State · 状态 — Design Patterns

UML 类图

classDiagram class state { <<interface>> +clicklock(player player) +clickplay(player player) +clicknext(player player) } class lockedstate { <<stateless>> +clicklock(player player) +clickplay(player player) +clicknext(player player) } class readystate { <<stateless>> +clicklock(player player) +clickplay(player player) +clicknext(player player) } class playingstate { <<stateless>> +clicklock(player player) +clickplay(player player) +clicknext(player player) } class player { -state state +changestate(state state) +clicklock() +clickplay() +clicknext() } state <|.. lockedstate state <|.. readystate state <|.. playingstate player *-- state state --> player

player 将行为委托给当前 state，状态对象负责行为和转换规则

案例演示

点击 Run 运行状态机...

          
          
          
          
          
          
        
/** 抽象状态接口：所有具体状态实现此接口 */
public interface State {
    void clickLock(Player player);
    void clickPlay(Player player);
    void clickNext(Player player);
}
/** 锁定状态：只能响应 clickLock 切到 ReadyState */
public class LockedState implements State {

    public void clickLock(Player player) {
        player.changeState(new ReadyState());
        System.out.println("Locked → Ready");
    }

    public void clickPlay(Player player) {
        System.out.println("Locked: do nothing");
    }

    public void clickNext(Player player) {
        System.out.println("Locked: do nothing");
    }
}
/** 就绪状态：clickPlay → PlayingState，clickLock → LockedState */
public class ReadyState implements State {

    public void clickLock(Player player) {
        player.changeState(new LockedState());
        System.out.println("Ready → Locked");
    }

    public void clickPlay(Player player) {
        player.changeState(new PlayingState());
        System.out.println("Ready → Playing");
    }

    public void clickNext(Player player) {
        System.out.println("Ready: do nothing");
    }
}
/** 播放状态：clickPlay/clickNext → 就绪，clickLock → 锁定 */
public class PlayingState implements State {

    public void clickLock(Player player) {
        player.changeState(new LockedState());
        System.out.println("Playing → Locked");
    }

    public void clickPlay(Player player) {
        player.changeState(new ReadyState());
        System.out.println("Playing → Ready (paused)");
    }

    public void clickNext(Player player) {
        System.out.println("Playing: next track");
    }
}
/** 播放器：持有当前状态，将操作委托给当前状态对象 */
public class Player {
    private State state;

    public Player() {
        state = new LockedState();
    }

    public void changeState(State s) { state = s; }

    public void clickLock()  { state.clickLock(this); }
    public void clickPlay() { state.clickPlay(this); }
    public void clickNext() { state.clickNext(this); }
}
public class Demo {
    public static void main(String[] args) {
        Player player = new Player();
        player.clickLock();   // Locked → Ready
        player.clickPlay();  // Ready → Playing
        player.clickNext(); // Playing: next track
        player.clickPlay();  // Playing → Ready
    }
}

核心要点

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状态决定行为，消除条件分支

Player.clickPlay() 本身不包含任何 if/else；行为完全由当前 State 对象决定，每个状态类只实现自己该有的行为。

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状态转换逻辑在状态类内部

ReadyState.clickPlay() 决定切换到 PlayingState；LockedState.clickLock() 决定切换到 ReadyState；状态转换逻辑集中在状态类而非 Player。

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状态模式 vs. 策略模式

状态模式：状态对象决定行为，且状态间相互感知（LockedState 知道如何切 ReadyState）；策略模式：客户选择算法，策略之间无关联。

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典型应用场景

媒体播放器（Locked/Ready/Playing）、订单系统（待支付/已支付/已取消）、TCP 连接（LISTEN/SYN_SENT/ESTABLISHED）、导航状态机、游戏角色状态。