### Question:
FelixTaskExecutor implements TaskExecutor { @Override public void execute(Runnable task) { if (task == null) { throw new IllegalArgumentException("Method argument task must not be null."); } task.run(); }  @Override void execute(Runnable task);  }### Answer:
@Test public void testSynchronousExecution() { execute(() -> executed = true); assertThat(this.executed).isTrue(); }
@Test(expected = IllegalArgumentException.class) public void testHandlingOfNullTask() { execute(null); }