状态模式

定义

允许对象在内部转态改变时改变它的行为，对象看起来好像修改了它的类。

使用场景

• 对象的行为取决于它的状态，并且它会在运行时根据状态改变行为
• 大量的操作会引入很多依赖对象状态的条件语句，对象的状态可由一个或多个枚举常量表示，并且许多操作通常包含相同的条件结构。 状态模式将每个条件分支放在单独的类中。 这使我们可以将对象的状态视为一个对象，而且此对象可以独立于其他对象进行变化。

例子

如下是一个糖果贩卖机的状态转换图，投入0.25元转动曲柄可以获得糖果。

Paste_Image.png

首先定义各种状态，并且实现各种状态的方法，如投币、拿回币、转动曲柄、分发糖果；有些状态的方法其实是无效的：

public interface State{
    public void insertQuarter();
    public void ejectQuarter();
    public void turnCrank();
    public void dispense();
}

// 投入硬币的状态
public class HasQuarterState implements State{
    GumballMachine gumballMachine;
    public HasQuarterState(GumballMachine gumballMachine){
        this.gumballMachine = gumballMachine;
    }
    public void insertQuarter(){
        System.out.println("You can not insert another quarter");
    }
    public void ejectQuarter(){
        System.out.println("Quarter returned");
        gumballMachine.setState(gumballMachine.getNoQuarterState());
    }
    public void turnCrank(){ // 摇动曲柄
        System.out.println("You turned...");
        gumballMachine.setState(gumballMachine.getSoldState());
    }
    public void dispense(){
        System.out.println(" No gumball dispensed");
    }
}

// 转动曲柄 销售状态
public class SoldState implements State{
    GumballMachine gumballMachine;
    public SoldState(GumballMachine gumballMachine){
        this.gumballMachine = gumballMachine;
    }
    public void insertQuarter(){
        System.out.println("Please wait, we are already giving you a gumball");
    }
    public void ejectQuarter(){
        System.out.println("Sorry, you have turned the crank");
    }
    public void turnCrank(){ // 摇动曲柄
        System.out.println("Turing twice doesn't get you another gumball");
    }
    public void dispense(){
        gumballMachine.releaseBall();
        if(gumballMachine.getCount() > 0){
            gumballMachine.setState(gumballMachine.getNoQuarterState());
        }else{
            System.out.println("out of gumballs");
            gumballMachine.setState(gumballMachine.getSoldOutState());
        }
    }
}

// NoQuarterState SoldOutState

糖果机的实现：

public class GumballMachine{
    State soldOutState;
    State noQuarterState;
    State hasQuarterState;
    State soldState;

    State state = soldState;
    int count = 0;

    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 = noQuarterState;
        }
    }

    public void insertQuarter(){
        state.insertQuarter();
    }
    public void ejectQuarter(){
        state.ejectQuarter();
    }
    public void turnCrank(){
        state.turnCrank();
        state.dispense();
    }

    void setState(State state){
        this.state = state;
    }
    void releaseBall(){
        System.out.println("A gumball comes rolling out the slot...");
        if(count !=0 ){
            count = count-1;
        }
    }

    // getNoQuarterState等各种getter
}

分析

状态模式：封装基于状态的行为，并将行为委托到当前状态。
策略模式：将可以互换的行为封装起来，然后使用委托的方法决定使用哪一个行为

参考

iluwatar/java-design-patterns