观察者模式应用广泛，尤其是在一些网络社交应用的推送功能中的应用更是十分广泛；因为这种模式对于此类应用具有先天优势，它能很方便的进行广播；
使用场景主要有：1.一种对象的操作依赖于另一方面的变化
2.更改一个对象是连带需要更改其一些他对象
一句话来说就是广播模型。
java本身内置了观察者模式的接口（java.util下），但在更为复杂的环境中，我们往往要自定义接口，以满足多样的需求。
同时，一个观察者，有时需要观察多个对象，当然，这个是要根据问题具体情况;
以下的demo是个一个观察者观察两个对象--天气，商品，同时通过标记来区别对待观察者
public interface WeatherSubject
{
public static final int QING = 4;
public static final int YU = 1;
public static final int QING2YU = 2;
public static final int FENG = 3;
public static final int ERROR = 0;
public void attach(WeatherObserver observer);
public void detach(WeatherObserver observer);
public void notifyWeatherObserver();
}
public interface WeatherObserver
{
public void updateWeather(WeatherSubject subject);
public boolean isInWeatherFlags(int f);
}
public interface ShopSubject
{
public static final int ERROR=0;
public static final int FOOD=1;
public static final int TOOL=2;
public static final int BED=3;
public void attach(ShopObserver observer);
public void detach(ShopObserver observer);
public void notifyShopObserver();
}
public interface ShopObserver
{
public void updateShop(ShopSubject subject);
public boolean isInShopFlags(int f);
}
public class ChinaWeatherSubject implements WeatherSubject
{
List<WeatherObserver> observers = new ArrayList<WeatherObserver>();
String weatherContent;
int flag = ERROR;
private String calculateWeather()
{
Random random = new Random();
switch(random.nextInt(5))
{
case 1:
flag = QING;
return "今天，天气晴好";
case 2:
flag = YU;
return "今天，狂风暴雨";
case 3:
flag = QING2YU;
return "雨夹雪转晴";
case 4:
flag = FENG;
return "今天下午，6级大风";
}
flag = ERROR;
return "获取天气失败";
}
public String getWeatherContent() {
return weatherContent;
}
public void setWeatherContent() {
this.weatherContent = calculateWeather();
notifyWeatherObserver();
}
public void attach(WeatherObserver observer) {
observers.add(observer);
}
public void detach(WeatherObserver observer) {
observers.remove(observer);
}
public void notifyWeatherObserver() {
for(WeatherObserver o : observers)
{
if(o.isInWeatherFlags(flag)) o.updateWeather(this);
}
}
}
public class TianMaoShopSubject implements ShopSubject
{
List<ShopObserver> observers = new ArrayList<ShopObserver>();
String shopContent;
int flag = ERROR;
private String calculateShop()
{
Random random = new Random();
switch(random.nextInt(4))
{
case 1:
flag = BED;
return "床上用品";
case 2:
flag = FOOD;
return "食物";
case 3:
flag = TOOL;
return "日用工具";
}
flag = ERROR;
return "获取商品失败";
}
public void attach(ShopObserver observer) {
observers.add(observer);
}
public void detach(ShopObserver observer) {
observers.remove(observer);
}
public void notifyShopObserver() {
for(ShopObserver o : observers)
{
if(o.isInShopFlags(flag)) o.updateShop(this);
}
}
public String getShopContent() {
return shopContent;
}
public void setShopContent() {
this.shopContent = calculateShop();
notifyShopObserver();
}
}
public class AppObserver implements WeatherObserver,ShopObserver
{
String username;
List<Integer> weatherFlags = new ArrayList<Integer>();
List<Integer> shopFlags = new ArrayList<Integer>();
public AppObserver(String uname,int[] wFlag,int[] sFlag) {
username = uname;
setWeatherFlags(wFlag);
setShopFlags(sFlag);
}
public void setWeatherFlags(int[] wFlag)
{
for(int i=0;i<wFlag.length;i++)
{
weatherFlags.add(wFlag[i]);
}
}
public void setShopFlags(int[] sFlag)
{
for(int i=0;i<sFlag.length;i++)
{
shopFlags.add(sFlag[i]);
}
}
public boolean isInWeatherFlags(int f) {
return weatherFlags.contains(f);
}
public boolean isInShopFlags(int f) {
return shopFlags.contains(f);
}
public void updateWeather(WeatherSubject subject) {
System.out.println(username+"收到天气消息 ; 天气情况是:"+((ChinaWeatherSubject)subject).getWeatherContent());
}
public void updateShop(ShopSubject subject) {
System.out.println(username+"收到商店消息 ; 货物情况是:"+((TianMaoShopSubject)subject).getShopContent());
}
}
public class ObserverTest {
public static void main(String[] args) {
ChinaWeatherSubject subject = new ChinaWeatherSubject();
TianMaoShopSubject subject0 = new TianMaoShopSubject();
AppObserver o1 = new AppObserver("凌风",new int[]{
WeatherSubject.FENG,
},new int[]{
ShopSubject.BED,
});
AppObserver o2 = new AppObserver("雪情",new int[]{
WeatherSubject.QING2YU,
},new int[]{
ShopSubject.FOOD,
});
AppObserver o3 = new AppObserver("莎莎",new int[]{
WeatherSubject.YU,
WeatherSubject.QING2YU,
WeatherSubject.QING,
WeatherSubject.FENG,
},new int[]{
ShopSubject.BED,
ShopSubject.FOOD,
ShopSubject.TOOL,
});
subject.attach(o1);
subject.attach(o2);
subject.attach(o3);
subject0.attach(o1);
subject0.attach(o2);
subject0.attach(o3);
subject.setWeatherContent();
subject0.setShopContent();
}
}运行效果：