/** 
* Sets the changed flag for this {@code Observable}. After calling 
* {@code setChanged()}, {@code hasChanged()} will return {@code true}. 
*/ 
protected void setChanged() { 
changed = true; 
} 
②notifyObservers()方法 / notifyObservers(Object data)方法
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/** 
* If {@code hasChanged()} returns {@code true}, calls the {@code update()} 
* method for every observer in the list of observers using null as the 
* argument. Afterwards, calls {@code clearChanged()}. 
* <p> 
* Equivalent to calling {@code notifyObservers(null)}. 
*/ 
public void notifyObservers() { 
notifyObservers(null); 
} 
/** 
* If {@code hasChanged()} returns {@code true}, calls the {@code update()} 
* method for every Observer in the list of observers using the specified 
* argument. Afterwards calls {@code clearChanged()}. 
* 
* @param data 
* the argument passed to {@code update()}. 
*/ 
@SuppressWarnings("unchecked") 
public void notifyObservers(Object data) { 
int size = 0; 
Observer[] arrays = null; 
synchronized (this) { 
if (hasChanged()) { 
clearChanged(); 
size = observers.size(); 
arrays = new Observer[size]; 
observers.toArray(arrays); 
} 
} 
if (arrays != null) { 
for (Observer observer : arrays) { 
observer.update(this, data); 
} 
} 
} 

import java.util.Observable; 
/** 
* 被观察者类 
*/ 
public class SimpleObservable extends Observable 
{ 
private int data = 0; 
public int getData(){ 
return data; 
} 
public void setData(int i){ 
if(this.data != i) { 
this.data = i; 
setChanged(); 
//只有在setChange()被调用后，notifyObservers()才会去调用update()，否则什么都不干。 
notifyObservers(); 
} 
} 
} 

/** 
* 观察者类 
*/ 
public class SimpleObserver implements Observer 
{ 
public SimpleObserver(SimpleObservable simpleObservable){ 
simpleObservable.addObserver(this ); 
} 

public void update(Observable observable ,Object data){ // data为任意对象，用于传递参数 
System.out.println(“Data has changed to” + (SimpleObservable)observable.getData()); 
} 
} 

public class SimpleTest 
{ 
public static void main(String[] args){ 
SimpleObservable doc = new SimpleObservable (); 
SimpleObserver view = new SimpleObserver (doc); 
doc.setData(1); 
doc.setData(2); 
doc.setData(2); 
doc.setData(3); 
} 
} 

// 添加一个Observer到列表observers中 
public void addObserver(Observer observer) { 
if (observer == null) { 
throw new NullPointerException(); 
} 
synchronized (this) { 
if (!observers.contains(observer)) 
observers.add(observer); 
} 
} 
// 从列表observers中删除一个observer 
public synchronized void deleteObserver(Observer observer) { 
observers.remove(observer); 
} 
// 清空列表observers 
public synchronized void deleteObservers() { 
observers.clear(); 
} 
// 返回列表observers中observer的个数 
public int countObservers() { 
return observers.size(); 
} 
// 重置数据改变标志位为未改变 
protected void clearChanged() { 
changed = false; 
} 
// 将数据改变标志位设置为改变 
protected void setChanged() { 
changed = true; 
} 
// 判断标志位的值 
public boolean hasChanged() { 
return changed; 
} 
// 通知所有observer（无参） 
public void notifyObservers() { 
notifyObservers(null); 
} 
// 通知所有observer（有参） 
@SuppressWarnings("unchecked") 
public void notifyObservers(Object data) { 
int size = 0; 
Observer[] arrays = null; 
synchronized (this) { 
if (hasChanged()) { 
clearChanged(); 
size = observers.size(); 
arrays = new Observer[size]; 
observers.toArray(arrays); 
} 
} 
if (arrays != null) { 
for (Observer observer : arrays) { 
observer.update(this, data); 
} 
} 
}