iterate to next position
+					this.incrementPosition();
+
+					return nextEntry;
+				}
+
+				/**
+				 * @return the next element in the iterator, without iterating over it
+				 * @since 2019-05-03
+				 */
+				private Entry<String, Unit> peek() {
+					if (!this.hasNext())
+						throw new NoSuchElementException("No units left!");
+
+					// if I have prefixes, ensure I'm not using a nonlinear unit
+					// since all of the unprefixed stuff is done, just remove nonlinear units
+					if (!this.prefixCoordinates.isEmpty()) {
+						while (this.unitNamePosition < this.unitNames.size()
+								&& !(this.map.get(this.unitNames.get(this.unitNamePosition)) instanceof LinearUnit)) {
+							this.unitNames.remove(this.unitNamePosition);
+						}
+					}
+
+					final String nextName = this.getCurrentUnitName();
+
+					return new PrefixedUnitEntry(nextName, this.map.get(nextName));
+				}
+
+				/**
+				 * Returns a string representation of the object. The exact details of the representation are
+				 * unspecified and subject to change.
+				 * 
+				 * @since 2019-05-03
+				 */
+				@Override
+				public String toString() {
+					return String.format("Iterator iterating over name-unit entries; next value is \"%s\"",
+							this.peek());
+				}
+			}
+
+			// the map that created this set
+			private final PrefixedUnitMap map;
+
+			/**
+			 * Creates the {@code PrefixedUnitNameSet}.
+			 * 
+			 * @param map
+			 *            map that created this set
+			 * @since 2019-04-13
+			 * @since v0.2.0
+			 */
+			public PrefixedUnitEntrySet(final PrefixedUnitMap map) {
+				this.map = map;
+			}
+
+			@Override
+			public boolean add(final Map.Entry<String, Unit> e) {
+				throw new UnsupportedOperationException("Cannot add to an immutable set");
+			}
+
+			@Override
+			public boolean addAll(final Collection<? extends Map.Entry<String, Unit>> c) {
+				throw new UnsupportedOperationException("Cannot add to an immutable set");
+			}
+
+			@Override
+			public void clear() {
+				throw new UnsupportedOperationException("Cannot clear an immutable set");
+			}
+
+			@Override
+			public boolean contains(final Object o) {
+				// get the entry
+				final Entry<String, Unit> entry;
+
+				try {
+					// This is OK because I'm in a try-catch block, catching the exact exception that would be thrown.
+					@SuppressWarnings("unchecked")
+					final Entry<String, Unit> tempEntry = (Entry<String, Unit>) o;
+					entry = tempEntry;
+				} catch (final ClassCastException e) {
+					throw new IllegalArgumentException("Attempted to test for an entry using a non-entry.");
+				}
+
+				return this.map.containsKey(entry.getKey()) && this.map.get(entry.getKey()).equals(entry.getValue());
+			}
+
+			@Override
+			public boolean containsAll(final Collection<?> c) {
+				for (final Object o : c)
+					if (!this.contains(o))
+						return false;
+				return true;
+			}
+
+			@Override
+			public boolean isEmpty() {
+				return this.map.isEmpty();
+			}
+
+			@Override
+			public Iterator<Entry<String, Unit>> iterator() {
+				return new PrefixedUnitEntryIterator(this.map);
+			}
+
+			@Override
+			public boolean remove(final Object o) {
+				throw new UnsupportedOperationException("Cannot remove from an immutable set");
+			}
+
+			@Override
+			public boolean removeAll(final Collection<?> c) {
+				throw new UnsupportedOperationException("Cannot remove from an immutable set");
+			}
+
+			@Override
+			public boolean removeIf(final Predicate<? super Entry<String, Unit>> filter) {
+				throw new UnsupportedOperationException("Cannot remove from an immutable set");
+			}
+
+			@Override
+			public boolean retainAll(final Collection<?> c) {
+				throw new UnsupportedOperationException("Cannot remove from an immutable set");
+			}
+
+			@Override
+			public int size() {
+				if (this.map.units.isEmpty())
+					return 0;
+				else {
+					if (this.map.prefixes.isEmpty())
+						return this.map.units.size();
+					else
+						// infinite set
+						return Integer.MAX_VALUE;
+				}
+			}
+
+			/**
+			 * @throws IllegalStateException
+			 *             if the set is infinite in size
+			 */
+			@Override
+			public Object[] toArray() {
+				if (this.map.units.isEmpty() || this.map.prefixes.isEmpty())
+					return super.toArray();
+				else
+					// infinite set
+					throw new IllegalStateException("Cannot make an infinite set into an array.");
+			}
+
+			/**
+			 * @throws IllegalStateException
+			 *             if the set is infinite in size
+			 */
+			@Override
+			public <T> T[] toArray(final T[] a) {
+				if (this.map.units.isEmpty() || this.map.prefixes.isEmpty())
+					return super.toArray(a);
+				else
+					// infinite set
+					throw new IllegalStateException("Cannot make an infinite set into an array.");
+			}
+
+			@Override
+			public String toString() {
+				if (this.map.units.isEmpty() || this.map.prefixes.isEmpty())
+					return super.toString();
+				else
+					return String.format("Infinite set of name-unit entries created from units %s and prefixes %s",
+							this.map.units, this.map.prefixes);
+			}
+		}
+
+		/**
+		 * The class used for unit name sets.
+		 * 
+		 * <p>
+		 * If the map that created this set is infinite in size (has at least one unit and at least one prefix), this
+		 * set is infinite as well. If this set is infinite in size, {@link #toArray} will fail with a
+		 * {@code IllegalStateException} instead of creating an infinite-sized array.
+		 * </p>
+		 * 
+		 * @author Adrien Hopkins
+		 * @since 2019-04-13
+		 * @since v0.2.0
+		 */
+		private static final class PrefixedUnitNameSet extends AbstractSet<String> {
+			/**
+			 * An iterator that iterates over the units of a {@code PrefixedUnitNameSet}.
+			 * 
+			 * @author Adrien Hopkins
+			 * @since 2019-04-14
+			 * @since v0.2.0
+			 */
+			private static final class PrefixedUnitNameIterator implements Iterator<String> {
+				// position in the unit list
+				private int unitNamePosition = 0;
+				// the indices of the prefixes attached to the current unit
+				private final List<Integer> prefixCoordinates = new ArrayList<>();
+
+				// values from the unit name set
+				private final Map<String, Unit> map;
+				private transient final List<String> unitNames;
+				private transient final List<String> prefixNames;
+
+				/**
+				 * Creates the {@code UnitsDatabase.PrefixedUnitMap.PrefixedUnitNameSet.PrefixedUnitNameIterator}.
+				 * 
+				 * @since 2019-04-14
+				 * @since v0.2.0
+				 */
+				public PrefixedUnitNameIterator(final PrefixedUnitMap map) {
+					this.map = map;
+					this.unitNames = new ArrayList<>(map.units.keySet());
+					this.prefixNames = new ArrayList<>(map.prefixes.keySet());
+				}
+
+				/**
+				 * @return current unit name
+				 * @since 2019-04-14
+				 * @since v0.2.0
+				 */
+				private String getCurrentUnitName() {
+					final StringBuilder unitName = new StringBuilder();
+					for (final int i : this.prefixCoordinates) {
+						unitName.append(this.prefixNames.get(i));
+					}
+					unitName.append(this.unitNames.get(this.unitNamePosition));
+
+					return unitName.toString();
+				}
+
+				@Override
+				public boolean hasNext() {
+					if (this.unitNames.isEmpty())
+						return false;
+					else {
+						if (this.prefixNames.isEmpty())
+							return this.unitNamePosition >= this.unitNames.size() - 1;
+						else
+							return true;
+					}
+				}
+
+				/**
+				 * Changes this iterator's position to the next available one.
+				 * 
+				 * @since 2019-04-14
+				 * @since v0.2.0
+				 */
+				private void incrementPosition() {
+					this.unitNamePosition++;
+
+					if (this.unitNamePosition >= this.unitNames.size()) {
+						// we have used all of our units, go to a different prefix
+						this.unitNamePosition = 0;
+
+						// if the prefix coordinates are empty, then set it to [0]
+						if (this.prefixCoordinates.isEmpty()) {
+							this.prefixCoordinates.add(0, 0);
+						} else {
+							// get the prefix coordinate to increment, then increment
+							int i = this.prefixCoordinates.size() - 1;
+							this.prefixCoordinates.set(i, this.prefixCoordinates.get(i) + 1);
+
+							// fix any carrying errors
+							while (i >= 0 && this.prefixCoordinates.get(i) >= this.prefixNames.size()) {
+								// carry over
+								this.prefixCoordinates.set(i--, 0); // null and decrement at the same time
+
+								if (i < 0) { // we need to add a new coordinate
+									this.prefixCoordinates.add(0, 0);
+								} else { // increment an existing one
+									this.prefixCoordinates.set(i, this.prefixCoordinates.get(i) + 1);
+								}
+							}
+						}
+					}
+				}
+
+				@Override
+				public String next() {
+					final String nextName = this.peek();
+
+					this.incrementPosition();
+
+					return nextName;
+				}
+
+				/**
+				 * @return the next element in the iterator, without iterating over it
+				 * @since 2019-05-03
+				 */
+				private String peek() {
+					if (!this.hasNext())
+						throw new NoSuchElementException("No units left!");
+					// if I have prefixes, ensure I'm not using a nonlinear unit
+					// since all of the unprefixed stuff is done, just remove nonlinear units
+					if (!this.prefixCoordinates.isEmpty()) {
+						while (this.unitNamePosition < this.unitNames.size()
+								&& !(this.map.get(this.unitNames.get(this.unitNamePosition)) instanceof LinearUnit)) {
+							this.unitNames.remove(this.unitNamePosition);
+						}
+					}
+
+					return this.getCurrentUnitName();
+				}
+
+				/**
+				 * Returns a string representation of the object. The exact details of the representation are
+				 * unspecified and subject to change.
+				 * 
+				 * @since 2019-05-03
+				 */
+				@Override
+				public String toString() {
+					return String.format("Iterator iterating over unit names; next value is \"%s\"", this.peek());
+				}
+			}
+
+			// the map that created this set
+			private final PrefixedUnitMap map;
+
+			/**
+			 * Creates the {@code PrefixedUnitNameSet}.
+			 * 
+			 * @param map
+			 *            map that created this set
+			 * @since 2019-04-13
+			 * @since v0.2.0
+			 */
+			public PrefixedUnitNameSet(final PrefixedUnitMap map) {
+				this.map = map;
+			}
+
+			@Override
+			public boolean add(final String e) {
+				throw new UnsupportedOperationException("Cannot add to an immutable set");
+			}
+
+			@Override
+			public boolean addAll(final Collection<? extends String> c) {
+				throw new UnsupportedOperationException("Cannot add to an immutable set");
+			}
+
+			@Override
+			public void clear() {
+				throw new UnsupportedOperationException("Cannot clear an immutable set");
+			}
+
+			@Override
