Java Observable countObservers() 方法

描述

Java Observable countObservers() 方法返回此Observable物件的觀察者數量。

宣告

以下是Java.util.Observable.countObservers()方法的宣告

public int countObservers()

引數

無

返回值

此方法返回此物件的觀察者數量。

異常

無

字串值更改示例的觀察者計數

以下示例顯示了 java.util.Observable.countObservers() 方法的用法。我們透過擴充套件 Observable 類建立了一個 ObservedObject 類，然後覆蓋了其 setValue() 方法。在主類中，我們使用 addObserver() 方法添加了觀察者。然後使用 countObservers() 方法，我們計算新增的觀察者，並在輸出中列印 ObservedObject 值發生的更新。

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);
   }
}

輸出

讓我們編譯並執行上述程式，這將產生以下結果：

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

整數值更改示例的觀察者計數

以下示例顯示了 java.util.Observable.countObservers() 方法的用法。我們透過擴充套件 Observable 類建立了一個 ObservedObject 類，然後覆蓋了其 setValue() 方法。在主類中，我們使用 addObserver() 方法添加了觀察者。然後使用 countObservers() 方法，我們計算新增的觀察者，並在輸出中列印 ObservedObject 值發生的更新。

package com.tutorialspoint;

import java.util.Observable;
import java.util.Observer;

class ObservedObject extends Observable {
   private Integer watchedValue;
   
   public ObservedObject(Integer value) {
      watchedValue = value;
   }
   
   public void setValue(Integer 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(1);
      
      // 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(2);
      
      // remove first observer
      watched.deleteObserver(watcher1);
      count = watched.countObservers();
      System.out.println(count+" observers listening");
      
      // trigger value change
      watched.setValue(3);
   }
   
   public void update(Observable obj, Object arg) {
      System.out.println(name+" called with Arguments: "+arg);
   }
}

輸出

讓我們編譯並執行上述程式，這將產生以下結果：

2 observers listening
Value changed to new value: 2
Watcher2 called with Arguments: 2
Watcher1 called with Arguments: 2
1 observers listening
Value changed to new value: 3
Watcher2 called with Arguments: 3

物件值更改示例的觀察者計數

以下示例顯示了 java.util.Observable.countObservers() 方法的用法。我們透過擴充套件 Observable 類建立了一個 ObservedObject 類，然後覆蓋了其 setValue() 方法。在主類中，我們使用 addObserver() 方法添加了觀察者。然後使用 countObservers() 方法，我們計算新增的觀察者，並在輸出中列印 ObservedObject 值發生的更新。

package com.tutorialspoint;

import java.util.Observable;
import java.util.Observer;

class ObservedObject extends Observable {
   private Student watchedValue;
   
   public ObservedObject(Student value) {
      watchedValue = value;
   }
   
   public void setValue(Student 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(new Student(1, "Julie"));
      
      // 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 Student(2, "Robert"));
      
      // remove first observer
      watched.deleteObserver(watcher1);
      count = watched.countObservers();
      System.out.println(count+" observers listening");
      
      // trigger value change
      watched.setValue(new Student(3, "Adam"));
   }
   
   public void update(Observable obj, Object arg) {
      System.out.println(name+" called with Arguments: "+arg);
   }
}
class Student {
   int rollNo;
   String name;

   Student(int rollNo, String name){
      this.rollNo = rollNo;
      this.name = name;
   }

   @Override
   public String toString() {
      return "[ " + this.rollNo + ", " + this.name + " ]";
   }
}

輸出

讓我們編譯並執行上述程式，這將產生以下結果：

2 observers listening
Value changed to new value: [ 2, Robert ]
Watcher2 called with Arguments: [ 2, Robert ]
Watcher1 called with Arguments: [ 2, Robert ]
1 observers listening
Value changed to new value: [ 3, Adam ]
Watcher2 called with Arguments: [ 3, Adam ]