设计模式---状态模式



public class GumballMachine {
    final static int SOLD_OUT = 0;
    final static int NO_QUARTER = 1;
    final static int HAS_QUARTER = 2;
    final static int SOLD = 3;
                                                  //以上四个变量用来设计四个状态
    int state = SOLD_OUT;                       //用来追踪当前状态
    int count = 0;                               //用来保存糖果数量
    
    public GumballMachine(int count) {
        this.count = count;
        if (count > 0) {
            state = NO_QUARTER;
        }
    }
    
    public void insertQuarter () {
        if (state = HAS_QUARTER) {
            System.out.println("You can't insert another quarter");
        } else if (state = NO_QUARTER) {
            state = HAS_QUARTER;
            System.out.println("You inserted a aquarter");
        } else if (state = SOLD_OUT) {
            System.out.println("You can't insert a quarter , the machine if sold out");
        } else if (state = SOLD) {
            System.out.println("Please wait, we're already giving you'a gumball");
        }
    }
}

恕我直言，这个要求真的是无语，没办法，毕竟是需求方的要求，只能帮他们实现了，这个时候，就发现我们之前写的代码不怎么易于维护和修改了吧，这个时候我们就需要重新构造他了。

让我们实现上面这个模式吧。（这里的类图少了一个WinnerState类，就是成功的能有两颗糖，不要在意，我们会在下面的代码中实现的）

State类：

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

GumballMachine类：

public class GumballMachine {
    State soldOutState;
    State noQuarterState;
    State hasQuarterState;
    State soldState;
    State winnerState;
                                      //以上是所有的状态
    State state = soldOutState;     //追踪当前状态
    int count = 0;                   //记录糖果数

    public void insertQuarter() {
        state.insertQuarter();
    }

    public void ejectQuarter() {
        state.ejectQuarter();
    }

    public void turnCrank() {
        state.turnCrank();
        state.dispense();                 //我们不需要单独实现dispense方法，可以把这个方法写入turnCrank里面
    }

    public void setState (State state) {
        this.state = state;
    }

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

    public State getSoldOutState() {
        return soldOutState;
    }

    public State getNoQuarterState() {
        return noQuarterState;
    }

    public State getHasQuarterState() {
        return hasQuarterState;
    }

    public State getSoldState() {
        return soldState;
    }

    public State getWinnerState() {
        return winnerState;
    }

    public State getState() {
        return state;
    }

    public int getCount() {
        return count;
    }
}

HasQuarterState类：

public class HasQuarterState implements State {
    GumballMachine gumballMachine;

    public HasQuarterState(GumballMachine gumballMachine) {
        this.gumballMachine = gumballMachine;
    }

    public void insertQuarter() {
        System.out.println("You can't insert another quarter");
    }

    public void ejectQuarter() {
        System.out.println("Quarter returned");
        gumballMachine.setState(gumballMachine.getNoQuarterState());
    }

    public void turnCrank() {
        System.out.println("You turned..");
        gumballMachine.setState(gumballMachine.getSoldState());
    }

    public void dispense() {
        System.out.println("No gumball dispensed");
    }
}

WinnerState类：

public class WinnerState implements State {
    GumballMachine gumballMachine;

    public WinnerState(GumballMachine gumballMachine) {
        this.gumballMachine = gumballMachine;
    }

    public void insertQuarter() {
        System.out.println("Please wait, we're already giving you a gumball");
    }

    public void ejectQuarter() {
        System.out.println("Sorry, you already turned the crank");
    }

    public void turnCrank() {
        System.out.println("Turning twice doesn't get you another gumball");
    }

    public void dispense() {
        System.out.println("You're a winner ! you get two gumballs for your quarter");
        gumballMachine.releaseBall();
        if (gumballMachine.getCount() == 0) {
            gumballMachine.setState(gumballMachine.getSoldOutState());
        } else {
            gumballMachine.releaseBall();
            if (gumballMachine.getCount() > 0) {
                gumballMachine.setState(gumballMachine.getNoQuarterState());
            } else {
                System.out.println("Oops, out of gumballs");
                gumballMachine.setState(gumballMachine.getSoldOutState());
            }
        }
    }
}

定义状态模式：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。

状态模式的类图：

总结：

1：状态模式允许一个对象基于内部状态而拥有不同的行为。

2：状态模式用类代表状态。

3：通过将每个状态封装进一个类，我们把以后需要做的任何改变局部化了。

4：状态模式和策略模式有相同的类图，但是他们的意图不一样。

5：使用状态模式通常会导致设计中类的数目大量增加。