public class User implements Observer {public String name;public User(String name) {this.name = name;}@Overridepublic void update(Observable o, Object arg) {System.out.println("Hi "+name +",公众号更新了内容："+arg);}
}

public class Gamedaily extends Observable {public void postNewArticle(String content){//内容发生改变
        setChanged();//通知所有订阅者改变的内容
        notifyObservers(content);}
}

public class Client {public static void main(String[] args) {Gamedaily gamedaily = new Gamedaily();User user1 = new User("user1");User user2 = new User("user2");User user3 = new User("user3");//将观察者注册到可观察者的通知列表中。
        gamedaily.addObserver(user1);gamedaily.addObserver(user2);gamedaily.addObserver(user3);gamedaily.postNewArticle("新文章来了");}
}

package android.widget;
public abstract class BaseAdapter implements ListAdapter, SpinnerAdapter {private final DataSetObservable mDataSetObservable = new DataSetObservable();public void registerDataSetObserver(DataSetObserver observer) {mDataSetObservable.registerObserver(observer);}public void unregisterDataSetObserver(DataSetObserver observer) {mDataSetObservable.unregisterObserver(observer);}......public void notifyDataSetChanged() {mDataSetObservable.notifyChanged();}......
}

package android.database;
public class DataSetObservable extends Observable<DataSetObserver> {   public void notifyChanged() {synchronized(mObservers) {for (int i = mObservers.size() - 1; i >= 0; i--) {mObservers.get(i).onChanged();}}}......
}

public class ListView extends AbsListView {public void setAdapter(ListAdapter adapter) {//如果已经有了一个adapter，注销这个adapter之前的观察者，if (mAdapter != null && mDataSetObserver != null) {mAdapter.unregisterDataSetObserver(mDataSetObserver);}......if (mHeaderViewInfos.size() > 0|| mFooterViewInfos.size() > 0) {mAdapter = wrapHeaderListAdapterInternal(mHeaderViewInfos, mFooterViewInfos, adapter);} else {//将新的adapter赋给mAdaptermAdapter = adapter;}......super.setAdapter(adapter);if (mAdapter != null) {mAreAllItemsSelectable = mAdapter.areAllItemsEnabled();//保存之前的数据个数mOldItemCount = mItemCount;//获取新的个数mItemCount = mAdapter.getCount();checkFocus();//创建数据集观察者mDataSetObserver = new AdapterDataSetObserver();//注册观察者
            mAdapter.registerDataSetObserver(mDataSetObserver);...}} else {...}requestLayout();}
}

package android.widget;
public abstract class AbsListView extends AdapterView<ListAdapter> implements TextWatcher,ViewTreeObserver.OnGlobalLayoutListener, Filter.FilterListener,ViewTreeObserver.OnTouchModeChangeListener,RemoteViewsAdapter.RemoteAdapterConnectionCallback {class AdapterDataSetObserver extends AdapterView<ListAdapter>.AdapterDataSetObserver {@Overridepublic void onChanged() {super.onChanged();if (mFastScroll != null) {mFastScroll.onSectionsChanged();}}······}
}

package android.widget;
public abstract class AdapterView<T extends Adapter> extends ViewGroup {class AdapterDataSetObserver extends DataSetObserver {private Parcelable mInstanceState = null;@Overridepublic void onChanged() {mDataChanged = true;mOldItemCount = mItemCount;mItemCount = getAdapter().getCount();// Detect the case where a cursor that was previously invalidated has// been repopulated with new data.if (AdapterView.this.getAdapter().hasStableIds() && mInstanceState != null&& mOldItemCount == 0 && mItemCount > 0) {AdapterView.this.onRestoreInstanceState(mInstanceState);mInstanceState = null;} else {rememberSyncState();}checkFocus();//重新布局
            requestLayout();}......}
}