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diff --git a/src/main/java/org/unitConverter/math/ConditionalExistenceCollections.java b/src/main/java/org/unitConverter/math/ConditionalExistenceCollections.java
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+/**
+ * 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.math;
+
+import java.util.AbstractCollection;
+import java.util.AbstractMap;
+import java.util.AbstractSet;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.NoSuchElementException;
+import java.util.Set;
+import java.util.function.Predicate;
+
+/**
+ * Elements in these wrapper collections only exist if they pass a condition.
+ * <p>
+ * All of the collections in this class are "views" of the provided collections.
+ * They are mutable if the provided collections are mutable, they allow null if
+ * the provided collections allow null, they will reflect changes in the
+ * provided collection, etc.
+ * <p>
+ * The modification operations will always run the corresponding operations,
+ * even if the conditional existence collection doesn't change. For example, if
+ * you have a set that ignores even numbers, add(2) will still add a 2 to the
+ * backing set (but the conditional existence set will say it doesn't exist).
+ * <p>
+ * The returned collections do <i>not</i> pass the hashCode and equals
+ * operations through to the backing collections, but rely on {@code Object}'s
+ * {@code equals} and {@code hashCode} methods. This is necessary to preserve
+ * the contracts of these operations in the case that the backing collections
+ * are sets or lists.
+ * <p>
+ * Other than that, <i>the only difference between the provided collections and
+ * the returned collections are that elements don't exist if they don't pass the
+ * provided condition</i>.
+ * 
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-10-17
+ */
+// TODO add conditional existence Lists and Sorted/Navigable Sets/Maps
+public final class ConditionalExistenceCollections {
+	/**
+	 * Elements in this collection only exist if they meet a condition.
+	 * 
+	 * @author Adrien Hopkins
+	 * @since 2019-10-17
+	 * @param <E> type of element in collection
+	 */
+	static final class ConditionalExistenceCollection<E>
+			extends AbstractCollection<E> {
+		final Collection<E> collection;
+		final Predicate<E> existenceCondition;
+		
+		/**
+		 * Creates the {@code ConditionalExistenceCollection}.
+		 * 
+		 * @param collection
+		 * @param existenceCondition
+		 * @since 2019-10-17
+		 */
+		private ConditionalExistenceCollection(final Collection<E> collection,
+				final Predicate<E> existenceCondition) {
+			this.collection = collection;
+			this.existenceCondition = existenceCondition;
+		}
+		
+		@Override
+		public boolean add(final E e) {
+			return this.collection.add(e) && this.existenceCondition.test(e);
+		}
+		
+		@Override
+		public void clear() {
+			this.collection.clear();
+		}
+		
+		@Override
+		public boolean contains(final Object o) {
+			if (!this.collection.contains(o))
+				return false;
+				
+			// this collection can only contain instances of E
+			// since the object is in the collection, we know that it must be an
+			// instance of E
+			// therefore this cast will always work
+			@SuppressWarnings("unchecked")
+			final E e = (E) o;
+			
+			return this.existenceCondition.test(e);
+		}
+		
+		@Override
+		public Iterator<E> iterator() {
+			return conditionalExistenceIterator(this.collection.iterator(),
+					this.existenceCondition);
+		}
+		
+		@Override
+		public boolean remove(final Object o) {
+			// remove() must be first in the && statement, otherwise it may not
+			// execute
+			final boolean containedObject = this.contains(o);
+			return this.collection.remove(o) && containedObject;
+		}
+		
+		@Override
+		public int size() {
+			return (int) this.collection.stream().filter(this.existenceCondition)
+					.count();
+		}
+		
+		@Override
+		public Object[] toArray() {
+			// ensure the toArray operation is supported
+			this.collection.toArray();
+			
+			// if it works, do it for real
+			return super.toArray();
+		}
+		
+		@Override
+		public <T> T[] toArray(T[] a) {
+			// ensure the toArray operation is supported
+			this.collection.toArray();
+			
+			// if it works, do it for real
+			return super.toArray(a);
+		}
+	}
+	
+	/**
+	 * Elements in this wrapper iterator only exist if they pass a condition.
+	 * 
+	 * @author Adrien Hopkins
+	 * @since 2019-10-17
+	 * @param <E> type of elements in iterator
+	 */
+	static final class ConditionalExistenceIterator<E> implements Iterator<E> {
+		final Iterator<E> iterator;
+		final Predicate<E> existenceCondition;
+		E nextElement;
+		boolean hasNext;
+		
+		/**
+		 * Creates the {@code ConditionalExistenceIterator}.
