public void timePasses() {
    if (state.getClass().equals(PeacefulState.class)) {
      changeStateTo(new AngryState(this));
    } else {
      changeStateTo(new PeacefulState(this));
    }
  }
@Test
  void testTimePasses() {
    final var mammoth = new Mammoth();

    mammoth.observe();
    assertEquals("The mammoth is calm and peaceful.", appender.getLastMessage());
    assertEquals(1, appender.getLogSize());

    mammoth.timePasses();
    assertEquals("The mammoth gets angry!", appender.getLastMessage());
    assertEquals(2, appender.getLogSize());

    mammoth.observe();
    assertEquals("The mammoth is furious!", appender.getLastMessage());
    assertEquals(3, appender.getLogSize());

    mammoth.timePasses();
    assertEquals("The mammoth calms down.", appender.getLastMessage());
    assertEquals(4, appender.getLogSize());

    mammoth.observe();
    assertEquals("The mammoth is calm and peaceful.", appender.getLastMessage());
    assertEquals(5, appender.getLogSize());

  }