java.util.Observable.countObservers()%uA0返回此Observable对象的observers数量。
声明
以下是java.util.Observable.countObservers()方法的声明
public int countObservers()
参数
NA返回值
这个方法返回当前对象的observers数量。
异常
NA例子
下面的例子显示java.util.Observable.countObservers()方法的用法。
package com.yiibai import java.util.Observable import java.util.Observer class ObservedObject extends Observable { private String watchedValue public ObservedObject(String value) { watchedValue = value } public void setValue(String value) { // if value has changed notify observers if(!watchedValue.equals(value)) { System.out.println("Value changed to new value: "+value) watchedValue = value // mark as value changed setChanged() // trigger notification notifyObservers(value) } } } public class ObservableDemo implements Observer { public String name public ObservableDemo(String name) { this.name = name } public static void main(String[] args) { // create watched and watcher objects ObservedObject watched = new ObservedObject("Original Value") // watcher object listens to object change ObservableDemo watcher1 = new ObservableDemo("Watcher1") ObservableDemo watcher2 = new ObservableDemo("Watcher2") // add observer to the watched object watched.addObserver(watcher1) watched.addObserver(watcher2) // count observers int count = watched.countObservers() System.out.println(count+" observers listening") // trigger value change watched.setValue("New Value") // remove first observer watched.deleteObserver(watcher1) count = watched.countObservers() System.out.println(count+" observers listening") // trigger value change watched.setValue("Latest Value") } public void update(Observable obj, Object arg) { System.out.println(name+" called with Arguments: "+arg) } }
让我们来编译和运行上面的程序，这将产生以下结果：
2 observers listening Value changed to new value: New Value Watcher2 called with Arguments: New Value Watcher1 called with Arguments: New Value 1 observers listening Value changed to new value: Latest Value Watcher2 called with Arguments: Latest Value