EasyMock and modifing a mutable method parameter

Question

How does one use EasyMock to modify a mocked method's mutable method parameter?

For example, I have class that uses a BlockingQueue. I want to mock the BlockingQueue member for unit testing. My class calls the method queue.drainTo(Collection c). Calling this method removes elements from the queue and adds them to the collection. How would I mock this behavior using EasyMock? Examples would be great.

Answer 1

You can use andAnswer and getCurrentArguments:

public void testDrainToQueue() {
  BlockingQueue<Foo> queue = EasyMock.createMock(BlockingQueue.class);
  EasyMock.expect(queue.drainTo(EasyMock.isA(List.class)))
      .andAnswer(new IAnswer<Integer>() {
        public Integer answer() {
          ((List) EasyMock.getCurrentArguments()[0]).add(new Foo(123));
          return 1; // 1 element drained
        }
      });
  EasyMock.replay(queue);
  ...
}

It sometimes helps to extract a helper class or method:

private static IAnswer<Integer> fakeDrainReturning(final List drainedElements) {
  return new IAnswer<Integer() {
    @Override public Integer answer() {
      ((List) EasyMock.getCurrentArguments()[0]).addAll(drainedElements);
      return drainedElements.size();
    }
  };
}

Then you can do:

List<Foo> drainedElements = Arrays.asList(new Foo(123), new Foo(42));
EasyMock.expect(queue.drainTo(EasyMock.isA(List.class)))
    .andAnswer(fakeDrainReturning(drainedElements));

It might be better to use a real BlockingQueue and find a way to insert the desired value into the queue before the method that you expect to remove data from the queue.