[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.
elastic · Oct 19, 2021 · afcefdd · afcefdd
1 parent d8c1428
commit afcefdd
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Showing 2 changed files with 19 additions and 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
@@ -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
@@ -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));
+        }
     }
 }