+			public boolean contains(final Object o) {
+				return this.map.containsKey(o);
+			}
+
+			@Override
+			public boolean containsAll(final Collection<?> c) {
+				for (final Object o : c)
+					if (!this.contains(o))
+						return false;
+				return true;
+			}
+
+			@Override
+			public boolean isEmpty() {
+				return this.map.isEmpty();
+			}
+
+			@Override
+			public Iterator<String> iterator() {
+				return new PrefixedUnitNameIterator(this.map);
+			}
+
+			@Override
+			public boolean remove(final Object o) {
+				throw new UnsupportedOperationException("Cannot remove from an immutable set");
+			}
+
+			@Override
+			public boolean removeAll(final Collection<?> c) {
+				throw new UnsupportedOperationException("Cannot remove from an immutable set");
+			}
+
+			@Override
+			public boolean removeIf(final Predicate<? super String> filter) {
+				throw new UnsupportedOperationException("Cannot remove from an immutable set");
+			}
+
+			@Override
+			public boolean retainAll(final Collection<?> c) {
+				throw new UnsupportedOperationException("Cannot remove from an immutable set");
+			}
+
+			@Override
+			public int size() {
+				if (this.map.units.isEmpty())
+					return 0;
+				else {
+					if (this.map.prefixes.isEmpty())
+						return this.map.units.size();
+					else
+						// infinite set
+						return Integer.MAX_VALUE;
+				}
+			}
+
+			/**
+			 * @throws IllegalStateException
+			 *             if the set is infinite in size
+			 */
+			@Override
+			public Object[] toArray() {
+				if (this.map.units.isEmpty() || this.map.prefixes.isEmpty())
+					return super.toArray();
+				else
+					// infinite set
+					throw new IllegalStateException("Cannot make an infinite set into an array.");
+
+			}
+
+			/**
+			 * @throws IllegalStateException
+			 *             if the set is infinite in size
+			 */
+			@Override
+			public <T> T[] toArray(final T[] a) {
+				if (this.map.units.isEmpty() || this.map.prefixes.isEmpty())
+					return super.toArray(a);
+				else
+					// infinite set
+					throw new IllegalStateException("Cannot make an infinite set into an array.");
+			}
+
+			@Override
+			public String toString() {
+				if (this.map.units.isEmpty() || this.map.prefixes.isEmpty())
+					return super.toString();
+				else
+					return String.format("Infinite set of name-unit entries created from units %s and prefixes %s",
+							this.map.units, this.map.prefixes);
+			}
+		}
+
+		/**
+		 * The units stored in this collection, without prefixes.
+		 * 
+		 * @since 2019-04-13
+		 * @since v0.2.0
+		 */
+		private final Map<String, Unit> units;
+
+		/**
+		 * The available prefixes for use.
+		 * 
+		 * @since 2019-04-13
+		 * @since v0.2.0
+		 */
+		private final Map<String, UnitPrefix> prefixes;
+
+		// caches
+		private transient Collection<Unit> values = null;
+		private transient Set<String> keySet = null;
+		private transient Set<Entry<String, Unit>> entrySet = null;
+
+		/**
+		 * Creates the {@code PrefixedUnitMap}.
+		 * 
+		 * @param units
+		 *            map mapping unit names to units
+		 * @param prefixes
+		 *            map mapping prefix names to prefixes
+		 * @since 2019-04-13
+		 * @since v0.2.0
+		 */
+		public PrefixedUnitMap(final Map<String, Unit> units, final Map<String, UnitPrefix> prefixes) {
+			// I am making unmodifiable maps to ensure I don't accidentally make changes.
+			this.units = Collections.unmodifiableMap(units);
+			this.prefixes = Collections.unmodifiableMap(prefixes);
+		}
+
+		@Override
+		public void clear() {
+			throw new UnsupportedOperationException("Cannot clear an immutable map");
+		}
+
+		@Override
+		public Unit compute(final String key,
+				final BiFunction<? super String, ? super Unit, ? extends Unit> remappingFunction) {
+			throw new UnsupportedOperationException("Cannot edit an immutable map");
+		}
+
+		@Override
+		public Unit computeIfAbsent(final String key, final Function<? super String, ? extends Unit> mappingFunction) {
+			throw new UnsupportedOperationException("Cannot edit an immutable map");
+		}
+
+		@Override
+		public Unit computeIfPresent(final String key,
+				final BiFunction<? super String, ? super Unit, ? extends Unit> remappingFunction) {
+			throw new UnsupportedOperationException("Cannot edit an immutable map");
+		}
+
+		@Override
+		public boolean containsKey(final Object key) {
+			// First, test if there is a unit with the key
+			if (this.units.containsKey(key))
+				return true;
+
+			// Next, try to cast it to String
+			if (!(key instanceof String))
+				throw new IllegalArgumentException("Attempted to test for a unit using a non-string name.");
+			final String unitName = (String) key;
+
+			// Then, look for the longest prefix that is attached to a valid unit
+			String longestPrefix = null;
+			int longestLength = 0;
+
+			for (final String prefixName : this.prefixes.keySet()) {
+				// a prefix name is valid if:
+				// - it is prefixed (i.e. the unit name starts with it)
+				// - it is longer than the existing largest prefix (since I am looking for the longest valid prefix)
+				// - the part after the prefix is a valid unit name
+				// - the unit described that name is a linear unit (since only linear units can have prefixes)
+				if (unitName.startsWith(prefixName) && prefixName.length() > longestLength) {
+					final String rest = unitName.substring(prefixName.length());
+					if (this.containsKey(rest) && this.get(rest) instanceof LinearUnit) {
+						longestPrefix = prefixName;
+						longestLength = prefixName.length();
+					}
+				}
+			}
+
+			return longestPrefix != null;
+		}
+
+		/**
+		 * {@inheritDoc}
+		 * 
+		 * <p>
+		 * Because of ambiguities between prefixes (i.e. kilokilo = mega), this method only tests for prefixless units.
+		 * </p>
+		 */
+		@Override
+		public boolean containsValue(final Object value) {
+			return this.units.containsValue(value);
+		}
+
+		@Override
+		public Set<Entry<String, Unit>> entrySet() {
+			if (this.entrySet == null) {
+				this.entrySet = new PrefixedUnitEntrySet(this);
+			}
+			return this.entrySet;
+		}
+
+		@Override
+		public Unit get(final Object key) {
+			// First, test if there is a unit with the key
+			if (this.units.containsKey(key))
+				return this.units.get(key);
+
+			// Next, try to cast it to String
+			if (!(key instanceof String))
+				throw new IllegalArgumentException("Attempted to obtain a unit using a non-string name.");
+			final String unitName = (String) key;
+
+			// Then, look for the longest prefix that is attached to a valid unit
+			String longestPrefix = null;
+			int longestLength = 0;
+
+			for (final String prefixName : this.prefixes.keySet()) {
+				// a prefix name is valid if:
+				// - it is prefixed (i.e. the unit name starts with it)
+				// - it is longer than the existing largest prefix (since I am looking for the longest valid prefix)
+				// - the part after the prefix is a valid unit name
+				// - the unit described that name is a linear unit (since only linear units can have prefixes)
+				if (unitName.startsWith(prefixName) && prefixName.length() > longestLength) {
+					final String rest = unitName.substring(prefixName.length());
+					if (this.containsKey(rest) && this.get(rest) instanceof LinearUnit) {
+						longestPrefix = prefixName;
+						longestLength = prefixName.length();
+					}
+				}
+			}
+
+			// if none found, returns null
+			if (longestPrefix == null)
+				return null;
+			else {
+				// get necessary data
+				final String rest = unitName.substring(longestLength);
+				// this cast will not fail because I verified that it would work before selecting this prefix
+				final LinearUnit unit = (LinearUnit) this.get(rest);
+				final UnitPrefix prefix = this.prefixes.get(longestPrefix);
+
+				return unit.withPrefix(prefix);
+			}
+		}
+
+		@Override
+		public boolean isEmpty() {
+			return this.units.isEmpty();
+		}
+
+		@Override
+		public Set<String> keySet() {
+			if (this.keySet == null) {
+				this.keySet = new PrefixedUnitNameSet(this);
+			}
+			return this.keySet;
+		}
+
+		@Override
+		public Unit merge(final String key, final Unit value,
+				final BiFunction<? super Unit, ? super Unit, ? extends Unit> remappingFunction) {
+			throw new UnsupportedOperationException("Cannot merge into an immutable map");
+		}
+
+		@Override
+		public Unit put(final String key, final Unit value) {
+			throw new UnsupportedOperationException("Cannot add entries to an immutable map");
+		}
+
+		@Override
+		public void putAll(final Map<? extends String, ? extends Unit> m) {
+			throw new UnsupportedOperationException("Cannot add entries to an immutable map");
+		}
+
+		@Override
+		public Unit putIfAbsent(final String key, final Unit value) {
+			throw new UnsupportedOperationException("Cannot add entries to an immutable map");
+		}
+
+		@Override
+		public Unit remove(final Object key) {
+			throw new UnsupportedOperationException("Cannot remove entries from an immutable map");
+		}
+
+		@Override
+		public boolean remove(final Object key, final Object value) {
+			throw new UnsupportedOperationException("Cannot remove entries from an immutable map");
+		}
+
+		@Override
+		public Unit replace(final String key, final Unit value) {
+			throw new UnsupportedOperationException("Cannot replace entries in an immutable map");
+		}
+
+		@Override
+		public boolean replace(final String key, final Unit oldValue, final Unit newValue) {
+			throw new UnsupportedOperationException("Cannot replace entries in an immutable map");
+		}
+
+		@Override
+		public void replaceAll(final BiFunction<? super String, ? super Unit, ? extends Unit> function) {
+			throw new UnsupportedOperationException("Cannot replace entries in an immutable map");
+		}
+
+		@Override
+		public int size() {
+			if (this.units.isEmpty())
+				return 0;
+			else {
+				if (this.prefixes.isEmpty())
+					return this.units.size();
+				else
+					// infinite set
+					return Integer.MAX_VALUE;
+			}
+		}
+
+		@Override
+		public String toString() {
+			if (this.units.isEmpty() || this.prefixes.isEmpty())
+				return super.toString();
+			else
+				return String.format("Infinite map of name-unit entries created from units %s and prefixes %s",
+						this.units, this.prefixes);
+		}
+
+		/**
+		 * {@inheritDoc}
+		 * 
+		 * <p>
+		 * Because of ambiguities between prefixes (i.e. kilokilo = mega), this method ignores prefixes.
+		 * </p>
+		 */
+		@Override
+		public Collection<Unit> values() {
+			if (this.values == null) {
+				this.values = Collections.unmodifiableCollection(this.units.values());
+			}
+			return this.values;
+		}
+	}
+
+	/**
