java - working - only void methods can donothing()

How to make mock to void methods with mockito (7)

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

public class World {

    List<Listener> listeners;

    void addListener(Listener item) {

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

    private string state;
    //setter getter state

public class WorldTest implements Listener {

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


interface Listener {
    void doAction();

The system are not triggered with mock. =( I want to show above mentioned system state. And make assertion according to them.

@ashley : works for me

public class AssetChangeListenerImpl extends
AbstractAssetChangeListener implements AssetChangeListener {

  public void onChangeEvent(final EventMessage message) throws EventHubClientException {

public class AbstractAssetChangeListener {
  protected  void execute( final EventMessage message ) throws EventHubClientException {
    executor.execute( new PublishTask(getClient(), message) );
} } @RunWith(MockitoJUnitRunner.class) public class AssetChangeListenerTest extends AbstractAssetChangeListenerTest {

 public void testExecute() throws EventHubClientException {
    EventMessage message = createEventMesage(EventType.CREATE);
    verify(assetChangeListener, times(1)).execute(message);
} }

How to mock void methods with mockito - there are two options:

  1. doAnswer - If we want our mocked void method to do something (mock the behavior despite being void).
  2. doThrow - Then there is Mockito.doThrow() if you want to throw an exception from the mocked void method.

Following is an example of how to use it (not an ideal usecase but just wanted to illustrate the basic usage).

public void testUpdate() {

    doAnswer(new Answer<Void>() {

        public Void answer(InvocationOnMock invocation) throws Throwable {
            Object[] arguments = invocation.getArguments();
            if (arguments != null && arguments.length > 1 && arguments[0] != null && arguments[1] != null) {

                Customer customer = (Customer) arguments[0];
                String email = (String) arguments[1];

            return null;
    }).when(daoMock).updateEmail(any(Customer.class), any(String.class));

    // calling the method under test
    Customer customer = service.changeEmail("[email protected]", "[email protected]");

    //some asserts
    assertThat(customer, is(notNullValue()));
    assertThat(customer.getEmail(), is(equalTo("[email protected]")));


@Test(expected = RuntimeException.class)
public void testUpdate_throwsException() {

    doThrow(RuntimeException.class).when(daoMock).updateEmail(any(Customer.class), any(String.class));

    // calling the method under test
    Customer customer = service.changeEmail("[email protected]", "[email protected]");


You could find more details on how to mock and test void methods with Mockito in my post How to mock with Mockito (A comprehensive guide with examples)

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);

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:


First of all: you should always import mockito static, this way the code will be much more readable (and intuitive):

import static org.mockito.Mockito.*;

For partial mocking and still keeping original functionality on the rest mockito offers "Spy".

You can use it as follows:

private World world = spy(World.class);

To eliminate a method from being executed you could use something like this:


to give some custom behaviour to a method use "when" with an "thenReturn":


For more examples please find the mockito samples in the doc.

I think your problems are due to your test structure. I've found it difficult to mix mocking with the traditional method of implementing interfaces in the test class (as you've done here).

If you implement the listener as a Mock you can then verify the interaction.

Listener listener = mock(Listener.class);

This should satisfy you that the 'World' is doing the right thing.

In Java 8 this can be made a little cleaner, assuming you have a static import for org.mockito.Mockito.doAnswer:

doAnswer(i -> {
  // Do stuff with i.getArguments() here
  return null;

The return null; is important and without it the compile will fail with some fairly obscure errors as it won't be able to find a suitable override for doAnswer.

For example an ExecutorService that just immediately executes any Runnable passed to execute() could be implemented using:

doAnswer(i -> {
  ((Runnable) i.getArguments()[0]).run();
  return null;

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.