From 8916770c8c02d3217546d96ae815e6ba0dbedfa4 Mon Sep 17 00:00:00 2001
From: OldTruckDriver <stayintouchwithdrq@gmail.com>
Date: Sat, 20 Jun 2026 00:02:53 +1000
Subject: [PATCH] [COLLECTIONS-891] Fix IndexedCollection contains equality

contains(Object) returned index.containsKey(transform(object)), so it reported true
whenever any element shared the transformed key, breaking Collection.contains for a
non-injective transformer. Look up the key's bucket and check actual object equality
within it. Add tests for unique and non-unique indexes.

Reviewed-by: OpenAI Codex
Reviewed-by: Anthropic Claude Code
---
 src/changes/changes.xml                         |  1 +
 .../collection/IndexedCollection.java           |  3 ++-
 .../collection/IndexedCollectionTest.java       | 17 +++++++++++++++++
 3 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/src/changes/changes.xml b/src/changes/changes.xml
index d631a8bc57..169f2f5f3b 100644
--- a/src/changes/changes.xml
+++ b/src/changes/changes.xml
@@ -52,6 +52,7 @@
     <action type="fix" dev="ggregory" due-to="Dexter.k, Gary Gregory">Re-validate entries in PredicatedMap/PredicatedCollection readObject (#682).</action>
     <action type="fix" dev="ggregory" due-to="Ruiqi Dong, Gary Gregory" issue="COLLECTIONS-889">IndexedCollection.remove(Object) removes all values for a non-unique index key.</action>
     <action type="fix" dev="ggregory" due-to="Ruiqi Dong, Gary Gregory" issue="COLLECTIONS-890">TransformIterator throws NullPointerException when its transformer is null.</action>
+    <action type="fix" dev="ggregory" due-to="Ruiqi Dong, Gary Gregory" issue="COLLECTIONS-891">IndexedCollection.contains(Object) can return true for objects that were never added.</action>
     <action type="fix" dev="ggregory" due-to="Maxim Safronov, Gary Gregory" issue="COLLECTIONS-878">MapUtils.invertMap() improves HashMap construction (#652).</action>
     <action type="fix" dev="ggregory" due-to="Dexter.k, Gary Gregory">Validate order and uniqueness invariants in decorator readObject() (#684).</action>
     <!-- ADD -->
diff --git a/src/main/java/org/apache/commons/collections4/collection/IndexedCollection.java b/src/main/java/org/apache/commons/collections4/collection/IndexedCollection.java
index c534703a37..17882daabf 100644
--- a/src/main/java/org/apache/commons/collections4/collection/IndexedCollection.java
+++ b/src/main/java/org/apache/commons/collections4/collection/IndexedCollection.java
@@ -159,7 +159,8 @@ public void clear() {
     @SuppressWarnings("unchecked")
     @Override
     public boolean contains(final Object object) {
-        return index.containsKey(keyTransformer.apply((C) object));
+        final Collection<C> values = (Collection<C>) index.get(keyTransformer.apply((C) object));
+        return values != null && values.contains(object);
     }
 
     /**
diff --git a/src/test/java/org/apache/commons/collections4/collection/IndexedCollectionTest.java b/src/test/java/org/apache/commons/collections4/collection/IndexedCollectionTest.java
index db427309dd..b608d1b4f6 100644
--- a/src/test/java/org/apache/commons/collections4/collection/IndexedCollectionTest.java
+++ b/src/test/java/org/apache/commons/collections4/collection/IndexedCollectionTest.java
@@ -18,6 +18,7 @@
 
 import static java.util.Arrays.asList;
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertThrows;
@@ -116,6 +117,22 @@ void testAddedObjectsCanBeRetrievedByKey() throws Exception {
         assertEquals("4", indexed.get(4));
     }
 
+    @Test
+    void testContainsUsesObjectEqualityNotOnlyTransformedKey() {
+        final Collection<String> coll = makeUniqueTestCollection();
+        coll.add("01");
+
+        assertFalse(coll.contains("1"));
+    }
+
+    @Test
+    void testContainsUsesObjectEqualityWithNonUniqueIndex() {
+        final Collection<String> coll = makeTestCollection();
+        coll.add("01");
+
+        assertFalse(coll.contains("1"));
+    }
+
     @Test
     void testDecoratedCollectionIsIndexedOnCreation() throws Exception {
         final Collection<String> original = makeFullCollection();