細(xì)心!用心!耐心!

如果您不瞭解TCP的連線方式，在看 Gof 的書(shū)介紹State模式時(shí)，大概會(huì)看得一頭霧水吧！TCP的連線狀態(tài)圖，光是要瞭解就要花點(diǎn)精神了，它的連線狀態(tài)很多，用來(lái)說(shuō)明狀態(tài)模式確實(shí)很適合，但不適合教導(dǎo)初學(xué)模式的人。

由簡(jiǎn)單的開(kāi)始會(huì)比較好理解狀態(tài)模式的作用，先來(lái)看一個(gè)例子，如果您有一個(gè)只能順時(shí)針轉(zhuǎn)動(dòng)的瓦斯開(kāi)關(guān)，轉(zhuǎn)動(dòng)一次的狀態(tài)為off、 small fire、medium fire與large fire，您如何在程式中控制狀態(tài)的變化與行為呢？一個(gè)最簡(jiǎn)單的方式就是用if..else或是switch流程來(lái)控制，例如：

• State.java

public class State { 
    private int state;

    public State() { 
        state = 0; 
    } 

    public void switchFire() { 
        if (state == 0) { 
            state = 1; 
            System.out.println( "small fire" ); 
        } else if (state == 1) { 
            state = 2; 
            System.out.println( "medium fire" ); 
        } else if (state == 2) { 
            state = 3; 
            System.out.println( "large fire" ); 
        } else { 
            state = 0; 
            System.out.println( "turning off" ); 
        } 
    } 
}  

• Main.java

public class Main { 
     public static void main(String[] args) { 
        State state = new State();

        state.switchFire(); 
        state.switchFire(); 
        state.switchFire(); 
        state.switchFire(); 
    } 
} 

這個(gè)方法很簡(jiǎn)單，每個(gè)人都會(huì)，但如果您的狀態(tài)變化並不是流水式的變化，而是像TCP連線狀態(tài)一樣，會(huì)是一個(gè)網(wǎng)絡(luò)圖的時(shí)候，用 if...else或switch來(lái)寫(xiě)的話，您的程式就會(huì)亂的不像話了；來(lái)考慮如何讓物件控制自己的狀態(tài)轉(zhuǎn)換與所應(yīng)表現(xiàn)的行為，這個(gè)程式可以這樣改寫(xiě)：

• IState.java

public interface IState { 
    public void switchFire(FireSwitch sw); 
} 

• OffState

public class OffState implements IState { 
    public void switchFire(FireSwitch sw) { 
        sw.setState(new SmallState()); 
        System.out.println( "small fire" ); 
    } 
} 

• SmallState.java

public class SmallState implements IState { 
    public void switchFire(FireSwitch sw) { 
        sw.setState(new MediumState()); 
        System.out.println( "medium fire" ); 
    } 
}  

• MediumState.java

public class MediumState implements IState { 
    public void switchFire(FireSwitch sw) { 
        sw.setState(new LargeState()); 
        System.out.println( "large fire" ); 
    } 
}  

• LargeState.java

public class LargeState implements IState { 
    public void switchFire(FireSwitch sw) { 
        sw.setState(new OffState()); 
        System.out.println( "off fire" ); 
    } 
}  

• FireSwitch.java

public class FireSwitch { 
    private State current;

    public FireSwitch() { 
        current = new OffState(); 
    }

    public void setState(State s) { 
        current = s; 
    }

    public void switchFire() { 
        current.switchFire(this); 
    }
} 

• Main.java

public class Main { 
    public static void main(String[] args) { 
        FireSwitch fireSwitch = new FireSwitch();
        fireSwitch.switchFire(); 
        fireSwitch.switchFire(); 
        fireSwitch.switchFire(); 
        fireSwitch.switchFire(); 
    } 
} 

