8340572: ConcurrentModificationException when sorting ArrayList sublists

Reviewed-by: smarks

src/java.base/share/classes/java/util/ArrayList.java

@@ -1808,6 +1808,7 @@ public class ArrayList<E> extends AbstractList<E>
     @Override
     public void sort(Comparator<? super E> c) {
         sortRange(c, 0, size);
+        modCount++;
     }
 
     @SuppressWarnings("unchecked")
@@ -1816,7 +1817,6 @@ public class ArrayList<E> extends AbstractList<E>
         Arrays.sort((E[]) elementData, fromIndex, toIndex, c);
         if (modCount != expectedModCount)
             throw new ConcurrentModificationException();
-        modCount++;
     }
 
     void checkInvariants() {

test/jdk/java/util/ArrayList/SortingModCount.java

@@ -0,0 +1,59 @@
+/*
+ * Copyright (c) 2024, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code 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 General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.ConcurrentModificationException;
+import java.util.List;
+
+/**
+ * @test
+ * @bug 8340572
+ * @summary ConcurrentModificationException when sorting ArrayList sublists
+ */
+
+public class SortingModCount {
+    public static void main(String[] args) {
+        testSortingSubListsDoesNotIncrementModCount();
+        testSortingListDoesIncrementModCount();
+    }
+
+    private static void testSortingSubListsDoesNotIncrementModCount() {
+        List<Integer> l = new ArrayList<>(List.of(1, 2, 3, 4));
+        var a = l.subList(0, 2);
+        var b = l.subList(2, 4);
+        Collections.sort(a);
+        Collections.sort(b);
+    }
+
+    private static void testSortingListDoesIncrementModCount() {
+        List<Integer> l = new ArrayList<>(List.of(1, 2, 3, 4));
+        var b = l.subList(2, 4);
+        Collections.sort(l);
+        try {
+            Collections.sort(b);
+            throw new Error("expected ConcurrentModificationException not thrown");
+        } catch (ConcurrentModificationException expected) {
+        }
+    }
+}