+	 * A regular expression that separates names and expressions in unit files.
+	 */
+	private static final Pattern NAME_EXPRESSION = Pattern.compile("(\\S+)\\s+(\\S.*)");
+
+	/**
+	 * The exponent operator
+	 * 
+	 * @param base
+	 *            base of exponentiation
+	 * @param exponentUnit
+	 *            exponent
+	 * @return result
+	 * @since 2019-04-10
+	 * @since v0.2.0
+	 */
+	private static final LinearUnit exponentiateUnits(final LinearUnit base, final LinearUnit exponentUnit) {
+		// exponent function - first check if o2 is a number,
+		if (exponentUnit.getBase().equals(SI.ONE.getBase())) {
+			// then check if it is an integer,
+			final double exponent = exponentUnit.getConversionFactor();
+			if (DecimalComparison.equals(exponent % 1, 0))
+				// then exponentiate
+				return base.toExponent((int) (exponent + 0.5));
+			else
+				// not an integer
+				throw new UnsupportedOperationException("Decimal exponents are currently not supported.");
+		} else
+			// not a number
+			throw new IllegalArgumentException("Exponents must be numbers.");
+	}
+
+	/**
+	 * The units in this system, excluding prefixes.
+	 * 
+	 * @since 2019-01-07
+	 * @since v0.1.0
+	 */
+	private final Map<String, Unit> prefixlessUnits;
+
+	/**
+	 * The unit prefixes in this system.
+	 * 
+	 * @since 2019-01-14
+	 * @since v0.1.0
+	 */
+	private final Map<String, UnitPrefix> prefixes;
+
+	/**
+	 * The dimensions in this system.
+	 * 
+	 * @since 2019-03-14
+	 * @since v0.2.0
+	 */
+	private final Map<String, ObjectProduct<BaseDimension>> dimensions;
+
+	/**
+	 * A map mapping strings to units (including prefixes)
+	 * 
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	private final Map<String, Unit> units;
+
+	/**
+	 * A parser that can parse unit expressions.
+	 * 
+	 * @since 2019-03-22
+	 * @since v0.2.0
+	 */
+	private final ExpressionParser<LinearUnit> unitExpressionParser = new ExpressionParser.Builder<>(
+			this::getLinearUnit).addBinaryOperator("+", (o1, o2) -> o1.plus(o2), 0)
+					.addBinaryOperator("-", (o1, o2) -> o1.minus(o2), 0)
+					.addBinaryOperator("*", (o1, o2) -> o1.times(o2), 1).addSpaceFunction("*")
+					.addBinaryOperator("/", (o1, o2) -> o1.dividedBy(o2), 1)
+					.addBinaryOperator("^", UnitDatabase::exponentiateUnits, 2).build();
+
+	/**
+	 * A parser that can parse unit prefix expressions
+	 * 
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	private final ExpressionParser<UnitPrefix> prefixExpressionParser = new ExpressionParser.Builder<>(this::getPrefix)
+			.addBinaryOperator("*", (o1, o2) -> o1.times(o2), 0).addSpaceFunction("*")
+			.addBinaryOperator("/", (o1, o2) -> o1.dividedBy(o2), 0)
+			.addBinaryOperator("^", (o1, o2) -> o1.toExponent(o2.getMultiplier()), 1).build();
+
+	/**
+	 * A parser that can parse unit dimension expressions.
+	 * 
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	private final ExpressionParser<ObjectProduct<BaseDimension>> unitDimensionParser = new ExpressionParser.Builder<>(
+			this::getDimension).addBinaryOperator("*", (o1, o2) -> o1.times(o2), 0).addSpaceFunction("*")
+					.addBinaryOperator("/", (o1, o2) -> o1.dividedBy(o2), 0).build();
+
+	/**
+	 * Creates the {@code UnitsDatabase}.
+	 * 
+	 * @since 2019-01-10
+	 * @since v0.1.0
+	 */
+	public UnitDatabase() {
+		this.prefixlessUnits = new HashMap<>();
+		this.prefixes = new HashMap<>();
+		this.dimensions = new HashMap<>();
+		this.units = new PrefixedUnitMap(this.prefixlessUnits, this.prefixes);
+	}
+
+	/**
+	 * Adds a unit dimension to the database.
+	 * 
+	 * @param name
+	 *            dimension's name
+	 * @param dimension
+	 *            dimension to add
+	 * @throws NullPointerException
+	 *             if name or dimension is null
+	 * @since 2019-03-14
+	 * @since v0.2.0
+	 */
+	public void addDimension(final String name, final ObjectProduct<BaseDimension> dimension) {
+		this.dimensions.put(Objects.requireNonNull(name, "name must not be null."),
+				Objects.requireNonNull(dimension, "dimension must not be null."));
+	}
+
+	/**
+	 * Adds to the list from a line in a unit dimension file.
+	 * 
+	 * @param line
+	 *            line to look at
+	 * @param lineCounter
+	 *            number of line, for error messages
+	 * @since 2019-04-10
+	 * @since v0.2.0
+	 */
+	private void addDimensionFromLine(final String line, final long lineCounter) {
+		// ignore lines that start with a # sign - they're comments
+		if (line.isEmpty())
+			return;
+		if (line.contains("#")) {
+			this.addDimensionFromLine(line.substring(0, line.indexOf("#")), lineCounter);
+			return;
+		}
+
+		// divide line into name and expression
+		final Matcher lineMatcher = NAME_EXPRESSION.matcher(line);
+		if (!lineMatcher.matches())
+			throw new IllegalArgumentException(String.format(
+					"Error at line %d: Lines of a dimension file must consist of a dimension name, then spaces or tabs, then a dimension expression.",
+					lineCounter));
+		final String name = lineMatcher.group(1);
+		final String expression = lineMatcher.group(2);
+
+		if (name.endsWith(" ")) {
+			System.err.printf("Warning - line %d's dimension name ends in a space", lineCounter);
+		}
+
+		// if expression is "!", search for an existing dimension
+		// if no unit found, throw an error
+		if (expression.equals("!")) {
+			if (!this.containsDimensionName(name))
+				throw new IllegalArgumentException(
+						String.format("! used but no dimension found (line %d).", lineCounter));
+		} else {
+			// it's a unit, get the unit
+			final ObjectProduct<BaseDimension> dimension;
+			try {
+				dimension = this.getDimensionFromExpression(expression);
+			} catch (final IllegalArgumentException e) {
+				System.err.printf("Parsing error on line %d:%n", lineCounter);
+				throw e;
+			}
+
+			this.addDimension(name, dimension);
+		}
+	}
+
+	/**
+	 * Adds a unit prefix to the database.
+	 * 
+	 * @param name
+	 *            prefix's name
+	 * @param prefix
+	 *            prefix to add
+	 * @throws NullPointerException
+	 *             if name or prefix is null
+	 * @since 2019-01-14
+	 * @since v0.1.0
+	 */
+	public void addPrefix(final String name, final UnitPrefix prefix) {
+		this.prefixes.put(Objects.requireNonNull(name, "name must not be null."),
+				Objects.requireNonNull(prefix, "prefix must not be null."));
+	}
+
+	/**
+	 * Adds a unit to the database.
+	 * 
+	 * @param name
+	 *            unit's name
+	 * @param unit
+	 *            unit to add
+	 * @throws NullPointerException
+	 *             if unit is null
+	 * @since 2019-01-10
+	 * @since v0.1.0
+	 */
+	public void addUnit(final String name, final Unit unit) {
+		this.prefixlessUnits.put(Objects.requireNonNull(name, "name must not be null."),
+				Objects.requireNonNull(unit, "unit must not be null."));
+	}
+
+	/**
+	 * Adds to the list from a line in a unit file.
+	 * 
+	 * @param line
+	 *            line to look at
+	 * @param lineCounter
+	 *            number of line, for error messages
+	 * @since 2019-04-10
+	 * @since v0.2.0
+	 */
+	private void addUnitOrPrefixFromLine(final String line, final long lineCounter) {
+		// ignore lines that start with a # sign - they're comments
+		if (line.isEmpty())
+			return;
+		if (line.contains("#")) {
+			this.addUnitOrPrefixFromLine(line.substring(0, line.indexOf("#")), lineCounter);
+			return;
+		}
+
+		// divide line into name and expression
+		final Matcher lineMatcher = NAME_EXPRESSION.matcher(line);
+		if (!lineMatcher.matches())
+			throw new IllegalArgumentException(String.format(
+					"Error at line %d: Lines of a unit file must consist of a unit name, then spaces or tabs, then a unit expression.",
+					lineCounter));
+		final String name = lineMatcher.group(1);
+		final String expression = lineMatcher.group(2);
+
+		if (name.endsWith(" ")) {
+			System.err.printf("Warning - line %d's unit name ends in a space", lineCounter);
+		}
+
+		// if expression is "!", search for an existing unit
+		// if no unit found, throw an error
+		if (expression.equals("!")) {
+			if (!this.containsUnitName(name))
+				throw new IllegalArgumentException(String.format("! used but no unit found (line %d).", lineCounter));
+		} else {
+			if (name.endsWith("-")) {
+				final UnitPrefix prefix;
+				try {
+					prefix = this.getPrefixFromExpression(expression);
+				} catch (final IllegalArgumentException e) {
+					System.err.printf("Parsing error on line %d:%n", lineCounter);
+					throw e;
+				}
+				this.addPrefix(name.substring(0, name.length() - 1), prefix);
+			} else {
+				// it's a unit, get the unit
+				final Unit unit;
+				try {
+					unit = this.getUnitFromExpression(expression);
+				} catch (final IllegalArgumentException e) {
+					System.err.printf("Parsing error on line %d:%n", lineCounter);
+					throw e;
+				}
+
+				this.addUnit(name, unit);
+			}
+		}
+	}
+
+	/**
+	 * Tests if the database has a unit dimension with this name.
+	 * 
+	 * @param name
+	 *            name to test
+	 * @return if database contains name
+	 * @since 2019-03-14
+	 * @since v0.2.0
+	 */
+	public boolean containsDimensionName(final String name) {
+		return this.dimensions.containsKey(name);
+	}
+
+	/**
+	 * Tests if the database has a unit prefix with this name.
+	 * 
+	 * @param name
+	 *            name to test
+	 * @return if database contains name
+	 * @since 2019-01-13
+	 * @since v0.1.0
+	 */
+	public boolean containsPrefixName(final String name) {
+		return this.prefixes.containsKey(name);
+	}
+
+	/**
+	 * Tests if the database has a unit with this name, taking prefixes into consideration
+	 * 
+	 * @param name
+	 *            name to test
+	 * @return if database contains name
+	 * @since 2019-01-13
+	 * @since v0.1.0
+	 */
+	public boolean containsUnitName(final String name) {
+		return this.units.containsKey(name);
+	}
+
+	/**
+	 * @return a map mapping dimension names to dimensions
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	public Map<String, ObjectProduct<BaseDimension>> dimensionMap() {
+		return Collections.unmodifiableMap(this.dimensions);
+	}
+
+	/**
+	 * Gets a unit dimension from the database using its name.
+	 * 
+	 * <p>
+	 * This method accepts exponents, like "L^3"
+	 * </p>
+	 * 
+	 * @param name
+	 *            dimension's name
+	 * @return dimension
+	 * @since 2019-03-14
+	 * @since v0.2.0
+	 */
+	public ObjectProduct<BaseDimension> getDimension(final String name) {
+		Objects.requireNonNull(name, "name must not be null.");
+		if (name.contains("^")) {
+			final String[] baseAndExponent = name.split("\\^");
+
+			final ObjectProduct<BaseDimension> base = this.getDimension(baseAndExponent[0]);
+
+			final int exponent;
+			try {
+				exponent = Integer.parseInt(baseAndExponent[baseAndExponent.length - 1]);