程式執(zhí)行結(jié)果與上一個(gè)例子是一樣的，但這次並沒(méi)有用流程控制來(lái)進(jìn)行狀態(tài)轉(zhuǎn)換，而由物件自行控制自己的狀態(tài)，與必須表現(xiàn)的行為，這個(gè)方式就是State 模式，將這個(gè)例子的 UML 類別結(jié)構(gòu)畫(huà)出就如下所示：

再進(jìn)一步考慮開(kāi)關(guān)可以順時(shí)針與逆時(shí)針轉(zhuǎn)動(dòng)，這時(shí)如果您仍以if...else或switch來(lái)寫(xiě)，就會(huì)讓流程顯示複雜，來(lái)看看如何使用狀態(tài)模式來(lái)撰寫(xiě)：

• IState.java

public interface IState { 
    public void switchClockWise(FireSwitch sw); 
    public void switchCountClock(FireSwitch sw); 
}  

• OffState.java

public class OffState implements IState { 
    public void switchClockWise(FireSwitch sw) { 
        sw.setState(new SmallState()); 
        System.out.println("small fire"); 
    }

    public void switchCountClock(FireSwitch sw) { 
        sw.setState(new LargeState()); 
        System.out.println("large fire"); 
    } 
}  

• SmallState.java

public class SmallState implements IState { 
    public void switchClockWise(FireSwitch sw) { 
        sw.setState(new MediumState()); 
        System.out.println("medium fire"); 
    } 

    public void switchCountClock(FireSwitch sw) { 
        sw.setState(new OffState()); 
        System.out.println("off fire"); 
    } 
}  

• MediumState.java

public class MediumState implements IState { 
    public void switchClockWise(FireSwitch sw) { 
        sw.setState(new LargeState()); 
        System.out.println("large fire"); 
    }
 
    public void switchCountClock(FireSwitch sw) { 
        sw.setState(new SmallState()); 
        System.out.println("small fire"); 
    } 
}  

• LargeState.java

public class LargeState implements State { 
    public void switchClockWise(FireSwitch sw) { 
        sw.setState(new OffState()); 
        System.out.println("off fire"); 
    }

    public void switchCountClock(FireSwitch sw) { 
        sw.setState(new MediumState()); 
        System.out.println("medium fire"); 
    } 
}  

• FireSwitch.java

public class FireSwitch { 
    private State current;

    public FireSwitch() { 
        current = new OffState(); 
    }

    public void setState(State s) { 
        current = s; 
    }

    public void switchClockWise() { 
        current.switchClockWise(this); 
    }

    public void switchCountClock() { 
       current.switchCountClock(this); 
    } 
}  

• Main.java

public class Main { 
    public static void main(String[] args) { 
        FireSwitch fireSwitch = new FireSwitch();

        fireSwitch.switchClockWise(); 
        fireSwitch.switchClockWise(); 
        fireSwitch.switchClockWise(); 
        fireSwitch.switchClockWise(); 

        System.out.println();

        fireSwitch.switchCountClock(); 
        fireSwitch.switchCountClock(); 
        fireSwitch.switchCountClock(); 
        fireSwitch.switchCountClock(); 
    } 
} 

接下來(lái)您可以任意的轉(zhuǎn)動(dòng)開(kāi)關(guān)了，無(wú)論是順時(shí)針轉(zhuǎn)動(dòng)或是逆時(shí)針轉(zhuǎn)動(dòng)，狀態(tài)的轉(zhuǎn)換都由物件自己來(lái)表現(xiàn)，這是雙向狀態(tài)轉(zhuǎn)換下的例子，如果一個(gè)狀態(tài)可能轉(zhuǎn)換至三個(gè)以上的狀態(tài)，使用State模式就更可以看出它的好處了，就像Gof的TCP連線例子一樣，如果您瞭解TCP連線，可以看看原書(shū)是如何實(shí)現(xiàn)TCP連線之間的狀態(tài)轉(zhuǎn)換的。

State模式的UML結(jié)構(gòu)圖如下：