+		 * 
+		 * @param iterator
+		 * @param condition
+		 * @since 2019-10-17
+		 */
+		private ConditionalExistenceIterator(final Iterator<E> iterator,
+				final Predicate<E> condition) {
+			this.iterator = iterator;
+			this.existenceCondition = condition;
+			this.getAndSetNextElement();
+		}
+		
+		/**
+		 * Gets the next element, and sets nextElement and hasNext accordingly.
+		 * 
+		 * @since 2019-10-17
+		 */
+		private void getAndSetNextElement() {
+			do {
+				if (!this.iterator.hasNext()) {
+					this.nextElement = null;
+					this.hasNext = false;
+					return;
+				}
+				this.nextElement = this.iterator.next();
+			} while (!this.existenceCondition.test(this.nextElement));
+			this.hasNext = true;
+		}
+		
+		@Override
+		public boolean hasNext() {
+			return this.hasNext;
+		}
+		
+		@Override
+		public E next() {
+			if (this.hasNext()) {
+				final E next = this.nextElement;
+				this.getAndSetNextElement();
+				return next;
+			} else
+				throw new NoSuchElementException();
+		}
+		
+		@Override
+		public void remove() {
+			this.iterator.remove();
+		}
+	}
+	
+	/**
+	 * Mappings in this map only exist if the entry passes some condition.
+	 * 
+	 * @author Adrien Hopkins
+	 * @since 2019-10-17
+	 * @param <K> key type
+	 * @param <V> value type
+	 */
+	static final class ConditionalExistenceMap<K, V> extends AbstractMap<K, V> {
+		Map<K, V> map;
+		Predicate<Entry<K, V>> entryExistenceCondition;
+		
+		/**
+		 * Creates the {@code ConditionalExistenceMap}.
+		 * 
+		 * @param map
+		 * @param entryExistenceCondition
+		 * @since 2019-10-17
+		 */
+		private ConditionalExistenceMap(final Map<K, V> map,
+				final Predicate<Entry<K, V>> entryExistenceCondition) {
+			this.map = map;
+			this.entryExistenceCondition = entryExistenceCondition;
+		}
+		
+		@Override
+		public boolean containsKey(final Object key) {
+			if (!this.map.containsKey(key))
+				return false;
+				
+			// only instances of K have mappings in the backing map
+			// since we know that key is a valid key, it must be an instance of K
+			@SuppressWarnings("unchecked")
+			final K keyAsK = (K) key;
+			
+			// get and test entry
+			final V value = this.map.get(key);
+			final Entry<K, V> entry = new SimpleEntry<>(keyAsK, value);
+			return this.entryExistenceCondition.test(entry);
+		}
+		
+		@Override
+		public Set<Entry<K, V>> entrySet() {
+			return conditionalExistenceSet(this.map.entrySet(),
+					this.entryExistenceCondition);
+		}
+		
+		@Override
+		public V get(final Object key) {
+			return this.containsKey(key) ? this.map.get(key) : null;
+		}
+		
+		private final Entry<K, V> getEntry(K key) {
+			return new Entry<>() {
+				@Override
+				public K getKey() {
+					return key;
+				}
+				
+				@Override
+				public V getValue() {
+					return ConditionalExistenceMap.this.map.get(key);
+				}
+				
+				@Override
+				public V setValue(V value) {
+					return ConditionalExistenceMap.this.map.put(key, value);
+				}
+			};
+		}
+		
+		@Override
+		public Set<K> keySet() {
+			return conditionalExistenceSet(super.keySet(),
+					k -> this.entryExistenceCondition.test(this.getEntry(k)));
+		}
+		
+		@Override
+		public V put(final K key, final V value) {
+			final V oldValue = this.map.put(key, value);
+			
+			// get and test entry
+			final Entry<K, V> entry = new SimpleEntry<>(key, oldValue);
+			return this.entryExistenceCondition.test(entry) ? oldValue : null;
+		}
+		
+		@Override
+		public V remove(final Object key) {
+			final V oldValue = this.map.remove(key);
+			return this.containsKey(key) ? oldValue : null;
+		}
+		
+		@Override
+		public Collection<V> values() {
+			// maybe change this to use ConditionalExistenceCollection
+			return super.values();
+		}
+	}
+	
+	/**
+	 * Elements in this set only exist if a certain condition is true.
+	 * 
+	 * @author Adrien Hopkins
+	 * @since 2019-10-17
+	 * @param <E> type of element in set
+	 */
+	static final class ConditionalExistenceSet<E> extends AbstractSet<E> {
+		private final Set<E> set;
+		private final Predicate<E> existenceCondition;
+		
+		/**
+		 * Creates the {@code ConditionalNonexistenceSet}.
