Clarify the error message when an AsyncFunction returns null (*not* a…

… Future that contains null).

Add a similar message for FutureFallback.
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=84565778

guava-gwt/src-super/com/google/common/util/concurrent/super/com/google/common/util/concurrent/Futures.java

@@ -451,9 +451,9 @@ public void run() {
           return;
         }
 
-        ListenableFuture<? extends O> outputFuture =
-            Preconditions.checkNotNull(function.apply(sourceResult),
-                "AsyncFunction may not return null.");
+        ListenableFuture<? extends O> outputFuture = function.apply(sourceResult);
+        checkNotNull(outputFuture, "AsyncFunction.apply returned null instead of a Future. "
+            + "Did you mean to return immediateFuture(null)?");
         setFuture(outputFuture);
       } catch (UndeclaredThrowableException e) {
         // Set the cause of the exception as this future's exception

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

@@ -685,8 +685,26 @@ public void testWithFallback_resultCancelledAfterFallback() throws Exception {
     mocksControl.verify();
   }
 
+  public void testWithFallback_nullInsteadOfFuture() throws Exception {
+    ListenableFuture<?> inputFuture = immediateFailedFuture(new Exception());
+    ListenableFuture<?> chainedFuture =
+        Futures.withFallback(inputFuture, new FutureFallback<Integer>() {
+          @Override public ListenableFuture<Integer> create(Throwable t) {
+            return null;
+          }
+        });
+    try {
+      chainedFuture.get();
+      fail();
+    } catch (ExecutionException expected) {
+      NullPointerException cause = (NullPointerException) expected.getCause();
+      assertThat(cause).hasMessage("FutureFallback.create returned null instead of a Future. "
+          + "Did you mean to return immediateFuture(null)?");
+    }
+  }
+
   public void testTransform_genericsWildcard_AsyncFunction() throws Exception {
-    ListenableFuture<?> nullFuture = Futures.immediateFuture(null);
+    ListenableFuture<?> nullFuture = immediateFuture(null);
     ListenableFuture<?> chainedFuture =
         Futures.transform(nullFuture, constantAsyncFunction(nullFuture));
     assertNull(chainedFuture.get());
@@ -746,6 +764,20 @@ public void testTransform_asyncFunction_error() throws InterruptedException {
     }
   }
 
+  public void testTransform_asyncFunction_nullInsteadOfFuture() throws Exception {
+    ListenableFuture<?> inputFuture = immediateFuture("a");
+    ListenableFuture<?> chainedFuture =
+        Futures.transform(inputFuture, constantAsyncFunction(null));
+    try {
+      chainedFuture.get();
+      fail();
+    } catch (ExecutionException expected) {
+      NullPointerException cause = (NullPointerException) expected.getCause();
+      assertThat(cause).hasMessage("AsyncFunction.apply returned null instead of a Future. "
+          + "Did you mean to return immediateFuture(null)?");
+    }
+  }
+
   public void testTransform_asyncFunction_cancelledWhileApplyingFunction()
       throws InterruptedException, ExecutionException {
     final CountDownLatch inFunction = new CountDownLatch(1);
@@ -1031,8 +1063,8 @@ public void testAllAsList_resultCancelled_withSecondaryListFuture()
     SettableFuture<String> future2 = SettableFuture.create();
     ListenableFuture<List<String>> compound =
         Futures.allAsList(future1, future2);
-    ListenableFuture<List<String>> otherCompound =
-        Futures.allAsList(future1, future2);
+    // This next call is "unused," but it is an important part of the test. Don't remove it!
+    Futures.allAsList(future1, future2);
 
     assertTrue(compound.cancel(false));
     assertTrue(future1.isCancelled());

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

@@ -495,7 +495,10 @@ private static class FallbackFuture<V> extends AbstractFuture.TrustedFuture<V> {
             throwable = e;
           }
           try {
-            setFuture(fallback.create(throwable));
+            ListenableFuture<? extends V> replacement = fallback.create(throwable);
+            checkNotNull(replacement, "FutureFallback.create returned null instead of a Future. "
+                + "Did you mean to return immediateFuture(null)?");
+            setFuture(replacement);
           } catch (Throwable e) {
             setException(e);
           }
@@ -911,9 +914,9 @@ public void run() {
           return;
         }
 
-        ListenableFuture<? extends O> outputFuture =
-            Preconditions.checkNotNull(function.apply(sourceResult),
-                "AsyncFunction may not return null.");
+        ListenableFuture<? extends O> outputFuture = function.apply(sourceResult);
+        checkNotNull(outputFuture, "AsyncFunction.apply returned null instead of a Future. "
+            + "Did you mean to return immediateFuture(null)?");
         setFuture(outputFuture);
       } catch (UndeclaredThrowableException e) {
         // Set the cause of the exception as this future's exception