8254090: Collectors.toUnmodifiableList exposes shared secret

src/java.base/share/classes/java/util/stream/Collectors.java

@@ -277,7 +277,7 @@ public Set<Characteristics> characteristics() {
      */
     public static <T>
     Collector<T, ?, List<T>> toList() {
-        return new CollectorImpl<>((Supplier<List<T>>) ArrayList::new, List::add,
+        return new CollectorImpl<>(ArrayList::new, List::add,
                                    (left, right) -> { left.addAll(right); return left; },
                                    CH_ID);
     }
@@ -293,13 +293,18 @@ public Set<Characteristics> characteristics() {
      * <a href="../List.html#unmodifiable">unmodifiable List</a> in encounter order
      * @since 10
      */
-    @SuppressWarnings("unchecked")
     public static <T>
     Collector<T, ?, List<T>> toUnmodifiableList() {
-        return new CollectorImpl<>((Supplier<List<T>>) ArrayList::new, List::add,
+        return new CollectorImpl<>(ArrayList::new, List::add,
                                    (left, right) -> { left.addAll(right); return left; },
-                                   list -> (List<T>)SharedSecrets.getJavaUtilCollectionAccess()
-                                                                 .listFromTrustedArray(list.toArray()),
+                                   list -> {
+                                       if (list.getClass() == ArrayList.class) { // ensure it's trusted
+                                           return SharedSecrets.getJavaUtilCollectionAccess()
+                                                               .listFromTrustedArray(list.toArray());
+                                       } else {
+                                           throw new IllegalArgumentException();
+                                       }
+                                   },
                                    CH_NOID);
     }
 
@@ -319,7 +324,7 @@ public Set<Characteristics> characteristics() {
      */
     public static <T>
     Collector<T, ?, Set<T>> toSet() {
-        return new CollectorImpl<>((Supplier<Set<T>>) HashSet::new, Set::add,
+        return new CollectorImpl<>(HashSet::new, Set::add,
                                    (left, right) -> {
                                        if (left.size() < right.size()) {
                                            right.addAll(left); return right;
@@ -348,7 +353,7 @@ public Set<Characteristics> characteristics() {
     @SuppressWarnings("unchecked")
     public static <T>
     Collector<T, ?, Set<T>> toUnmodifiableSet() {
-        return new CollectorImpl<>((Supplier<Set<T>>) HashSet::new, Set::add,
+        return new CollectorImpl<>(HashSet::new, Set::add,
                                    (left, right) -> {
                                        if (left.size() < right.size()) {
                                            right.addAll(left); return right;

test/jdk/java/util/stream/test/org/openjdk/tests/java/util/stream/CollectorToUnmodListTest.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) 2020, 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.
+ */
+package org.openjdk.tests.java.util.stream;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.function.Function;
+import java.util.stream.Collector;
+import java.util.stream.Collectors;
+
+import org.testng.annotations.Test;
+import static org.testng.Assert.*;
+
+/*
+ * @test
+ * @bug 8254090
+ * @summary Test for Collectors.toUnmodifiableList().
+ */
+public class CollectorToUnmodListTest {
+    @SuppressWarnings("unchecked")
+    @Test
+    public void testFinisher() {
+        String[] array = { "x", "y", "z" };
+        List<String> in = new ArrayList<>() {
+            public Object[] toArray() {
+                return array;
+            }
+        };
+        var finisher = (Function<List<String>, List<String>>)Collectors.<String>toUnmodifiableList().finisher();
+        assertThrows(IllegalArgumentException.class, () -> finisher.apply(in));
+  }
+}