How to mock void methods with Mockito

How to mock methods with void return type?

I implemented an observer pattern but I can't mock it with Mockito because I don't know how.

And I tried to find an example on the Internet but didn't succeed.

My class looks like this:

public class World {

    List<Listener> listeners;

    void addListener(Listener item) {
        listeners.add(item);
    }

    void doAction(Action goal,Object obj) {
        setState("i received");
        goal.doAction(obj);
        setState("i finished");
    }

    private string state;
    //setter getter state
} 

public class WorldTest implements Listener {

    @Test public void word{
    World  w= mock(World.class);
    w.addListener(this);
    ...
    ...

    }
}

interface Listener {
    void doAction();
}

The system is not triggered with mock.

I want to show the above-mentioned system state. And make assertions according to them.

Solution 1:

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());

Solution 2:

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);

Solution 3:

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());

Solution 4:

The solution of so-called problem is to use a spy Mockito.spy(...) instead of a mock Mockito.mock(..).

Spy enables us to partial mocking. Mockito is good at this matter. Because you have class which is not complete, in this way you mock some required place in this class.