How to test a function with a callback?

Question

Suppose we have an interface for a callback function as such:

public interface Callback
{
    void foo();
}

And we have a method that calls a callback that was given as an argument:

public void doSomething(final Callback callback)
{
    // Do something, like asynchronously fetch something from a server
    asyncStuff.doSomethingAsync(new AsyncResponseHandler()
    {
        @Override
        public void asyncStuffDone()
        {
            // Call the callback
            callback.foo();
        }
    });
}

Now, to the question: Given this kind of scenario, how would one test that the callback indeed gets called?

Answer 1

Supply a mock Callback object for the test and validate that the method was invoked. For example, using Mockito as a mocking library, you would create a mock object (arrange):

Callback myMock = mock(Callback.class);

Then supply it to the code being tested (act):

someObject.doSomething(myMock);

And validate that the method was invoked (assert):

verify(myMock, times(1)).foo();