public class GumballMachine {
    final static int SOLD_OUT = 0;
    final static int NO_COIN = 1;
    final static int HAS_COIN = 2;
    final static int SOLD = 3;

    int state = SOLD_OUT;
    int count = 0;
    // ...

    public void insertCoin() {
        // 痛点：基于状态的巨大 if-else 或 switch 语句
        if (state == HAS_COIN) {
            System.out.println("You can't insert another coin");
        } else if (state == NO_COIN) {
            state = HAS_COIN;
            System.out.println("You inserted a coin");
        } else if (state == SOLD_OUT) {
            System.out.println("You can't insert a coin, the machine is sold out");
        } else if (state == SOLD) {
            System.out.println("Please wait, we're already giving you a gumball");
        }
    }
    // ... turnCrank() 和其他方法也充满了这样的条件判断 ...
}

// 1. 状态接口 (State)
interface State {
    void insertCoin();
    void ejectCoin();
    void turnCrank();
    void dispense();
}

// --- 3. 上下文 (Context) ---
class GumballMachine {
    // 持有所有状态的实例
    private State soldOutState;
    private State noCoinState;
    private State hasCoinState;
    private State soldState;

    private State currentState;
    private int count = 0; // 糖果数量

    public GumballMachine(int numberGumballs) {
        soldOutState = new SoldOutState(this);
        noCoinState = new NoCoinState(this);
        hasCoinState = new HasCoinState(this);
        soldState = new SoldState(this);

        this.count = numberGumballs;
        if (numberGumballs > 0) {
            currentState = noCoinState; // 初始状态
        } else {
            currentState = soldOutState;
        }
    }

    // 动作被委托给当前状态
    public void insertCoin() { currentState.insertCoin(); }
    public void ejectCoin() { currentState.ejectCoin(); }
    public void turnCrank() {
        currentState.turnCrank();
        currentState.dispense(); // 紧跟着分发
    }

    // Setter for state transition
    void setState(State state) { this.currentState = state; }

    // Getters for states to use
    public State getSoldOutState() { return soldOutState; }
    public State getNoCoinState() { return noCoinState; }
    public State getHasCoinState() { return hasCoinState; }
    public State getSoldState() { return soldState; }
    public int getCount() { return count; }
    public void releaseBall() {
        System.out.println("A gumball comes rolling out the slot...");
        if (count > 0) {
            count = count - 1;
        }
    }
}

// --- 2. 具体状态 (Concrete State) ---
// 以 HasCoinState 为例
class HasCoinState implements State {
    GumballMachine gumballMachine;

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

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

    @Override
    public void ejectCoin() {
        System.out.println("Coin returned");
        gumballMachine.setState(gumballMachine.getNoCoinState()); // 转换到 NoCoinState
    }

    @Override
    public void turnCrank() {
        System.out.println("You turned...");
        gumballMachine.setState(gumballMachine.getSoldState()); // 转换到 SoldState
    }

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

// 其他状态类 (NoCoinState, SoldState, SoldOutState) ... 篇幅所限，省略具体实现
class NoCoinState implements State {
    GumballMachine machine;
    public NoCoinState(GumballMachine m) { this.machine = m; }
    public void insertCoin() {
        System.out.println("You inserted a coin");
        machine.setState(machine.getHasCoinState());
    }
    public void ejectCoin() { System.out.println("You haven't inserted a coin"); }
    public void turnCrank() { System.out.println("You turned, but there's no coin"); }
    public void dispense() { System.out.println("You need to pay first"); }
}

// ... SoldState 和 SoldOutState 的实现

public class StatePatternDemo {
    public static void main(String[] args) {
        GumballMachine gumballMachine = new GumballMachine(5);

        System.out.println(gumballMachine);

        gumballMachine.insertCoin();
        gumballMachine.turnCrank();

        System.out.println(gumballMachine);

        gumballMachine.insertCoin();
        gumballMachine.ejectCoin();
        gumballMachine.turnCrank(); // this will fail

        System.out.println(gumballMachine);
    }
}