状态模式（State）

目的

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所属的类。

类图

实现

糖果销售机有多种状态，每种状态下销售机有不同的行为，状态可以发生转移，使得销售机的行为也发生改变。

public interface State {
    /**
     * 投入 25 分钱
     */
    void insertQuarter();
    /**
     * 退回 25 分钱
     */
    void ejectQuarter();
    /**
     * 转动曲柄
     */
    void turnCrank();
    /**
     * 发放糖果
     */
    void dispense();
}
public class HasQuarterState implements State {
    private GumballMachine gumballMachine;
    public HasQuarterState(GumballMachine gumballMachine) {
        this.gumballMachine = gumballMachine;
    }
    @Override
    public void insertQuarter() {
        System.out.println("You can't insert another quarter");
    }
    @Override
    public void ejectQuarter() {
        System.out.println("Quarter returned");
        gumballMachine.setState(gumballMachine.getNoQuarterState());
    }
    @Override
    public void turnCrank() {
        System.out.println("You turned...");
        gumballMachine.setState(gumballMachine.getSoldState());
    }
    @Override
    public void dispense() {
        System.out.println("No gumball dispensed");
    }
}
public class NoQuarterState implements State {
    GumballMachine gumballMachine;
    public NoQuarterState(GumballMachine gumballMachine) {
        this.gumballMachine = gumballMachine;
    }
    @Override
    public void insertQuarter() {
        System.out.println("You insert a quarter");
        gumballMachine.setState(gumballMachine.getHasQuarterState());
    }
    @Override
    public void ejectQuarter() {
        System.out.println("You haven't insert a quarter");
    }
    @Override
    public void turnCrank() {
        System.out.println("You turned, but there's no quarter");
    }
    @Override
    public void dispense() {
        System.out.println("You need to pay first");
    }
}
public class SoldOutState implements State {
    GumballMachine gumballMachine;
    public SoldOutState(GumballMachine gumballMachine) {
        this.gumballMachine = gumballMachine;
    }
    @Override
    public void insertQuarter() {
        System.out.println("You can't insert a quarter, the machine is sold out");
    }
    @Override
    public void ejectQuarter() {
        System.out.println("You can't eject, you haven't inserted a quarter yet");
    }
    @Override
    public void turnCrank() {
        System.out.println("You turned, but there are no gumballs");
    }
    @Override
    public void dispense() {
        System.out.println("No gumball dispensed");
    }
}
public class SoldState implements State {
    GumballMachine gumballMachine;
    public SoldState(GumballMachine gumballMachine) {
        this.gumballMachine = gumballMachine;
    }
    @Override
    public void insertQuarter() {
        System.out.println("Please wait, we're already giving you a gumball");
    }
    @Override
    public void ejectQuarter() {
        System.out.println("Sorry, you already turned the crank");
    }
    @Override
    public void turnCrank() {
        System.out.println("Turning twice doesn't get you another gumball!");
    }
    @Override
    public void dispense() {
        gumballMachine.releaseBall();
        if (gumballMachine.getCount() > 0) {
            gumballMachine.setState(gumballMachine.getNoQuarterState());
        } else {
            System.out.println("Oops, out of gumballs");
            gumballMachine.setState(gumballMachine.getSoldOutState());
        }
    }
}
public class GumballMachine {
    private State soldOutState;
    private State noQuarterState;
    private State hasQuarterState;
    private State soldState;
    private State state;
    private int count = 0;
    public GumballMachine(int numberGumballs) {
        count = numberGumballs;
        soldOutState = new SoldOutState(this);
        noQuarterState = new NoQuarterState(this);
        hasQuarterState = new HasQuarterState(this);
        soldState = new SoldState(this);
        if (numberGumballs > 0) {
            state = noQuarterState;
        } else {
            state = soldOutState;
        }
    }
    public void insertQuarter() {
        state.insertQuarter();
    }
    public void ejectQuarter() {
        state.ejectQuarter();
    }
    public void turnCrank() {
        state.turnCrank();
        state.dispense();
    }
    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 -= 1;
        }
    }
    public State getSoldOutState() {
        return soldOutState;
    }
    public State getNoQuarterState() {
        return noQuarterState;
    }
    public State getHasQuarterState() {
        return hasQuarterState;
    }
    public State getSoldState() {
        return soldState;
    }
    public int getCount() {
        return count;
    }
}
public class Client {
    public static void main(String[] args) {
        GumballMachine gumballMachine = new GumballMachine(5);
        gumballMachine.insertQuarter();
        gumballMachine.turnCrank();
        gumballMachine.insertQuarter();
        gumballMachine.ejectQuarter();
        gumballMachine.turnCrank();
        gumballMachine.insertQuarter();
        gumballMachine.turnCrank();
        gumballMachine.insertQuarter();
        gumballMachine.turnCrank();
        gumballMachine.ejectQuarter();
        gumballMachine.insertQuarter();
        gumballMachine.insertQuarter();
        gumballMachine.turnCrank();
        gumballMachine.insertQuarter();
        gumballMachine.turnCrank();
        gumballMachine.insertQuarter();
        gumballMachine.turnCrank();
    }
}
运行结果
You insert a quarter
You turned...
A gumball comes rolling out the slot...
You insert a quarter
Quarter returned
You turned, but there's no quarter
You need to pay first
You insert a quarter
You turned...
A gumball comes rolling out the slot...
You insert a quarter
You turned...
A gumball comes rolling out the slot...
You haven't insert a quarter
You insert a quarter
You can't insert another quarter
You turned...
A gumball comes rolling out the slot...
You insert a quarter
You turned...
A gumball comes rolling out the slot...
Oops, out of gumballs
You can't insert a quarter, the machine is sold out
You turned, but there are no gumballs
No gumball dispensed