设计模式之状态模式

定义

状态模式是一种行为设计模式，它允许对象在内部状态改变时改变其行为，看起来好像这个对象改变了它的类。这种模式常用于处理对象在不同状态下的不同行为，使得代码更加结构化和易于维护。
在状态模式中，有以下几个关键角色：

• Context（上下文）：持有一个State对象的引用，通常会有一个方法来设置或获取当前状态。
• State（状态）：定义了状态接口，声明了所有可能的操作。
• ConcreteState（具体状态）：实现了State接口，提供了具体的行为。

示例代码

public interface VendingMachineState {
    void insertCoin();

    void ejectCoin();

    void turnCrank();

    void dispense();
}

public class NoMoneyState implements VendingMachineState {
    private VendingMachine vendingMachine;

    public NoMoneyState(VendingMachine vendingMachine) {
        this.vendingMachine = vendingMachine;
    }

    @Override
    public void insertCoin() {
        System.out.println("Coin inserted, new state is HasMoney");
        vendingMachine.setState(vendingMachine.getHasMoneyState());
    }

    @Override
    public void ejectCoin() {}

    @Override
    public void turnCrank() {}

    @Override
    public void dispense() {}
}

public class HasMoneyState implements VendingMachineState {
    private VendingMachine vendingMachine;

    public HasMoneyState(VendingMachine vendingMachine) {
        this.vendingMachine = vendingMachine;
    }

    @Override
    public void insertCoin() {
        System.out.println("Machine is already in HasMoney state, coin will be added to credit");
    }

    @Override
    public void ejectCoin() {

    }

    @Override
    public void turnCrank() {
        System.out.println("Machine is already in HasMoney state, turnCrank is not allowed");
    }

    @Override
    public void dispense() {

    }
}

public class VendingMachine {
    private VendingMachineState state;

    public VendingMachine() {
        this.state = new NoMoneyState(this);
    }

    public VendingMachineState getState() {
        return state;
    }

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

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

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

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

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

    public VendingMachineState getHasMoneyState() {
        return new HasMoneyState(this);
    }
}

public class StateTest {
    @Test
    public void test() {
        VendingMachine machine = new VendingMachine();
        machine.insertCoin(); // 输出 "Coin inserted, new state is HasMoney"
        machine.turnCrank(); // 根据HasMoneyState的行为执行
    }
}

源代码地址

https://gitee.com/youxiaxiaomage/java-practices/tree/master/yxxmg-gof-sample/src/main/java/com/yxxmg/gof/behavior/state