+			} catch (final NumberFormatException e2) {
+				throw new IllegalArgumentException("Exponent must be an integer.");
+			}
+
+			return base.toExponent(exponent);
+		}
+		return this.dimensions.get(name);
+	}
+
+	/**
+	 * Uses the database's data to parse an expression into a unit dimension
+	 * <p>
+	 * The expression is a series of any of the following:
+	 * <ul>
+	 * <li>The name of a unit dimension, which multiplies or divides the result based on preceding operators</li>
+	 * <li>The operators '*' and '/', which multiply and divide (note that just putting two unit dimensions next to each
+	 * other is equivalent to multiplication)</li>
+	 * <li>The operator '^' which exponentiates. Exponents must be integers.</li>
+	 * </ul>
+	 * 
+	 * @param expression
+	 *            expression to parse
+	 * @throws IllegalArgumentException
+	 *             if the expression cannot be parsed
+	 * @throws NullPointerException
+	 *             if expression is null
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	public ObjectProduct<BaseDimension> getDimensionFromExpression(final String expression) {
+		Objects.requireNonNull(expression, "expression must not be null.");
+
+		// attempt to get a dimension as an alias first
+		if (this.containsDimensionName(expression))
+			return this.getDimension(expression);
+
+		// force operators to have spaces
+		String modifiedExpression = expression;
+		modifiedExpression = modifiedExpression.replaceAll("\\*", " \\* ");
+		modifiedExpression = modifiedExpression.replaceAll("/", " / ");
+		modifiedExpression = modifiedExpression.replaceAll(" *\\^ *", "\\^");
+
+		// fix broken spaces
+		modifiedExpression = modifiedExpression.replaceAll(" +", " ");
+
+		return this.unitDimensionParser.parseExpression(modifiedExpression);
+	}
+
+	/**
+	 * Gets a unit. If it is linear, cast it to a LinearUnit and return it. Otherwise, throw an
+	 * {@code IllegalArgumentException}.
+	 * 
+	 * @param name
+	 *            unit's name
+	 * @return unit
+	 * @since 2019-03-22
+	 * @since v0.2.0
+	 */
+	private LinearUnit getLinearUnit(final String name) {
+		// see if I am using a function-unit like tempC(100)
+		if (name.contains("(") && name.contains(")")) {
+			// break it into function name and value
+			final List<String> parts = Arrays.asList(name.split("\\("));
+			if (parts.size() != 2)
+				throw new IllegalArgumentException("Format nonlinear units like: unit(value).");
+
+			// solve the function
+			final Unit unit = this.getUnit(parts.get(0));
+			final double value = Double.parseDouble(parts.get(1).substring(0, parts.get(1).length() - 1));
+			return LinearUnit.fromUnitValue(unit, value);
+		} else {
+			// get a linear unit
+			final Unit unit = this.getUnit(name);
+			if (unit instanceof LinearUnit)
+				return (LinearUnit) unit;
+			else
+				throw new IllegalArgumentException(String.format("%s is not a linear unit.", name));
+		}
+	}
+
+	/**
+	 * Gets a unit prefix from the database from its name
+	 * 
+	 * @param name
+	 *            prefix's name
+	 * @return prefix
+	 * @since 2019-01-10
+	 * @since v0.1.0
+	 */
+	public UnitPrefix getPrefix(final String name) {
+		try {
+			return UnitPrefix.valueOf(Double.parseDouble(name));
+		} catch (final NumberFormatException e) {
+			return this.prefixes.get(name);
+		}
+	}
+
+	/**
+	 * Gets a unit prefix from a prefix expression
+	 * <p>
+	 * Currently, prefix expressions are much simpler than unit expressions: They are either a number or the name of
+	 * another prefix
+	 * </p>
+	 * 
+	 * @param expression
+	 *            expression to input
+	 * @return prefix
+	 * @throws IllegalArgumentException
+	 *             if expression cannot be parsed
+	 * @throws NullPointerException
+	 *             if any argument is null
+	 * @since 2019-01-14
+	 * @since v0.1.0
+	 */
+	public UnitPrefix getPrefixFromExpression(final String expression) {
+		Objects.requireNonNull(expression, "expression must not be null.");
+
+		// attempt to get a unit as an alias first
+		if (this.containsUnitName(expression))
+			return this.getPrefix(expression);
+
+		// force operators to have spaces
+		String modifiedExpression = expression;
+		modifiedExpression = modifiedExpression.replaceAll("\\*", " \\* ");
+		modifiedExpression = modifiedExpression.replaceAll("/", " / ");
+		modifiedExpression = modifiedExpression.replaceAll("\\^", " \\^ ");
+
+		// fix broken spaces
+		modifiedExpression = modifiedExpression.replaceAll(" +", " ");
+
+		return this.prefixExpressionParser.parseExpression(modifiedExpression);
+	}
+
+	/**
+	 * Gets a unit from the database from its name, looking for prefixes.
+	 * 
+	 * @param name
+	 *            unit's name
+	 * @return unit
+	 * @since 2019-01-10
+	 * @since v0.1.0
+	 */
+	public Unit getUnit(final String name) {
+		try {
+			final double value = Double.parseDouble(name);
+			return SI.ONE.times(value);
+		} catch (final NumberFormatException e) {
+			return this.units.get(name);
+		}
+
+	}
+
+	/**
+	 * Uses the database's unit data to parse an expression into a unit
+	 * <p>
+	 * The expression is a series of any of the following:
+	 * <ul>
+	 * <li>The name of a unit, which multiplies or divides the result based on preceding operators</li>
+	 * <li>The operators '*' and '/', which multiply and divide (note that just putting two units or values next to each
+	 * other is equivalent to multiplication)</li>
+	 * <li>The operator '^' which exponentiates. Exponents must be integers.</li>
+	 * <li>A number which is multiplied or divided</li>
+	 * </ul>
+	 * This method only works with linear units.
+	 * 
+	 * @param expression
+	 *            expression to parse
+	 * @throws IllegalArgumentException
+	 *             if the expression cannot be parsed
+	 * @throws NullPointerException
+	 *             if expression is null
+	 * @since 2019-01-07
+	 * @since v0.1.0
+	 */
+	public Unit getUnitFromExpression(final String expression) {
+		Objects.requireNonNull(expression, "expression must not be null.");
+
+		// attempt to get a unit as an alias first
+		if (this.containsUnitName(expression))
+			return this.getUnit(expression);
+
+		// force operators to have spaces
+		String modifiedExpression = expression;
+		modifiedExpression = modifiedExpression.replaceAll("\\+", " \\+ ");
+		modifiedExpression = modifiedExpression.replaceAll("-", " - ");
+		modifiedExpression = modifiedExpression.replaceAll("\\*", " \\* ");
+		modifiedExpression = modifiedExpression.replaceAll("/", " / ");
+		modifiedExpression = modifiedExpression.replaceAll("\\^", " \\^ ");
+
+		// fix broken spaces
+		modifiedExpression = modifiedExpression.replaceAll(" +", " ");
+
+		// the previous operation breaks negative numbers, fix them!
+		// (i.e. -2 becomes - 2)
+		for (int i = 2; i < modifiedExpression.length(); i++) {
+			if (modifiedExpression.charAt(i) == '-'
+					&& Arrays.asList('+', '-', '*', '/', '^').contains(modifiedExpression.charAt(i - 2))) {
+				// found a broken negative number
+				modifiedExpression = modifiedExpression.substring(0, i + 1) + modifiedExpression.substring(i + 2);
+			}
+		}
+
+		return this.unitExpressionParser.parseExpression(modifiedExpression);
+	}
+
+	/**
+	 * Adds all dimensions from a file, using data from the database to parse them.
+	 * <p>
+	 * Each line in the file should consist of a name and an expression (parsed by getDimensionFromExpression) separated
+	 * by any number of tab characters.
+	 * <p>
+	 * <p>
+	 * Allowed exceptions:
+	 * <ul>
+	 * <li>Anything after a '#' character is considered a comment and ignored.</li>
+	 * <li>Blank lines are also ignored</li>
+	 * <li>If an expression consists of a single exclamation point, instead of parsing it, this method will search the
+	 * database for an existing unit. If no unit is found, an IllegalArgumentException is thrown. This is used to define
+	 * initial units and ensure that the database contains them.</li>
+	 * </ul>
+	 * 
+	 * @param file
+	 *            file to read
+	 * @throws IllegalArgumentException
+	 *             if the file cannot be parsed, found or read
+	 * @throws NullPointerException
+	 *             if file is null
+	 * @since 2019-01-13
+	 * @since v0.1.0
+	 */
+	public void loadDimensionFile(final File file) {
+		Objects.requireNonNull(file, "file must not be null.");
+		try (FileReader fileReader = new FileReader(file); BufferedReader reader = new BufferedReader(fileReader)) {
+			// while the reader has lines to read, read a line, then parse it, then add it
+			long lineCounter = 0;
+			while (reader.ready()) {
+				this.addDimensionFromLine(reader.readLine(), ++lineCounter);
+			}
+		} catch (final FileNotFoundException e) {
+			throw new IllegalArgumentException("Could not find file " + file, e);
+		} catch (final IOException e) {
+			throw new IllegalArgumentException("Could not read file " + file, e);
+		}
+	}
+
+	/**
+	 * Adds all units from a file, using data from the database to parse them.
+	 * <p>
+	 * Each line in the file should consist of a name and an expression (parsed by getUnitFromExpression) separated by
+	 * any number of tab characters.
+	 * <p>
+	 * <p>
+	 * Allowed exceptions:
+	 * <ul>
+	 * <li>Anything after a '#' character is considered a comment and ignored.</li>
+	 * <li>Blank lines are also ignored</li>
+	 * <li>If an expression consists of a single exclamation point, instead of parsing it, this method will search the
+	 * database for an existing unit. If no unit is found, an IllegalArgumentException is thrown. This is used to define
+	 * initial units and ensure that the database contains them.</li>
+	 * </ul>
+	 * 
+	 * @param file
+	 *            file to read
+	 * @throws IllegalArgumentException
+	 *             if the file cannot be parsed, found or read
+	 * @throws NullPointerException
+	 *             if file is null
+	 * @since 2019-01-13
+	 * @since v0.1.0
+	 */
+	public void loadUnitsFile(final File file) {
+		Objects.requireNonNull(file, "file must not be null.");
+		try (FileReader fileReader = new FileReader(file); BufferedReader reader = new BufferedReader(fileReader)) {
+			// while the reader has lines to read, read a line, then parse it, then add it
+			long lineCounter = 0;
+			while (reader.ready()) {
+				this.addUnitOrPrefixFromLine(reader.readLine(), ++lineCounter);
+			}
+		} catch (final FileNotFoundException e) {
+			throw new IllegalArgumentException("Could not find file " + file, e);
+		} catch (final IOException e) {
+			throw new IllegalArgumentException("Could not read file " + file, e);
+		}
+	}
+
+	/**
+	 * @return a map mapping prefix names to prefixes
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	public Map<String, UnitPrefix> prefixMap() {
