Address comments.

Author: reuvenlax

runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/DataflowRunner.java

@@ -1437,6 +1437,8 @@ public Map<PValue, ReplacementOutput> mapOutputs(
   @VisibleForTesting
   static class StreamingShardedWriteFactory<T>
       implements PTransformOverrideFactory<PCollection<T>, PDone, WriteFiles<T>> {
+    // We pick 10 as a a default, as it works well with the default number of workers started
+    // by Dataflow.
     static final int DEFAULT_NUM_SHARDS = 10;
     DataflowPipelineWorkerPoolOptions options;
 

runners/google-cloud-dataflow-java/src/test/java/org/apache/beam/runners/dataflow/DataflowRunnerTest.java

@@ -1142,6 +1142,16 @@ public void testWorkerHarnessContainerImage() {
   public void testStreamingWriteWithNoShardingReturnsNewTransform() {
     PipelineOptions options = TestPipeline.testingPipelineOptions();
     options.as(DataflowPipelineWorkerPoolOptions.class).setMaxNumWorkers(10);
+    testStreamingWriteOverride(options, 20);
+  }
+
+  @Test
+  public void testStreamingWriteWithNoShardingReturnsNewTransformMaxWorkersUnset() {
+    PipelineOptions options = TestPipeline.testingPipelineOptions();
+    testStreamingWriteOverride(options, StreamingShardedWriteFactory.DEFAULT_NUM_SHARDS);
+  }
+
+  private void testStreamingWriteOverride(PipelineOptions options, int expectedNumShards) {
     TestPipeline p = TestPipeline.fromOptions(options);
 
     StreamingShardedWriteFactory<Object> factory =
@@ -1155,7 +1165,7 @@ public void testStreamingWriteWithNoShardingReturnsNewTransform() {
     WriteFiles<Object> replacement = (WriteFiles<Object>)
         factory.getReplacementTransform(originalApplication).getTransform();
     assertThat(replacement, not(equalTo((Object) original)));
-    assertThat(replacement.getNumShards().get(), equalTo(20));
+    assertThat(replacement.getNumShards().get(), equalTo(expectedNumShards));
   }
 
   private static class TestSink extends FileBasedSink<Object> {