状态模式(State Pattern)

在状态模式中，一个类的行为基于它的状态的改变而改变。状态模式归属于行为型模式。

在下面的实例中，我们创建了一个接口State，定义了一个操作方法，两个实现类StartState和StopState。另外，创建了一个上下文类Context，这个类关联到State类。UML类图如下所示：

状态类State

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

实现类StartState.java

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";
   }
}

实现类StopState.java

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";
   }
}

上下文类Context.java

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

测试类StatePatternDemo.java

public class StatePatternDemo {
   public static void main(String[] args) {
      Context context = new Context();
 
      StartState startState = new StartState();
      startState.doAction(context);
 
      System.out.println(context.getState().toString());
 
      StopState stopState = new StopState();
      stopState.doAction(context);
 
      System.out.println(context.getState().toString());
   }
}

输出结果：

Player is in start state
Start State
Player is in stop state
Stop State