设计模式之状态模式

定义：允许对象在内部状态改变时改变它的行为。对象看起来好像修改了它的类。状态模式将状态封装成独立的类，并将动作委托到代表当前状态的对象。

看个类图：

 

来一个具体的例子：

public interface State{


    void insertQuarter():
    
    void ejectQuarter();

    void turnCrank();

    void dispense();



}


public class NoQuarterState implents State{


        GumballMachine gumballMachine;

    public NoQuarterState ( GumballMachine gumballMachine ){

        this. gumballMachine= gumballMachine;


    }

    public void insertQuarter(){


        System.out.println("你放入了钱");

        gumballMachine.setState（gumballMachine.getHasQuarterState()）

    }


    public void ejectQuarter(){

        System.out.println("你没有钱");


    }

    
    public void turnCrank(){

         System.out.println("白转没有钱");

    }

    
    public void dispense(){

        System.out.println("你需要先给钱");

    }

}

//糖果机
public class GumballMachine{


    State soldOutState;

    State noQuarterState;

    State hasQuarterState;

    State soldState;

    State state = soldOutState; //状态的实例变量引用
    
    int count=0;//对糖果的实例变量



    public GumballMachine (int numberGumballs){


        soldOutState = new SoldOutState(this);
        noQuarterState = new NoQuarterState(this);
        hasQuarterState = new HasQuarterState(this);
        soldState = new SoldState(this);
        this.count=numberGumballs;
        if(numberGumballs > 0 ){
            
            state = niQuarterState;


        }
        

    }


    //四个行为动作很容易实现了，我们只是委托到当前状态

    public void insertQuarter(){

        state.insertQuarter();


    }

    
    public void ejectQuarter(){

        state.ejectQuarter();

    
    }


    public void turnCrank(){

        state.turnCrank();
        state.dispense();
//我们不需要在机器中准备一个发放糖果的方法，因为这是一个内部动作
//用户不可以直接要求发放糖果。我们是在状态对象的turnCrank()中调用
    
    }


    void setState(Satet state){

        this.state=state;

    }


    void releaseBall(){


        System.out.println("出糖了");

        if(count ! = 0){

            count--;

        }

    }



}

public class HasQuarterState implments State{

       GumballMachine gumballMachine;

       public HasQuarterState(GumballMachine gumballMachine){



            this.gumballMachine=gumballMachine;


        }

    
    public void insertQuarter(){

        System.out.println("你不能再放钱了");


    }

    public void ejectQuarter(){

        System.out.println("退给你");

        gumballMachine.setState(gumballMachine.getNoQuarterState());
    }


     public void turnCrank(){

        System.out.println("你转动手柄");
         gumballMachine.setState(gumballMachine.getSoldState());

    }


    public void dispense(){

        System.out.println("没有糖果发放");


    }

}


//售出状态
public class SoldState implments State{


     GumballMachine gumballMachine;

       public HasQuarterState(GumballMachine gumballMachine){



            this.gumballMachine=gumballMachine;


        }


     public void insertQuarter(){

        System.out.println("等等刚给了你糖果");


    }

    public void ejectQuarter(){

        System.out.println("你已经转手柄了");

      
    }


     public void turnCrank(){

        System.out.println("你白转动手柄不会给你两个");
        

    }


    public void dispense(){

       gumballMachine.releaseBall();
       if(gumballMachine.getCount() > 0){

            gumballMachine.setState(gumballMachine.getNoQuarterState() );

        }else{
             System.out.println("卖完了");

             gumballMachine.setState(gumballMachine.getSoldOutState() );

        }


    }




}


//售完的状态

public class SoldOutState implments State{


    GumballMachine gumballMachine;

       public HasQuarterState(GumballMachine gumballMachine){



            this.gumballMachine=gumballMachine;


        }


    public void insertQuarter(){

        System.out.println("糖果已经卖完了");


    }

    public void ejectQuarter(){

        System.out.println("你不能退这没有糖果");

      
    }


     public void turnCrank(){

        System.out.println("你白转动手柄没有糖果");
        

    }


    public void dispense(){

        System.out.println("没有糖果");


    }


}

 总结一把：状态模式的类图和策略者模式的类图是一样的，二者的区别在于意图不同，策略模式通常会用行为或算法来配置Context类，状态模式允许Context随着状态的改变而改变行为。而任何的改变都是定义好的。当我的Context对象被创建之后，建我可以告诉他们从什么时候开始，然后他们会随着时间的流逝改变自己的状态。