Java.util.Observable.countObservers() Java.util.Observable.clearChanged() Java.util.Observable.deleteObserver() 描述 所述java.util.Observable.countObservers()返回此的观察员的人数可观察对象。 声明 以下是java.util.Observable.countObservers()方法的声明 public int countObservers() 参数 NA 返回值 此方法返回此对象的观察者数。 异常 NA 实例 以下示例显示了java.util.Observable.countObservers()方法的用法。 package com.tutorialspoint; 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); } } 让我们编译并运行上面的程序,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 Java.util.Observable.clearChanged() Java.util.Observable.deleteObserver()