+		return Collections.unmodifiableMap(this.prefixes);
+	}
+
+	@Override
+	public String toString() {
+		return String.format("Unit Database with %d units and %d unit prefixes", this.prefixlessUnits.size(),
+				this.prefixes.size());
+	}
+
+	/**
+	 * Returns a map mapping unit names to units, including units with prefixes.
+	 * <p>
+	 * The returned map is infinite in size if there is at least one unit and at least one prefix. If it is infinite,
+	 * some operations that only work with finite collections, like converting name/entry sets to arrays, will throw an
+	 * {@code IllegalStateException}.
+	 * </p>
+	 * <p>
+	 * Specifically, the operations that will throw an IllegalStateException if the map is infinite in size are:
+	 * <ul>
+	 * <li>{@code unitMap.entrySet().toArray()} (either overloading)</li>
+	 * <li>{@code unitMap.keySet().toArray()} (either overloading)</li>
+	 * </ul>
+	 * </p>
+	 * <p>
+	 * Because of ambiguities between prefixes (i.e. kilokilo = mega), the map's {@link PrefixedUnitMap#containsValue
+	 * containsValue} and {@link PrefixedUnitMap#values() values()} methods currently ignore prefixes.
+	 * </p>
+	 * 
+	 * @return a map mapping unit names to units, including prefixed names
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	public Map<String, Unit> unitMap() {
+		return this.units; // PrefixedUnitMap is immutable so I don't need to make an unmodifiable map.
+	}
+
+	/**
+	 * @return a map mapping unit names to units, ignoring prefixes
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	public Map<String, Unit> unitMapPrefixless() {
+		return Collections.unmodifiableMap(this.prefixlessUnits);
+	}
+}
diff --git a/src/org/unitConverter/unit/UnitDatabaseTest.java b/src/org/unitConverter/unit/UnitDatabaseTest.java
new file mode 100644
index 0000000..164172b
--- /dev/null
+++ b/src/org/unitConverter/unit/UnitDatabaseTest.java
@@ -0,0 +1,307 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.unit;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.Assertions.fail;
+
+import java.util.Iterator;
+import java.util.Map;
+import java.util.Map.Entry;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * A test for the {@link UnitDatabase} class. This is NOT part of this program's public API.
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-04-14
+ * @since v0.2.0
+ */
+class UnitDatabaseTest {
+	// some linear units and one nonlinear
+	private static final Unit U = SI.METRE;
+	private static final Unit V = SI.KILOGRAM;
+	private static final Unit W = SI.SECOND;
+
+	// used for testing expressions
+	// J = U^2 * V / W^2
+	private static final LinearUnit J = SI.KILOGRAM.times(SI.METRE.toExponent(2)).dividedBy(SI.SECOND.toExponent(2));
+	private static final LinearUnit K = SI.KELVIN;
+
+	private static final Unit NONLINEAR = Unit.fromConversionFunctions(SI.METRE.getBase(), o -> o + 1, o -> o - 1);
+
+	// make the prefix values prime so I can tell which multiplications were made
+	private static final UnitPrefix A = UnitPrefix.valueOf(2);
+	private static final UnitPrefix B = UnitPrefix.valueOf(3);
+	private static final UnitPrefix C = UnitPrefix.valueOf(5);
+	private static final UnitPrefix AB = UnitPrefix.valueOf(7);
+	private static final UnitPrefix BC = UnitPrefix.valueOf(11);
+
+	/**
+	 * Confirms that operations that shouldn't function for infinite databases throw an {@code IllegalStateException}.
+	 * 
+	 * @since 2019-05-03
+	 */
+	@Test
+	public void testInfiniteSetExceptions() {
+		// load units
+		final UnitDatabase infiniteDatabase = new UnitDatabase();
+
+		infiniteDatabase.addUnit("J", J);
+		infiniteDatabase.addUnit("K", K);
+
+		infiniteDatabase.addPrefix("A", A);
+		infiniteDatabase.addPrefix("B", B);
+		infiniteDatabase.addPrefix("C", C);
+
+		{
+			boolean exceptionThrown = false;
+			try {
+				infiniteDatabase.unitMap().entrySet().toArray();
+			} catch (final IllegalStateException e) {
+				exceptionThrown = true;
+				// pass!
+			} finally {
+				if (!exceptionThrown) {
+					fail("No IllegalStateException thrown");
+				}
+			}
+		}
+
+		{
+			boolean exceptionThrown = false;
+			try {
+				infiniteDatabase.unitMap().keySet().toArray();
+			} catch (final IllegalStateException e) {
+				exceptionThrown = true;
+				// pass!
+			} finally {
+				if (!exceptionThrown) {
+					fail("No IllegalStateException thrown");
+				}
+			}
+		}
+	}
+
+	/**
+	 * Test that prefixes correctly apply to units.
+	 * 
+	 * @since 2019-04-14
+	 * @since v0.2.0
+	 */
+	@Test
+	public void testPrefixes() {
+		final UnitDatabase database = new UnitDatabase();
+
+		database.addUnit("U", U);
+		database.addUnit("V", V);
+		database.addUnit("W", W);
+
+		database.addPrefix("A", A);
+		database.addPrefix("B", B);
+		database.addPrefix("C", C);
+
+		// get the product
+		final Unit abcuNonlinear = database.getUnit("ABCU");
+		assert abcuNonlinear instanceof LinearUnit;
+
+		final LinearUnit abcu = (LinearUnit) abcuNonlinear;
+		assertEquals(A.getMultiplier() * B.getMultiplier() * C.getMultiplier(), abcu.getConversionFactor(), 1e-15);
+	}
+
+	/**
+	 * Tests the functionnalites of the prefixless unit map.
+	 * 
+	 * <p>
+	 * The map should be an auto-updating view of the units in the database.
+	 * </p>
+	 * 
+	 * @since 2019-04-14
+	 * @since v0.2.0
+	 */
+	@Test
+	public void testPrefixlessUnitMap() {
+		final UnitDatabase database = new UnitDatabase();
+		final Map<String, Unit> prefixlessUnits = database.unitMapPrefixless();
+
+		database.addUnit("U", U);
+		database.addUnit("V", V);
+		database.addUnit("W", W);
+
+		// this should work because the map should be an auto-updating view
+		assertTrue(prefixlessUnits.containsKey("U"));
+		assertFalse(prefixlessUnits.containsKey("Z"));
+
+		assertTrue(prefixlessUnits.containsValue(U));
+		assertFalse(prefixlessUnits.containsValue(NONLINEAR));
+	}
+
+	/**
+	 * Tests that the database correctly stores and retrieves units, ignoring prefixes.
+	 * 
+	 * @since 2019-04-14
+	 * @since v0.2.0
+	 */
+	@Test
+	public void testPrefixlessUnits() {
+		final UnitDatabase database = new UnitDatabase();
+
+		database.addUnit("U", U);
+		database.addUnit("V", V);
+		database.addUnit("W", W);
+
+		assertTrue(database.containsUnitName("U"));
+		assertFalse(database.containsUnitName("Z"));
+
+		assertEquals(U, database.getUnit("U"));
+		assertEquals(null, database.getUnit("Z"));
+	}
+
+	/**
+	 * Test that unit expressions return the correct value.
+	 * 
+	 * @since 2019-04-14
+	 * @since v0.2.0
+	 */
+	@Test
+	public void testUnitExpressions() {
+		// load units
+		final UnitDatabase database = new UnitDatabase();
+
+		database.addUnit("U", U);
+		database.addUnit("V", V);
+		database.addUnit("W", W);
+		database.addUnit("fj", J.times(5));
+		database.addUnit("ej", J.times(8));
+
+		database.addPrefix("A", A);
+		database.addPrefix("B", B);
+		database.addPrefix("C", C);
+
+		// first test - test prefixes and operations
+		final Unit expected1 = J.withPrefix(A).withPrefix(B).withPrefix(C).withPrefix(C);
+		final Unit actual1 = database.getUnitFromExpression("ABV * CU^2 / W / W");
+
+		assertEquals(expected1, actual1);
+
+		// second test - test addition and subtraction
+		final Unit expected2 = J.times(58);
+		final Unit actual2 = database.getUnitFromExpression("2 fj + 6 ej");
+
+		assertEquals(expected2, actual2);
+	}
+
+	/**
+	 * Tests both the unit name iterator and the name-unit entry iterator
+	 * 
+	 * @since 2019-04-14
+	 * @since v0.2.0
+	 */
+	@Test
+	public void testUnitIterator() {
+		// load units
+		final UnitDatabase database = new UnitDatabase();
+
+		database.addUnit("J", J);
+		database.addUnit("K", K);
+
+		database.addPrefix("A", A);
+		database.addPrefix("B", B);
+		database.addPrefix("C", C);
+
+		final int NUM_UNITS = database.unitMapPrefixless().size();
+		final int NUM_PREFIXES = database.prefixMap().size();
+
+		final Iterator<String> nameIterator = database.unitMap().keySet().iterator();
+		final Iterator<Entry<String, Unit>> entryIterator = database.unitMap().entrySet().iterator();
+
+		int expectedLength = 1;
+		int unitsWithThisLengthSoFar = 0;
+
+		// loop 1000 times
+		for (int i = 0; i < 1000; i++) {
+			// expected length of next
+			if (unitsWithThisLengthSoFar >= NUM_UNITS * (int) Math.pow(NUM_PREFIXES, expectedLength - 1)) {
+				expectedLength++;
+				unitsWithThisLengthSoFar = 0;
+			}
+
+			// test that stuff is valid
+			final String nextName = nameIterator.next();
+			final Unit nextUnit = database.getUnit(nextName);
+			final Entry<String, Unit> nextEntry = entryIterator.next();
+
+			assertEquals(expectedLength, nextName.length());
+			assertEquals(nextName, nextEntry.getKey());
+			assertEquals(nextUnit, nextEntry.getValue());
+
+			unitsWithThisLengthSoFar++;
+		}
+
+		// test toString for consistency
+		final String entryIteratorString = entryIterator.toString();
+		for (int i = 0; i < 3; i++) {
+			assertEquals(entryIteratorString, entryIterator.toString());
+		}
+
+		final String nameIteratorString = nameIterator.toString();
+		for (int i = 0; i < 3; i++) {
+			assertEquals(nameIteratorString, nameIterator.toString());
+		}
+	}
+
+	/**
+	 * Determine, given a unit name that could mean multiple things, which meaning is chosen.
+	 * <p>
+	 * For example, "ABCU" could mean "A-B-C-U", "AB-C-U", or "A-BC-U". In this case, "AB-C-U" is the correct choice.
+	 * </p>
+	 * 
+	 * @since 2019-04-14
+	 * @since v0.2.0
+	 */
+	@Test
+	public void testUnitPrefixCombinations() {
+		// load units
+		final UnitDatabase database = new UnitDatabase();
+
+		database.addUnit("J", J);
+
+		database.addPrefix("A", A);
+		database.addPrefix("B", B);
+		database.addPrefix("C", C);
+		database.addPrefix("AB", AB);
+		database.addPrefix("BC", BC);
+
+		// test 1 - AB-C-J vs A-BC-J vs A-B-C-J
+		final Unit expected1 = J.withPrefix(AB).withPrefix(C);
+		final Unit actual1 = database.getUnit("ABCJ");
+
+		assertEquals(expected1, actual1);
+
+		// test 2 - ABC-J vs AB-CJ vs AB-C-J
+		database.addUnit("CJ", J.times(13));
+		database.addPrefix("ABC", UnitPrefix.valueOf(17));
+
+		final Unit expected2 = J.times(17);
+		final Unit actual2 = database.getUnit("ABCJ");
+
+		assertEquals(expected2, actual2);
+	}
+}
diff --git a/src/org/unitConverter/unit/UnitPrefix.java b/src/org/unitConverter/unit/UnitPrefix.java
index 9f9645d..514fa1c 100644
--- a/src/org/unitConverter/unit/UnitPrefix.java
+++ b/src/org/unitConverter/unit/UnitPrefix.java
@@ -1,5 +1,5 @@
 /**
- * Copyright (C) 2018 Adrien Hopkins
+ * Copyright (C) 2019 Adrien Hopkins
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as published by
@@ -16,14 +16,57 @@
  */
 package org.unitConverter.unit;
 
