1 files changed, 86 insertions, 1 deletions

diff --git a/src/org/unitConverter/math/ConditionalExistenceCollections.java b/src/org/unitConverter/math/ConditionalExistenceCollections.java
index c0a69f0..9522885 100644
--- a/src/org/unitConverter/math/ConditionalExistenceCollections.java
+++ b/src/org/unitConverter/math/ConditionalExistenceCollections.java
@@ -16,6 +16,7 @@
  */
 package org.unitConverter.math;
 
+import java.util.AbstractCollection;
 import java.util.AbstractMap;
 import java.util.AbstractSet;
 import java.util.Collection;
@@ -48,9 +49,75 @@ import java.util.function.Predicate;
  * @author Adrien Hopkins
  * @since 2019-10-17
  */
-// TODO add conditional existence Collections, Lists and Sorted/Navigable Sets/Maps
+// 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();
+		}
+	}
+
+	/**
 	 * Elements in this wrapper iterator only exist if they pass a condition.
 	 * 
 	 * @author Adrien Hopkins
@@ -258,6 +325,7 @@ public final class ConditionalExistenceCollections {
 
 		@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;
 		}
@@ -269,6 +337,23 @@ public final class ConditionalExistenceCollections {
 	}
 
 	/**
+	 * 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>