How to mock void methods with Mockito

Question

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.

Answer 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());