设计模式——行为型模式_状态模式

1. 类图

2. 代码

• State相关
  • 接口

    • public interface State {
         public void doAction(Context context);
      }

  • 实现类：此处定义Start和Stop两个状态

    • public class StartState implements State {
       
         public void doAction(Context context) {
            System.out.println("Player is in start state");
            context.setState(this); 
         }
       
         public String toString(){
            return "Start State";
         }
      }
      
      public class StopState implements State {
       
         public void doAction(Context context) {
            System.out.println("Player is in stop state");
            context.setState(this); 
         }
       
         public String toString(){
            return "Stop State";
         }
      }

• Context相关

  • public class Context {
       private State state;
     
       public Context(){
          state = null;
       }
     
       public void setState(State state){
          this.state = state;     
       }
     
       public State getState(){
          return state;
       }
    }

• 客户端调用

  • public class StatePatternDemo {
       public static void main(String[] args) {
          Context context = new Context();
     
          StartState startState = new StartState();
          startState.doAction(context);    //Player is in start state
     
          System.out.println(context.getState().toString());    //Start State
     
          StopState stopState = new StopState();
          stopState.doAction(context);    //Player is in stop state
     
          System.out.println(context.getState().toString());    //Stop State
       }
    }

3. 总结

优点：

• 避免了if else的判断
• 很好的体现了开闭原则和单一职责原则，想要增加状态就增加子类，想要修改状态就修改子类即可
• 封装性非常好，状态变化放置到了类的内部来实现，外部调用不需要知道类内部如何实现状态和行为的变换

缺点：

• 状态多的时候，子类爆炸