Reduce allocations for empty ArgumentValue

Closes gh-1175

Signed-off-by: James Bodkin <[email protected]>
[[email protected]: apply code conventions]
Signed-off-by: Brian Clozel <[email protected]>

Author: James Bodkin

Diff for: spring-graphql/src/main/java/org/springframework/graphql/data/ArgumentValue.java

@@ -49,6 +49,8 @@
  */
 public final class ArgumentValue<T> {
 
+	private static final ArgumentValue<?> EMPTY = new ArgumentValue<>(null, false);
+
 	private static final ArgumentValue<?> OMITTED = new ArgumentValue<>(null, true);
 
 
@@ -118,7 +120,7 @@ public void ifPresent(Consumer<? super T> action) {
 
 	@Override
 	public boolean equals(Object other) {
-		// This covers OMITTED constant
+		// This covers EMPTY and OMITTED constant
 		if (this == other) {
 			return true;
 		}
@@ -142,8 +144,9 @@ public int hashCode() {
 	 * @param <T> the type of value
 	 * @param value the value to hold in the instance
 	 */
+	@SuppressWarnings("unchecked")
 	public static <T> ArgumentValue<T> ofNullable(@Nullable T value) {
-		return new ArgumentValue<>(value, false);
+		return (value != null) ? new ArgumentValue<>(value, false) : (ArgumentValue<T>) EMPTY;
 	}
 
 	/**

Diff for: spring-graphql/src/test/java/org/springframework/graphql/data/ArgumentValueTests.java

@@ -74,7 +74,6 @@ void ifPresentShouldSkipWhenNull() {
 	}
 
 	@Test
-
 	void ifPresentShouldSkipWhenOmitted() {
 		AtomicBoolean called = new AtomicBoolean();
 		ArgumentValue.omitted().ifPresent(value -> called.set(true));