+import org.unitConverter.math.DecimalComparison;
+
 /**
- * A prefix that can be attached onto the front of any unit, which multiplies it by a certain value
+ * A prefix that can be applied to a {@code LinearUnit} to multiply it by some value
  * 
  * @author Adrien Hopkins
- * @since 2019-01-14
- * @since v0.1.0
+ * @since 2019-10-16
  */
-public interface UnitPrefix {
+public final class UnitPrefix {
+	/**
+	 * Gets a {@code UnitPrefix} from a multiplier
+	 * 
+	 * @param multiplier
+	 *            multiplier of prefix
+	 * @return prefix
+	 * @since 2019-10-16
+	 */
+	public static UnitPrefix valueOf(final double multiplier) {
+		return new UnitPrefix(multiplier);
+	}
+
+	/**
+	 * The number that this prefix multiplies units by
+	 * 
+	 * @since 2019-10-16
+	 */
+	private final double multiplier;
+
+	/**
+	 * Creates the {@code DefaultUnitPrefix}.
+	 * 
+	 * @param multiplier
+	 * @since 2019-01-14
+	 * @since v0.2.0
+	 */
+	private UnitPrefix(final double multiplier) {
+		this.multiplier = multiplier;
+	}
+
+	/**
+	 * Divides this prefix by a scalar
+	 * 
+	 * @param divisor
+	 *            number to divide by
+	 * @return quotient of prefix and scalar
+	 * @since 2019-10-16
+	 */
+	public UnitPrefix dividedBy(final double divisor) {
+		return valueOf(this.getMultiplier() / divisor);
+	}
+
 	/**
 	 * Divides this prefix by {@code other}.
 	 * 
@@ -33,16 +76,42 @@ public interface UnitPrefix {
 	 * @since 2019-04-13
 	 * @since v0.2.0
 	 */
-	default UnitPrefix dividedBy(final UnitPrefix other) {
-		return new DefaultUnitPrefix(this.getMultiplier() / other.getMultiplier());
+	public UnitPrefix dividedBy(final UnitPrefix other) {
+		return valueOf(this.getMultiplier() / other.getMultiplier());
+	}
+
+	@Override
+	public boolean equals(final Object obj) {
+		if (this == obj)
+			return true;
+		if (obj == null)
+			return false;
+		if (!(obj instanceof UnitPrefix))
+			return false;
+		final UnitPrefix other = (UnitPrefix) obj;
+		return DecimalComparison.equals(this.getMultiplier(), other.getMultiplier());
+	}
+
+	public double getMultiplier() {
+		return this.multiplier;
+	}
+
+	@Override
+	public int hashCode() {
+		return DecimalComparison.hash(this.getMultiplier());
 	}
 
