From afcefdd8dc99f71391fb9fae71917ce8d406a7c7 Mon Sep 17 00:00:00 2001
From: Artem Prigoda <artem.prigoda@elastic.co>
Date: Tue, 19 Oct 2021 15:30:51 +0200
Subject: [PATCH] [7.x] Optimize allocations for `List.copyOf` and `Set.copyOf`
 (#79395)

We can optimize away the array allocation just
to call `List.of/Set.of` and instead create copies directly
based the supplied collection.
---
 .../src/main/java/org/elasticsearch/core/List.java | 11 +++++++++--
 .../src/main/java/org/elasticsearch/core/Set.java  | 14 ++++++++++----
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/libs/core/src/main/java/org/elasticsearch/core/List.java b/libs/core/src/main/java/org/elasticsearch/core/List.java
index 325405f945a72..ba8843dfd6ec3 100644
--- a/libs/core/src/main/java/org/elasticsearch/core/List.java
+++ b/libs/core/src/main/java/org/elasticsearch/core/List.java
@@ -8,6 +8,7 @@
 
 package org.elasticsearch.core;
 
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
@@ -75,8 +76,14 @@ public static <T> java.util.List<T> of(T... entries) {
      * @param coll a {@code Collection} from which elements are drawn, must be non-null
      * @return a {@code List} containing the elements of the given {@code Collection}
      */
-    @SuppressWarnings("unchecked")
     public static <T> java.util.List<T> copyOf(Collection<? extends T> coll) {
-        return (java.util.List<T>) List.of(coll.toArray());
+        switch (coll.size()) {
+            case 0:
+                return Collections.emptyList();
+            case 1:
+                return Collections.singletonList(coll.iterator().next());
+            default:
+                return Collections.unmodifiableList(new ArrayList<>(coll));
+        }
     }
 }
diff --git a/libs/core/src/main/java/org/elasticsearch/core/Set.java b/libs/core/src/main/java/org/elasticsearch/core/Set.java
index 7fca623126247..003ce934aa68c 100644
--- a/libs/core/src/main/java/org/elasticsearch/core/Set.java
+++ b/libs/core/src/main/java/org/elasticsearch/core/Set.java
@@ -61,9 +61,9 @@ public static <T> java.util.Set<T> of(T e1, T e2) {
     public static <T> java.util.Set<T> of(T... entries) {
         switch (entries.length) {
             case 0:
-                return Set.of();
+                return of();
             case 1:
-                return Set.of(entries[0]);
+                return of(entries[0]);
             default:
                 return Collections.unmodifiableSet(new HashSet<>(Arrays.asList(entries)));
         }
@@ -78,8 +78,14 @@ public static <T> java.util.Set<T> of(T... entries) {
      * @throws NullPointerException if coll is null, or if it contains any nulls
      * @since 10
      */
-    @SuppressWarnings("unchecked")
     public static <T> java.util.Set<T> copyOf(Collection<? extends T> coll) {
-        return (java.util.Set<T>) Set.of(new HashSet<>(coll).toArray());
+        switch (coll.size()) {
+            case 0:
+                return Collections.emptySet();
+            case 1:
+                return Collections.singleton(coll.iterator().next());
+            default:
+                return Collections.unmodifiableSet(new HashSet<>(coll));
+        }
     }
 }