How to Effectively Mock Void Methods in Mockito?

Mocking Void Methods with Mockito

In the Observer pattern, observers typically define methods with void return types. When attempting to mock such methods using Mockito, you may encounter challenges.

Challenge: Mocks of methods with no return values aren't automatically triggered. As a result, assertions related to state changes or method invocations may fail in unit tests.

Solution:

Utilize the doThrow(), doAnswer(), doNothing(), or doReturn() methods from Mockito to manipulate the behavior of void methods.

For instance, to trigger an exception when a method is called:

Mockito.doThrow(new Exception()).when(instance).methodName();

To combine it with subsequent actions:

Mockito.doThrow(new Exception()).doNothing().when(instance).methodName();

In the provided example, mocking the setState method can be achieved using the doAnswer method:

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());

With this technique, you can mock void methods, set expectations, and perform assertions to verify their behavior in your unit tests.

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