 	/**
-	 * @return this prefix's multiplier
-	 * @since 2019-01-14
-	 * @since v0.1.0
+	 * Multiplies this prefix by a scalar
+	 * 
+	 * @param multiplicand
+	 *            number to multiply by
+	 * @return product of prefix and scalar
+	 * @since 2019-10-16
 	 */
-	double getMultiplier();
+	public UnitPrefix times(final double multiplicand) {
+		return valueOf(this.getMultiplier() * multiplicand);
+	}
 
 	/**
 	 * Multiplies this prefix by {@code other}.
@@ -53,8 +122,8 @@ public interface UnitPrefix {
 	 * @since 2019-04-13
 	 * @since v0.2.0
 	 */
-	default UnitPrefix times(final UnitPrefix other) {
-		return new DefaultUnitPrefix(this.getMultiplier() * other.getMultiplier());
+	public UnitPrefix times(final UnitPrefix other) {
+		return valueOf(this.getMultiplier() * other.getMultiplier());
 	}
 
 	/**
@@ -66,7 +135,12 @@ public interface UnitPrefix {
 	 * @since 2019-04-13
 	 * @since v0.2.0
 	 */
-	default UnitPrefix toExponent(final double exponent) {
-		return new DefaultUnitPrefix(Math.pow(getMultiplier(), exponent));
+	public UnitPrefix toExponent(final double exponent) {
+		return valueOf(Math.pow(this.getMultiplier(), exponent));
+	}
+
+	@Override
+	public String toString() {
+		return String.format("Unit prefix equal to %s", this.multiplier);
 	}
 }
diff --git a/src/org/unitConverter/unit/UnitSystem.java b/src/org/unitConverter/unit/UnitSystem.java
deleted file mode 100644
index 550eff6..0000000
--- a/src/org/unitConverter/unit/UnitSystem.java
+++ /dev/null
@@ -1,53 +0,0 @@
-/**
- * Copyright (C) 2018 Adrien Hopkins
- *
- * This program is free software: you can redistribute it and/or modify
- * it under the terms of the GNU Affero General Public License as published by
- * the Free Software Foundation, either version 3 of the License, or
- * (at your option) any later version.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU Affero General Public License for more details.
- *
- * You should have received a copy of the GNU Affero General Public License
- * along with this program.  If not, see <https://www.gnu.org/licenses/>.
- */
-package org.unitConverter.unit;
-
-import java.util.Objects;
-
-import org.unitConverter.dimension.UnitDimension;
-
-/**
- * A system of units. Each unit should be aware of its system.
- * 
- * @author Adrien Hopkins
- * @since 2018-12-23
- * @since v0.1.0
- */
-public interface UnitSystem {
-	/**
-	 * Gets a base unit for this system and the provided dimension.
-	 * 
-	 * @param dimension
-	 *            dimension used by base unit
-	 * @return base unit
-	 * @throws NullPointerException
-	 *             if dimension is null
-	 * @since 2019-01-25
-	 * @since v0.1.0
-	 */
-	default BaseUnit getBaseUnit(final UnitDimension dimension) {
-		Objects.requireNonNull(dimension, "dimension must not be null.");
-		return new BaseUnit(dimension, this);
-	}
-
-	/**
-	 * @return name of system
-	 * @since 2018-12-23
-	 * @since v0.1.0
-	 */
-	String getName();
-}
diff --git a/src/org/unitConverter/unit/UnitTest.java b/src/org/unitConverter/unit/UnitTest.java
index 7ae5fbf..c078cfc 100644
--- a/src/org/unitConverter/unit/UnitTest.java
+++ b/src/org/unitConverter/unit/UnitTest.java
@@ -22,7 +22,6 @@ import java.util.Random;
 import java.util.concurrent.ThreadLocalRandom;
 
