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Java编程之状态模式

news 2025/7/2 5:20:57

引言

“人生如戏，全靠状态。”——设计模式也深知这一点。

一、什么是状态模式（State Pattern）

状态模式是一种行为型设计模式，允许对象在内部状态改变时，改变它的行为。也就是说，看起来像是修改了它的类。

通俗解释

就像一只电风扇，有关机状态、低速状态、高速状态。你按下按钮，它就在不同的状态间切换。状态决定了电风扇的行为。

二、示例讲解：

本篇我们以“自动售货机 Vending Machine”为例。它可能处于以下状态：

等待投币
已投币，等待选择
正在出货
售罄

用户的操作行为（如投币、选择商品、取货）会根据当前状态产生不同的响应。

三、UML 图结构（PlantUML）：

@startuml
interface State {+insertCoin()+selectProduct()+dispense()
}class WaitingForCoin implements State
class HasCoin implements State
class Dispensing implements State
class SoldOut implements Stateclass VendingMachine {-state: State+setState(s: State)+insertCoin()+selectProduct()+dispense()
}State <|.. WaitingForCoin
State <|.. HasCoin
State <|.. Dispensing
State <|.. SoldOut
VendingMachine --> State
@enduml

在这里插入图片描述

四、Java 实现代码

状态接口

public interface State {void insertCoin();void selectProduct();void dispense();
}

具体状态实现

等待投币状态

public class WaitingForCoinState implements State {private VendingMachine machine;public WaitingForCoinState(VendingMachine machine) {this.machine = machine;}@Overridepublic void insertCoin() {System.out.println("硬币已投入。");machine.setState(machine.getHasCoinState());}@Overridepublic void selectProduct() {System.out.println("请先投币！");}@Overridepublic void dispense() {System.out.println("不能发货，请先投币并选择商品。");}
}

已投币状态

public class HasCoinState implements State {private VendingMachine machine;public HasCoinState(VendingMachine machine) {this.machine = machine;}@Overridepublic void insertCoin() {System.out.println("已经有硬币了，请先选择商品。");}@Overridepublic void selectProduct() {System.out.println("商品已选择，正在出货...");machine.setState(machine.getDispensingState());}@Overridepublic void dispense() {System.out.println("请先选择商品！");}
}

出货中状态

public class DispensingState implements State {private VendingMachine machine;public DispensingState(VendingMachine machine) {this.machine = machine;}@Overridepublic void insertCoin() {System.out.println("正在出货，请稍候...");}@Overridepublic void selectProduct() {System.out.println("已经选择了，请等待出货。");}@Overridepublic void dispense() {System.out.println("出货完成！");if (machine.getProductCount() > 0) {machine.setState(machine.getWaitingForCoinState());} else {System.out.println("商品售罄！");machine.setState(machine.getSoldOutState());}}
}

售罄状态

public class SoldOutState implements State {private VendingMachine machine;public SoldOutState(VendingMachine machine) {this.machine = machine;}@Overridepublic void insertCoin() {System.out.println("商品已售罄，请勿投币。");}@Overridepublic void selectProduct() {System.out.println("无法选择商品，已售罄。");}@Overridepublic void dispense() {System.out.println("无法出货，商品售罄。");}
}

状态持有者：VendingMachine 类

public class VendingMachine {private State waitingForCoinState;private State hasCoinState;private State dispensingState;private State soldOutState;private State currentState;private int productCount;public VendingMachine(int initialCount) {waitingForCoinState = new WaitingForCoinState(this);hasCoinState = new HasCoinState(this);dispensingState = new DispensingState(this);soldOutState = new SoldOutState(this);this.productCount = initialCount;this.currentState = initialCount > 0 ? waitingForCoinState : soldOutState;}public void insertCoin() {currentState.insertCoin();}public void selectProduct() {currentState.selectProduct();}public void dispense() {currentState.dispense();if (productCount > 0 && currentState == dispensingState) {productCount--;}}// Getters & Setterspublic void setState(State state) {this.currentState = state;}public State getWaitingForCoinState() { return waitingForCoinState; }public State getHasCoinState() { return hasCoinState; }public State getDispensingState() { return dispensingState; }public State getSoldOutState() { return soldOutState; }public int getProductCount() { return productCount; }
}

测试代码

public class Main {public static void main(String[] args) {VendingMachine machine = new VendingMachine(2);machine.insertCoin();machine.selectProduct();machine.dispense();machine.insertCoin();machine.selectProduct();machine.dispense();// 尝试再买一次machine.insertCoin();machine.selectProduct();machine.dispense();}
}