状态模式.md

状态 State

重点等级：:star:

一句话

• 替代 if ... else 。把复杂的“判断逻辑”提取到不同的状态对象中，允许状态对象在其内部状态发生改变时改变其行为
• 对象的行为取决于其状态，并且必须根据该状态在运行时更改其行为

//各种动物的不同状态所对应不一样的处理
//状态接口类
public interface State {

  void onEnterState();

  void observe();
}

//安静状态实现类
public class PeacefulState implements State {

  private static final Logger LOGGER = LoggerFactory.getLogger(PeacefulState.class);

  private final Animal animal;

  public PeacefulState(Animal animal) {
    this.animal = animal;
  }

  @Override
  public void observe() {
    LOGGER.info("{} 平静祥和.", animal);
  }

  @Override
  public void onEnterState() {
    LOGGER.info("{} 安静下来.", animal);
  }
}

public class AngryState implements State {

  private static final Logger LOGGER = LoggerFactory.getLogger(AngryState.class);

  private final Animal animal;

  public AngryState(Animal animal) {
    this.animal = animal;
  }

  @Override
  public void observe() {
    LOGGER.info("{} 更生气了!", animal);
  }

  @Override
  public void onEnterState() {
    LOGGER.info("{} 生气了!", animal);
  }
}

public class Animal {

  private State state;
  private String name;

  public Animal(String name) {
    this.name = name;
    this.state = new PeacefulState(this);
  }

  public void timePasses() {
    if (state.getClass().equals(PeacefulState.class)) {
      changeStateTo(new AngryState(this));
    } else {
      changeStateTo(new PeacefulState(this));
    }
  }

  private void changeStateTo(State newState) {
    this.state = newState;
    this.state.onEnterState();
  }

  @Override
  public String toString() {
    return name;
  }

  public void observe() {
    this.state.observe();
  }
}

var animal = new Animal("小猫");
animal.observe();
animal.timePasses();
animal.observe();
animal.timePasses();
animal.observe();

//小猫 平静祥和.
//小猫 生气了!
//小猫 更生气了!
//小猫 安静下来.
//小猫 平静祥和.