garydgregory commented on code in PR #537:
URL:
https://github.com/apache/commons-collections/pull/537#discussion_r1740092109
##########
src/main/java/org/apache/commons/collections4/ListUtils.java:
##########
@@ -173,6 +174,39 @@ public static <E> List<E> fixedSizeList(final List<E>
list) {
return FixedSizeList.fixedSizeList(list);
}
+ /**
+ * Finds and returns a list of duplicate elements in the given list.
+ * <p>
+ * This method uses two sets: one for tracking seen elements and one for
+ * collecting duplicates. It iterates through the list once and collects
+ * duplicates in a result list.
+ * </p>
+ *
+ * @param <E> the type of elements in the list
+ * @param list the list to check for duplicates, must not be null
+ * @return a list of duplicate elements, or an empty list if no duplicates
are found
+ * @throws NullPointerException if the list is null
+ */
Review Comment:
Add `@since 4.5.0-M3`
##########
src/test/java/org/apache/commons/collections4/ListUtilsTest.java:
##########
@@ -102,6 +102,54 @@ public void testEquals() {
assertFalse(ListUtils.isEqualList(list1, list2));
}
+ @Test
+ public void testFindDuplicatesWithDuplicates() {
+ List<Integer> input = Arrays.asList(1, 2, 3, 2, 4, 5, 3);
+ List<Integer> expected = Arrays.asList(2, 3);
+ List<Integer> actual = ListUtils.findDuplicates(input);
+ assertEquals(expected, actual, "The list should contain only the
duplicate elements.");
+ }
+
+ @Test
+ public void testFindDuplicatesNoDuplicates() {
+ List<Integer> input = Arrays.asList(1, 2, 3, 4, 5);
+ List<Integer> expected = Arrays.asList();
+ List<Integer> actual = ListUtils.findDuplicates(input);
+ assertEquals(expected, actual, "The list should be empty as there are
no duplicates.");
+ }
+
+ @Test
+ public void testFindDuplicatesEmptyList() {
+ List<Integer> input = Arrays.asList();
+ List<Integer> expected = Arrays.asList();
+ List<Integer> actual = ListUtils.findDuplicates(input);
+ assertEquals(expected, actual, "The list should be empty as the input
list is empty.");
+ }
+
+ @Test
+ public void testFindDuplicatesSingleElement() {
+ List<Integer> input = Arrays.asList(1);
+ List<Integer> expected = Arrays.asList();
+ List<Integer> actual = ListUtils.findDuplicates(input);
+ assertEquals(expected, actual, "The list should be empty as there is
only one element.");
Review Comment:
Empty results can be simply asserted as
`assertTrue(ListUtils.duplicates(input).isEmpty());`
I'm going to create a CollectionUtils version of the method called
`duplicates`, I suggest the method be named the same here.
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