Add immediateVoidFuture() to Futures.java to create an immediately succeeding ListenableFuture<Void>.

RELNOTES=`util.concurrent`: Added `immediateVoidFuture`.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=303756624

Author: ashish1294

android/guava-tests/test/com/google/common/util/concurrent/FuturesTest.java

@@ -31,6 +31,7 @@
 import static com.google.common.util.concurrent.Futures.immediateCancelledFuture;
 import static com.google.common.util.concurrent.Futures.immediateFailedFuture;
 import static com.google.common.util.concurrent.Futures.immediateFuture;
+import static com.google.common.util.concurrent.Futures.immediateVoidFuture;
 import static com.google.common.util.concurrent.Futures.inCompletionOrder;
 import static com.google.common.util.concurrent.Futures.lazyTransform;
 import static com.google.common.util.concurrent.Futures.nonCancellationPropagating;
@@ -138,6 +139,14 @@ public void testImmediateFuture() throws Exception {
     assertThat(future.toString()).contains("[status=SUCCESS, result=[" + DATA1 + "]]");
   }
 
+  public void testImmediateVoidFuture() throws Exception {
+    ListenableFuture<Void> voidFuture = immediateVoidFuture();
+
+    assertThat(getDone(voidFuture)).isNull();
+    assertThat(getDoneFromTimeoutOverload(voidFuture)).isNull();
+    assertThat(voidFuture.toString()).contains("[status=SUCCESS, result=[null]]");
+  }
+
   public void testImmediateFailedFuture() throws Exception {
     Exception exception = new Exception();
     ListenableFuture<String> future = immediateFailedFuture(exception);

android/guava/src/com/google/common/util/concurrent/Futures.java

@@ -135,6 +135,17 @@ public static <V> ListenableFuture<V> immediateFuture(@NullableDecl V value) {
     return new ImmediateFuture<>(value);
   }
 
+  /**
+   * Returns a successful {@code ListenableFuture<Void>}. This method is equivalent to {@code
+   * immediateFuture(null)} except that it is restricted to produce futures of type {@code Void}.
+   *
+   * @since NEXT
+   */
+  @SuppressWarnings("unchecked")
+  public static ListenableFuture<Void> immediateVoidFuture() {
+    return (ListenableFuture<Void>) ImmediateFuture.NULL;
+  }
+
   /**
    * Returns a {@code ListenableFuture} which has an exception set immediately upon construction.
    *

guava-gwt/test/com/google/common/util/concurrent/FuturesTest_gwt.java

@@ -1746,6 +1746,33 @@ public void testImmediateFuture() throws Exception {
   }
 }
 
+public void testImmediateVoidFuture() throws Exception {
+  com.google.common.util.concurrent.FuturesTest testCase = new com.google.common.util.concurrent.FuturesTest();
+  testCase.setUp();
+  Throwable failure = null;
+  try {
+    testCase.testImmediateVoidFuture();
+  } catch (Throwable t) {
+    failure = t;
+  }
+  try {
+    testCase.tearDown();
+  } catch (Throwable t) {
+    if (failure == null) {
+      failure = t;
+    }
+  }
+  if (failure instanceof Exception) {
+    throw (Exception) failure;
+  }
+  if (failure instanceof Error) {
+    throw (Error) failure;
+  }
+  if (failure != null) {
+    throw new RuntimeException(failure);
+  }
+}
+
 public void testNonCancellationPropagating_delegateCancelled() throws Exception {
   com.google.common.util.concurrent.FuturesTest testCase = new com.google.common.util.concurrent.FuturesTest();
   testCase.setUp();

guava-tests/test/com/google/common/util/concurrent/FuturesTest.java

@@ -31,6 +31,7 @@
 import static com.google.common.util.concurrent.Futures.immediateCancelledFuture;
 import static com.google.common.util.concurrent.Futures.immediateFailedFuture;
 import static com.google.common.util.concurrent.Futures.immediateFuture;
+import static com.google.common.util.concurrent.Futures.immediateVoidFuture;
 import static com.google.common.util.concurrent.Futures.inCompletionOrder;
 import static com.google.common.util.concurrent.Futures.lazyTransform;
 import static com.google.common.util.concurrent.Futures.nonCancellationPropagating;
@@ -138,6 +139,14 @@ public void testImmediateFuture() throws Exception {
     assertThat(future.toString()).contains("[status=SUCCESS, result=[" + DATA1 + "]]");
   }
 
+  public void testImmediateVoidFuture() throws Exception {
+    ListenableFuture<Void> voidFuture = immediateVoidFuture();
+
+    assertThat(getDone(voidFuture)).isNull();
+    assertThat(getDoneFromTimeoutOverload(voidFuture)).isNull();
+    assertThat(voidFuture.toString()).contains("[status=SUCCESS, result=[null]]");
+  }
+
   public void testImmediateFailedFuture() throws Exception {
     Exception exception = new Exception();
     ListenableFuture<String> future = immediateFailedFuture(exception);

guava/src/com/google/common/util/concurrent/Futures.java

@@ -137,6 +137,17 @@ public static <V> ListenableFuture<V> immediateFuture(@Nullable V value) {
     return new ImmediateFuture<>(value);
   }
 
+  /**
+   * Returns a successful {@code ListenableFuture<Void>}. This method is equivalent to {@code
+   * immediateFuture(null)} except that it is restricted to produce futures of type {@code Void}.
+   *
+   * @since NEXT
+   */
+  @SuppressWarnings("unchecked")
+  public static ListenableFuture<Void> immediateVoidFuture() {
+    return (ListenableFuture<Void>) ImmediateFuture.NULL;
+  }
+
   /**
    * Returns a {@code ListenableFuture} which has an exception set immediately upon construction.
    *