4 files changed, 112 insertions, 2 deletions
diff --git a/src/unitConverter/unit/SI.java b/src/unitConverter/unit/SI.java
index 4486bf9..cda42e7 100644
--- a/src/unitConverter/unit/SI.java
+++ b/src/unitConverter/unit/SI.java
@@ -16,6 +16,10 @@
  */
 package unitConverter.unit;
 
+import java.util.HashSet;
+import java.util.Objects;
+import java.util.Set;
+
 import unitConverter.dimension.StandardDimensions;
 import unitConverter.dimension.UnitDimension;
 
@@ -28,6 +32,13 @@ import unitConverter.dimension.UnitDimension;
 public enum SI implements UnitSystem {
 	SI;
 
+	/**
+	 * This system's base units.
+	 * 
+	 * @since 2019-01-25
+	 */
+	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);
@@ -39,7 +50,19 @@ public enum SI implements UnitSystem {
 
 	@Override
 	public BaseUnit getBaseUnit(final UnitDimension dimension) {
-		return new BaseUnit(dimension, this);
+		// 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
diff --git a/src/unitConverter/unit/SIPrefix.java b/src/unitConverter/unit/SIPrefix.java
new file mode 100755
index 0000000..452d46f
--- /dev/null
+++ b/src/unitConverter/unit/SIPrefix.java
@@ -0,0 +1,49 @@
+/**
+ * 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 unitConverter.unit;
+
+/**
+ * @author Adrien Hopkins
+ * @since 2019-01-14
+ */
+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 value;
+
+	/**
+	 * Creates the {@code SIPrefix}.
+	 * 
+	 * @param value
+	 *            value of prefix
+	 * @since 2019-01-14
+	 */
+	private SIPrefix(final double value) {
+		this.value = value;
+	}
+
+	/**
+	 * @return value
+	 * @since 2019-01-14
+	 */
+	@Override
+	public final double getValue() {
+		return this.value;
+	}
+}
diff --git a/src/unitConverter/unit/UnitPrefix.java b/src/unitConverter/unit/UnitPrefix.java
new file mode 100755
index 0000000..0dbdc00
--- /dev/null
+++ b/src/unitConverter/unit/UnitPrefix.java
@@ -0,0 +1,31 @@
+/**
+ * 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 unitConverter.unit;
+
+/**
+ * A prefix that can be attached onto the front of any unit, which multiplies it by a certain value
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-01-14
+ */
+public interface UnitPrefix {
+	/**
+	 * @return value of this prefix
+	 * @since 2019-01-14
+	 */
+	double getValue();
+}
diff --git a/src/unitConverter/unit/UnitSystem.java b/src/unitConverter/unit/UnitSystem.java
index 0a50062..d641832 100755
--- a/src/unitConverter/unit/UnitSystem.java
+++ b/src/unitConverter/unit/UnitSystem.java
@@ -16,6 +16,8 @@
  */
 package unitConverter.unit;
 
+import java.util.Objects;
+
 import unitConverter.dimension.UnitDimension;
 
 /**
@@ -31,9 +33,14 @@ public interface UnitSystem {
 	 * @param dimension
 	 *            dimension used by base unit
 	 * @return base unit
+	 * @throws NullPointerException
+	 *             if dimension is null
 	 * @since 2019-01-25
 	 */
-	BaseUnit getBaseUnit(UnitDimension dimension);
+	default BaseUnit getBaseUnit(final UnitDimension dimension) {
+		Objects.requireNonNull(dimension, "dimension must not be null.");
+		return new BaseUnit(dimension, this);
+	}
 
 	/**
 	 * @return name of system