在Java中通過Observable類和Observer接口實現了觀察者模式。Observer對象是觀察者,Observable對象是被觀察者。
1. 實現觀察者模式
實現觀察者模式非常簡單,
[1]創建被觀察者類,它繼承自java.util.Observable類;
[2]創建觀察者類,它實現java.util.Observer接口;
[3]對於被觀察者類,
添加它的觀察者:
void addObserver(Observer o) |
addObserver()方法把觀察者對象添加到觀察者對象列表中。
當被觀察事件發生時,執行:
setChanged(); notifyObservers(); |
setChange()方法用來設置一個內部標誌位註明數據發生了變化;notifyObservers()方法會去調用觀察者對象列表中所有的Observer的update()方法,通知它們數據發生了變化。
只有在setChange()被調用後,notifyObservers()纔會去調用update()。
[4]對於觀察者類,實現Observer接口的唯一方法update
void update(Observable o, Object arg) |
形參Object arg,對應一個由notifyObservers(Object arg);傳遞來的參數,當執行的是notifyObservers();時,arg爲null。
2.實例一
NumObserable是一個被觀察者,當它的成員變量data的數值發生變化時,會通知所有的觀察者。
NumObserable.java
package com.zj.observer; import java.util.Observable;
public class NumObservable extends Observable { private int data = 0;
public int getData() { return data; }
public void setData(int i) { data = i; setChanged(); notifyObservers(); } } |
NumObserver是觀察者。當它的被觀察者(NumObserable)執行了notifyObservers()後,它會執行uodate()方法。
NumObserver.java
package com.zj.observer; import java.util.Observable; import java.util.Observer;
public class NumObserver implements Observer{ public void update(Observable o, Object arg) { NumObservable myObserable=(NumObservable) o; System.out.println("Data has changed to "+myObserable.getData()); } } |
測試類SingleTest,在這裏將觀察者加入到被觀察者的觀察列表中。
SingleTest.java
package com.zj.observer;
public class SingleTest { public static void main(String[] args) { NumObservable number = new NumObservable(); number.addObserver(new NumObserver()); number.setData(1); number.setData(2); number.setData(3); } } |
結果:
Data has changed to 1
Data has changed to 2
Data has changed to 3
3.實例二
這個實例中,還是對data進行觀察,擁有兩個觀察者,分別觀察奇數和偶數的變化,通過notifyObservers(arg)中的參數arg來識別通知信息。
被觀察者NumsObservable.java
package com.zj.observers; import java.util.Observable;
public class NumsObservable extends Observable { public final static Integer ODD = 1; public final static Integer EVEN = 2; private int data = 0;
public int getData() { return data; }
public void setData(int i) { data = i; Integer flag = EVEN; if ((data & 0x0001) == 1) flag = ODD; setChanged(); notifyObservers(flag); } } |
奇數觀察者OddObserver.java
package com.zj.observers; import java.util.Observable; import java.util.Observer;
public class OddObserver implements Observer { public void update(Observable o, Object arg) { if (arg == NumsObservable.ODD) { NumsObservable myObserable = (NumsObservable) o; System.out.println("OddObserver:Data has changed to " + myObserable.getData()); } } } |
偶數觀察者EvenObserver.java
package com.zj.observers; import java.util.Observable; import java.util.Observer;
public class EvenObserver implements Observer { public void update(Observable o, Object arg) { if (arg == NumsObservable.EVEN) { NumsObservable myObserable = (NumsObservable) o; System.out.println("EvenObserver:Data has changed to " + myObserable.getData()); } } } |
測試類MultiTest.java
package com.zj.observers;
public class MultiTest { public static void main(String[] args) { NumsObservable number = new NumsObservable(); number.addObserver(new OddObserver()); number.addObserver(new EvenObserver()); number.setData(1); number.setData(2); number.setData(3); } } |
結果:
OddObserver:Data has changed to 1
EvenObserver:Data has changed to 2
OddObserver:Data has changed to 3