状态模式

状态模式允许对象在对象内部状态改变时改变它的行为，对象看起来好像修改了他的类
现在又一个糖果机，我们需要写程序来控制它的出售行为，如图

当然我们可以用一堆if，else语句俩控制它。但是如果程序需要加入其他的状态，那么整个程序就会变得越来越乱。这时我们需要状态模式来重构程序。

java代码

// 状态接口
public interface State {

    void insertCoin();

    void ejectCoin();

    void turnCrank();

    void dispense();
}

// 没有投币状态
public class NoCoinState implements State {

    private GumballMachine machine;

    public NoCoinState(GumballMachine machine) {
        this.machine = machine;
    }

    @Override
    public void insertCoin() {
        machine.setState(machine.getHasCoinState());
        System.out.println("Your inserted a coin");
    }

    @Override
    public void ejectCoin() {
        System.out.println("Your have not inserted a coin");
    }

    @Override
    public void turnCrank() {
        System.out.println("You turned, but there is no coin");
    }

    @Override
    public void dispense() {
        System.out.println("You need to pay first");
    }
}

// 已投币状态
public class HasCoinState implements State {

    private GumballMachine machine;

    public HasCoinState(GumballMachine machine) {
        this.machine = machine;
    }

    @Override
    public void insertCoin() {
        System.out.println("You can not insert another coin");
    }

    @Override
    public void ejectCoin() {
        machine.setState(machine.getNoCoinState());
        System.out.println("Coin returned");
    }

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

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

// 已出售状态（正在出货）
public class SoldState implements State {

    private GumballMachine machine;

    public SoldState(GumballMachine machine) {
        this.machine = machine;
    }

    @Override
    public void insertCoin() {
        System.out.println("Please wait, we are already giving you a gumball");
    }

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

    @Override
    public void turnCrank() {
        System.out.println("Turning twice does not get you another gumball!");
    }

    @Override
    public void dispense() {
        if (machine.getGumballNum() > 0) {
            machine.setGumballNum(machine.getGumballNum() - 1);
            machine.setState(machine.getNoCoinState());
            System.out.println("A gumball comes rolling out the slot ... ");
        }
        if (machine.getGumballNum() <= 0) {
            machine.setState(machine.getSoldOutState());
            System.out.println("Oops, out of gumballs!");
        }
    }
}

// 售罄状态
public class SoldOutState implements State {

    private GumballMachine machine;

    public SoldOutState(GumballMachine machine) {
        this.machine = machine;
    }

    @Override
    public void insertCoin() {
        System.out.println("You can not insert a coin, this machine is sold out");
    }

    @Override
    public void ejectCoin() {
        System.out.println("Your have not inserted a coin");
    }

    @Override
    public void turnCrank() {
        System.out.println("You turned, but there are no gumballs");
    }

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

// 测试类
public class Test {

    public static void main(String[] args) {
        new Test().test();
    }

    private void test() {

        GumballMachine machine = new GumballMachine(5);
        machine.insertCoin();
        machine.turnCrank();
    }
}

注：

1. 当糖果的状态改变的时候，他的所有行为都已经改变

参考文章
1. Head First 设计模式