From a2a7703370caf07afd88b5cfe44e1a78eed699e9 Mon Sep 17 00:00:00 2001
From: Viktor Klang <viktor.klang@oracle.com>
Date: Thu, 26 Jan 2023 18:15:56 +0000
Subject: [PATCH] 8299444: java.util.Set.copyOf allocates needlessly for empty
 input collections

Reviewed-by: rriggs, shade, smarks
---
 .../share/classes/java/util/ImmutableCollections.java     | 8 +++++---
 src/java.base/share/classes/java/util/Map.java            | 2 ++
 src/java.base/share/classes/java/util/Set.java            | 4 +++-
 3 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/src/java.base/share/classes/java/util/ImmutableCollections.java b/src/java.base/share/classes/java/util/ImmutableCollections.java
index 4c8fd0cfc8f..3de7e1d5eae 100644
--- a/src/java.base/share/classes/java/util/ImmutableCollections.java
+++ b/src/java.base/share/classes/java/util/ImmutableCollections.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016, 2021, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2016, 2023, 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
@@ -166,10 +166,12 @@ class ImmutableCollections {
      */
     @SuppressWarnings("unchecked")
     static <E> List<E> listCopy(Collection<? extends E> coll) {
-        if (coll instanceof List12 || (coll instanceof ListN && ! ((ListN<?>)coll).allowNulls)) {
+        if (coll instanceof List12 || (coll instanceof ListN<?> c && !c.allowNulls)) {
             return (List<E>)coll;
+        } else if (coll.isEmpty()) { // implicit nullcheck of coll
+            return List.of();
         } else {
-            return (List<E>)List.of(coll.toArray()); // implicit nullcheck of coll
+            return (List<E>)List.of(coll.toArray());
         }
     }
 
diff --git a/src/java.base/share/classes/java/util/Map.java b/src/java.base/share/classes/java/util/Map.java
index f20f0a259a9..4e8a2496e2a 100644
--- a/src/java.base/share/classes/java/util/Map.java
+++ b/src/java.base/share/classes/java/util/Map.java
@@ -1738,6 +1738,8 @@ public interface Map<K, V> {
     static <K, V> Map<K, V> copyOf(Map<? extends K, ? extends V> map) {
         if (map instanceof ImmutableCollections.AbstractImmutableMap) {
             return (Map<K,V>)map;
+        } else if (map.isEmpty()) { // Implicit nullcheck of map
+            return Map.of();
         } else {
             return (Map<K,V>)Map.ofEntries(map.entrySet().toArray(new Entry[0]));
         }
diff --git a/src/java.base/share/classes/java/util/Set.java b/src/java.base/share/classes/java/util/Set.java
index 4c703008a87..5ce3bf04c7c 100644
--- a/src/java.base/share/classes/java/util/Set.java
+++ b/src/java.base/share/classes/java/util/Set.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2020, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2023, 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
@@ -728,6 +728,8 @@ public interface Set<E> extends Collection<E> {
     static <E> Set<E> copyOf(Collection<? extends E> coll) {
         if (coll instanceof ImmutableCollections.AbstractImmutableSet) {
             return (Set<E>)coll;
+        } else if (coll.isEmpty()) { // Implicit nullcheck of coll
+            return Set.of();
         } else {
             return (Set<E>)Set.of(new HashSet<>(coll).toArray());
         }