@Test
public void startShouldMakeWorkerWorkOnWorkItem(){
Worker worker = mock(Worker.class);
WorkItem workItem = mock(WorkItem.class);
SingularTask task = new SingularTask(worker);

task.start(workItem);

verify(worker).workOn(workItem);
}

##</code><code>##

public class SingularTask implements Task {
private Worker worker;
...
public void start(WorkItem workItem) {
worker.workOn(workItem);
}
...
}