当对象之间存在一对多关系时使用Observer模式,例如,如果修改了一个对象,则会自动通知其依赖对象.观察者模式属于行为模式类别.
观察者模式使用三个actor类.主题,观察者和客户. Subject是具有将观察者附加和分离到客户端对象的方法的对象.我们创建了一个抽象类 Observer 和一个扩展class Observer 的具体类 Subject .
ObserverPatternDemo ,我们的演示类,将使用 Subject 和具体的类对象来显示行动中的观察者模式.
创建主题类.
Subject.java
import java.util.ArrayList; import java.util.List; public class Subject { private List<Observer> observers = new ArrayList<Observer>(); private int state; public int getState() { return state; } public void setState(int state) { this.state = state; notifyAllObservers(); } public void attach(Observer observer){ observers.add(observer); } public void notifyAllObservers(){ for (Observer observer : observers) { observer.update(); } } }
创建Observer类.
Observer.java
public abstract class Observer { protected Subject subject; public abstract void update(); }
创建具体的观察者类
BinaryObserver.java
public class BinaryObserver extends Observer{ public BinaryObserver(Subject subject){ this.subject = subject; this.subject.attach(this); } @Override public void update() { System.out.println( "Binary String: " + Integer.toBinaryString( subject.getState() ) ); } }
HexaObserver.java
public class HexaObserver extends Observer{ public HexaObserver(Subject subject){ this.subject = subject; this.subject.attach(this); } @Override public void update() { System.out.println( "Hex String: " + Integer.toHexString( subject.getState() ).toUpperCase() ); } }
使用主题具体的观察者对象.
ObserverPatternDemo.java
public class ObserverPatternDemo { public static void main(String[] args) { Subject subject = new Subject(); new HexaObserver(subject); new OctalObserver(subject); new BinaryObserver(subject); System.out.println("First state change: 15"); subject.setState(15); System.out.println("Second state change: 10"); subject.setState(10); } }
验证输出.
First state change: 15 Hex String: F Octal String: 17 Binary String: 1111 Second state change: 10 Hex String: A Octal String: 12 Binary String: 1010