An example of using doAnswer when while testing an asynchronous method call.

AnswerAsync.java

@Test
public void getData_returnsData_whenAsyncThreadIsComplete() {
    String expectedData = "Here's your data!";
    doAnswer(new Answer<Void>() {
        @Override
        public Void answer(InvocationOnMock invocation) throws Throwable {
            Runnable runnable = (Runnable) invocation.getArguments()[0];
            runnable.run();
            return null;
        }
    }).when(client).processData(any(Runnable.class));


    String result = myService.getData().toString();

    Assertions.assertThat(result).isEqualTo(expectedData);
}