add method retainAll

primitiveCollections/src/main/java/org/lucares/collections/IntList.java

@@ -261,6 +261,26 @@ public final class IntList implements Serializable, Cloneable {
 		index = insertPosition;
 	}
 
+	/**
+	 * Retains all elements contained in the specified list.
+	 *
+	 * @param retain
+	 *            the elements to retain
+	 */
+	public void retainAll(final IntList retain) {
+		final int size = index;
+		int insertPosition = 0;
+		for (int i = 0; i < size; i++) {
+			final int current = data[i];
+			if (retain.indexOf(current) >= 0) {
+				// keep current element
+				data[insertPosition] = current;
+				insertPosition++;
+			}
+		}
+		index = insertPosition;
+	}
+
 	/**
 	 * Replaces all values in the list by applying {@code operator}.
 	 *