GwtMockitoCallingAnonymousClass.java

    private SampleWidget sampleWidget;
    private ClickHandler clickHandler;

    @Before
    public void setUp() {
        Mockito.when(button.addClickHandler(Mockito.any(ClickHandler.class))).thenAnswer(new Answer() {
            public Object answer(InvocationOnMock aInvocation) throws Throwable {
                clickHandler = (ClickHandler) aInvocation.getArguments()[0];
                return null;
            }
        });
        sampleWidget = new SampleWidget();
    }

    @Test
    public void clickOnButton() {
        clickHandler.onClick(new ClickEvent(){});
        //...
    }