public void changeStrategy(DragonSlayingStrategy strategy) {
    this.strategy = strategy;
  }
@Test
  void testChangeStrategy() {
    final var initialStrategy = mock(DragonSlayingStrategy.class);
    final var dragonSlayer = new DragonSlayer(initialStrategy);

    dragonSlayer.goToBattle();
    verify(initialStrategy).execute();

    final var newStrategy = mock(DragonSlayingStrategy.class);
    dragonSlayer.changeStrategy(newStrategy);

    dragonSlayer.goToBattle();
    verify(newStrategy).execute();

    verifyNoMoreInteractions(initialStrategy, newStrategy);
  }