+		 * 
+		 * @param set                set to use
+		 * @param existenceCondition condition where element exists
+		 * @since 2019-10-17
+		 */
+		private ConditionalExistenceSet(final Set<E> set,
+				final Predicate<E> existenceCondition) {
+			this.set = set;
+			this.existenceCondition = existenceCondition;
+		}
+		
+		/**
+		 * {@inheritDoc}
+		 * <p>
+		 * Note that this method returns {@code false} if {@code e} does not pass
+		 * the existence condition.
+		 */
+		@Override
+		public boolean add(final E e) {
+			return this.set.add(e) && this.existenceCondition.test(e);
+		}
+		
+		@Override
+		public void clear() {
+			this.set.clear();
+		}
+		
+		@Override
+		public boolean contains(final Object o) {
+			if (!this.set.contains(o))
+				return false;
+				
+			// this set can only contain instances of E
+			// since the object is in the set, we know that it must be an instance
+			// of E
+			// therefore this cast will always work
+			@SuppressWarnings("unchecked")
+			final E e = (E) o;
+			
+			return this.existenceCondition.test(e);
+		}
+		
+		@Override
+		public Iterator<E> iterator() {
+			return conditionalExistenceIterator(this.set.iterator(),
+					this.existenceCondition);
+		}
+		
+		@Override
+		public boolean remove(final Object o) {
+			// remove() must be first in the && statement, otherwise it may not
+			// execute
+			final boolean containedObject = this.contains(o);
+			return this.set.remove(o) && containedObject;
+		}
+		
+		@Override
+		public int size() {
+			return (int) this.set.stream().filter(this.existenceCondition).count();
+		}
+		
+		@Override
+		public Object[] toArray() {
+			// ensure the toArray operation is supported
+			this.set.toArray();
+			
+			// if it works, do it for real
+			return super.toArray();
+		}
+		
+		@Override
+		public <T> T[] toArray(T[] a) {
+			// ensure the toArray operation is supported
+			this.set.toArray();
+			
+			// if it works, do it for real
+			return super.toArray(a);
+		}
+	}
+	
+	/**
+	 * Elements in the returned wrapper collection are ignored if they don't pass
+	 * a condition.
+	 * 
+	 * @param <E>                type of elements in collection
+	 * @param collection         collection to wrap
+	 * @param existenceCondition elements only exist if this returns true
+	 * @return wrapper collection
+	 * @since 2019-10-17
+	 */
+	public static final <E> Collection<E> conditionalExistenceCollection(
+			final Collection<E> collection,
+			final Predicate<E> existenceCondition) {
+		return new ConditionalExistenceCollection<>(collection,
+				existenceCondition);
+	}
+	
+	/**
+	 * Elements in the returned wrapper iterator are ignored if they don't pass a
+	 * condition.
+	 * 
+	 * @param <E>                type of elements in iterator
+	 * @param iterator           iterator to wrap
+	 * @param existenceCondition elements only exist if this returns true
+	 * @return wrapper iterator
+	 * @since 2019-10-17
+	 */
+	public static final <E> Iterator<E> conditionalExistenceIterator(
+			final Iterator<E> iterator, final Predicate<E> existenceCondition) {
+		return new ConditionalExistenceIterator<>(iterator, existenceCondition);
+	}
+	
+	/**
+	 * Mappings in the returned wrapper map are ignored if the corresponding
+	 * entry doesn't pass a condition
+	 * 
+	 * @param <K>                     type of key in map
+	 * @param <V>                     type of value in map
+	 * @param map                     map to wrap
+	 * @param entryExistenceCondition mappings only exist if this returns true
+	 * @return wrapper map
+	 * @since 2019-10-17
+	 */
+	public static final <K, V> Map<K, V> conditionalExistenceMap(
+			final Map<K, V> map,
+			final Predicate<Entry<K, V>> entryExistenceCondition) {
+		return new ConditionalExistenceMap<>(map, entryExistenceCondition);
+	}
+	
+	/**
+	 * Elements in the returned wrapper set are ignored if they don't pass a
+	 * condition.
+	 * 
+	 * @param <E>                type of elements in set
+	 * @param set                set to wrap
+	 * @param existenceCondition elements only exist if this returns true
+	 * @return wrapper set
+	 * @since 2019-10-17
+	 */
+	public static final <E> Set<E> conditionalExistenceSet(final Set<E> set,
+			final Predicate<E> existenceCondition) {
+		return new ConditionalExistenceSet<>(set, existenceCondition);
+	}
+}