How to mock void methods with Mockito

Take a look at the Mockito API docs. As the linked document mentions (Point # 12) you can use any of the doThrow(),doAnswer(),doNothing(),doReturn() family of methods from Mockito framework to mock void methods.

For example,

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

or if you want to combine it with follow-up behavior,

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

Presuming that you are looking at mocking the setter setState(String s) in the class World below is the code uses doAnswer method to mock the setState.

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());

I think I've found a simpler answer to that question, to call the real method for just one method (even if it has a void return) you can do this:

Mockito.doCallRealMethod().when(<objectInstance>).<method>();<objectInstance>.<method>();

Or, you could call the real method for all methods of that class, doing this:

<Object> <objectInstance> = mock(<Object>.class, Mockito.CALLS_REAL_METHODS);

Adding to what @sateesh said, when you just want to mock a void method in order to prevent the test from calling it, you could use a Spy this way:

World world = new World();World spy = Mockito.spy(world);Mockito.doNothing().when(spy).methodToMock();

When you want to run your test, make sure you call the method in test on the spy object and not on the world object. For example:

assertEquals(0, spy.methodToTestThatShouldReturnZero());