How to Mock Void Methods in Mockito: A Guide to doThrow, doAnswer, doNothing, and doReturn?

Mocking Void Methods with Mockito

When dealing with observer patterns, mocking methods with void return types can present challenges. One such case is encountering difficulties in mocking a class with a method like addListener, which returns void.

To mock a method with no return value, Mockito provides a set of methods: doThrow(), doAnswer(), doNothing(), and doReturn(). These methods allow you to specify the behavior of the mocked method.

For instance, to specify that the addListener method throws an exception when called, you can use:

Mockito.doThrow(new Exception()).when(instance).addListener(any(Listener.class));

Alternatively, you can chain multiple behaviors together. For example, the following code throws an exception on the first call to addListener and then does nothing on subsequent calls:

Mockito.doThrow(new Exception()).doNothing().when(instance).addListener(any(Listener.class));

In the provided World class, you can mock the setState method using the doAnswer method, as shown below:

World mockWorld = mock(World.class);
doAnswer(new Answer<Void>() {
    public Void answer(InvocationOnMock invocation) {
        Object[] args = invocation.getArguments();
        System.out.println("called with arguments: " + Arrays.toString(args));
        return null;
    }
}).when(mockWorld).setState(anyString());

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