Java 状态模式详解

状态模式详解

一、状态模式概述

状态模式(State Pattern)是一种行为型设计模式，它允许一个对象在其内部状态改变时改变它的行为，使对象看起来似乎修改了它的类。

核心特点

状态封装：将每个状态的行为封装到独立的类中
状态转换：对象在不同状态间透明切换
消除条件判断：用多态代替状态条件判断
开闭原则：新增状态无需修改现有代码

二、状态模式的结构

主要角色

Context：上下文，维护当前状态
State：抽象状态，定义状态接口
ConcreteState：具体状态，实现特定状态行为

三、状态模式的实现

1. 基本实现

// 状态接口
public interface State {void handle(Context context);
}// 具体状态A
public class ConcreteStateA implements State {public void handle(Context context) {System.out.println("处理状态A的行为");context.setState(new ConcreteStateB());}
}// 具体状态B
public class ConcreteStateB implements State {public void handle(Context context) {System.out.println("处理状态B的行为");context.setState(new ConcreteStateA());}
}// 上下文类
public class Context {private State state;public Context(State state) {this.state = state;}public void setState(State state) {this.state = state;}public void request() {state.handle(this);}
}// 使用示例
Context context = new Context(new ConcreteStateA());
context.request(); // 执行状态A的行为，并切换到状态B
context.request(); // 执行状态B的行为，并切换回状态A

2. 更复杂的实现（订单状态）

// 订单状态接口
public interface OrderState {void next(Order order);void prev(Order order);void printStatus();
}// 新建状态
public class NewState implements OrderState {public void next(Order order) {order.setState(new PaidState());}public void prev(Order order) {System.out.println("订单已在初始状态");}public void printStatus() {System.out.println("订单状态：新建");}
}// 已支付状态
public class PaidState implements OrderState {public void next(Order order) {order.setState(new ShippedState());}public void prev(Order order) {order.setState(new NewState());}public void printStatus() {System.out.println("订单状态：已支付");}
}// 订单类（上下文）
public class Order {private OrderState state;public Order() {this.state = new NewState();}public void setState(OrderState state) {this.state = state;}public void nextState() {state.next(this);}public void previousState() {state.prev(this);}public void printStatus() {state.printStatus();}
}

四、状态模式的应用场景

1. 电梯状态控制

// 电梯状态接口
public interface ElevatorState {void openDoor();void closeDoor();void run();void stop();
}// 运行状态
public class RunningState implements ElevatorState {public void openDoor() {System.out.println("电梯运行中不能开门");}public void closeDoor() {System.out.println("电梯门已是关闭状态");}public void run() {System.out.println("电梯已在运行");}public void stop() {System.out.println("电梯停止运行");}
}// 停止状态
public class StoppedState implements ElevatorState {public void openDoor() {System.out.println("电梯门打开");}public void closeDoor() {System.out.println("电梯门关闭");}public void run() {System.out.println("电梯开始运行");}public void stop() {System.out.println("电梯已是停止状态");}
}

2. 游戏角色状态

// 游戏角色状态
public interface CharacterState {void attack();void defend();void move();
}// 正常状态
public class NormalState implements CharacterState {public void attack() {System.out.println("造成100%伤害");}public void defend() {System.out.println("受到100%伤害");}public void move() {System.out.println("移动速度100%");}
}// 中毒状态
public class PoisonedState implements CharacterState {public void attack() {System.out.println("造成80%伤害");}public void defend() {System.out.println("受到120%伤害");}public void move() {System.out.println("移动速度60%");}
}

3. TCP连接状态

// TCP状态接口
public interface TCPState {void open();void close();void acknowledge();
}// 已建立连接状态
public class EstablishedState implements TCPState {public void open() {System.out.println("连接已建立，无需再次打开");}public void close() {System.out.println("关闭TCP连接");}public void acknowledge() {System.out.println("发送ACK响应");}
}// 监听状态
public class ListenState implements TCPState {public void open() {System.out.println("建立TCP连接");}public void close() {System.out.println("未建立连接，无需关闭");}public void acknowledge() {System.out.println("未建立连接，不能发送ACK");}
}

五、状态模式的变体

1. 状态表驱动

public class StateMachine {private Map<State, Map<Event, State>> transitions = new HashMap<>();private State currentState;public void addTransition(State source, Event event, State target) {transitions.computeIfAbsent(source, k -> new HashMap<>()).put(event, target);}public void handleEvent(Event event) {currentState = transitions.get(currentState).get(event);}
}

2. 状态模式与策略模式结合

public class Context {private StateStrategy state;public void setState(StateStrategy state) {this.state = state;}public void execute() {state.execute();state = state.nextState();}
}public interface StateStrategy {void execute();StateStrategy nextState();
}