 import org.junit.jupiter.api.Test;
-import org.unitConverter.dimension.StandardDimensions;
 import org.unitConverter.math.DecimalComparison;
 
 /**
@@ -46,16 +45,8 @@ class UnitTest {
 	}
 
 	@Test
-	public void testBaseUnitExclusives() {
-		// this test should have a compile error if I am doing something wrong
-		final BaseUnit metrePerSecondSquared = SI.METRE.dividedBy(SI.SECOND.toExponent(2));
-
-		assertEquals(metrePerSecondSquared, SI.SI.getBaseUnit(StandardDimensions.ACCELERATION));
-	}
-
-	@Test
 	public void testConversion() {
-		final BaseUnit metre = SI.METRE;
+		final LinearUnit metre = SI.METRE;
 		final Unit inch = metre.times(0.0254);
 
 		assertEquals(1.9, inch.convertTo(metre, 75), 0.01);
@@ -77,8 +68,8 @@ class UnitTest {
 
 	@Test
 	public void testEquals() {
-		final BaseUnit metre = SI.METRE;
-		final Unit meter = SI.SI.getBaseUnit(StandardDimensions.LENGTH);
+		final LinearUnit metre = SI.METRE;
+		final Unit meter = SI.BaseUnits.METRE.asLinearUnit();
 
 		assertEquals(metre, meter);
 	}
@@ -87,7 +78,7 @@ class UnitTest {
 	public void testMultiplicationAndDivision() {
 		// test unit-times-unit multiplication
 		final LinearUnit generatedJoule = SI.KILOGRAM.times(SI.METRE.toExponent(2)).dividedBy(SI.SECOND.toExponent(2));
-		final LinearUnit actualJoule = SI.SI.getBaseUnit(StandardDimensions.ENERGY);
+		final LinearUnit actualJoule = SI.JOULE;
 
 		assertEquals(generatedJoule, actualJoule);
 
@@ -96,14 +87,14 @@ class UnitTest {
 		final LinearUnit hour = SI.SECOND.times(3600);
 		final LinearUnit generatedKPH = kilometre.dividedBy(hour);
 
-		final LinearUnit actualKPH = SI.SI.getBaseUnit(StandardDimensions.VELOCITY).dividedBy(3.6);
+		final LinearUnit actualKPH = SI.METRE.dividedBy(SI.SECOND).dividedBy(3.6);
 
 		assertEquals(generatedKPH, actualKPH);
 	}
 
 	@Test
 	public void testPrefixes() {
-		final LinearUnit generatedKilometre = SI.METRE.withPrefix(SIPrefix.KILO);
+		final LinearUnit generatedKilometre = SI.METRE.withPrefix(SI.KILO);
 		final LinearUnit actualKilometre = SI.METRE.times(1000);
 
 		assertEquals(generatedKilometre, actualKilometre);
diff --git a/src/org/unitConverter/unit/package-info.java b/src/org/unitConverter/unit/package-info.java
index dd5a939..2f0e097 100644
--- a/src/org/unitConverter/unit/package-info.java
+++ b/src/org/unitConverter/unit/package-info.java
@@ -15,10 +15,10 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 /**
- * All of the classes that correspond to the units being converted.
+ * Everything to do with the units that make up Unit Converter.
  * 
  * @author Adrien Hopkins
- * @since 2019-01-25
+ * @since 2019-10-16
  * @since v0.1.0
  */
 package org.unitConverter.unit;
\ No newline at end of file