Java implementation callback code example

JAVA Implementation Callback

Developers who are familiar with MS-Windows and X Windows event-driven design patterns usually pass a pointer to a method to the event source and call it when an event occurs. This method (also called a "callback"). Java's object-oriented model currently does not support method pointers and does not seem to be able to use this convenient mechanism.

Java supports interface, and the same callback can be achieved through interface. The trick is to define a simple interface and declare a method that you want to call back.

For example, assuming that we will be notified when a certain event occurs, we can define an interface:

public interface InterestingEvent {
 // 这只是一个普通的方法，可以接收参数、也可以返回值
 public void interestingEvent();
}

In this way, we will have any class that implements this interface The object's handle grip.

When an event occurs, the object that implements the InterestingEvent interface needs to be notified and the interestingEvent() method needs to be called.

class EventNotifier {
 private InterestingEvent ie;
 private boolean somethingHappened;
 
 public EventNotifier(InterestingEvent event) {
  ie = event;
  somethingHappened = false;
  }
public void doWork() {<br>        if (somethingHappened) {<br>            // 事件发生时，通过调用接口的这个方法来通知<br>            ie.interestingEvent();<br>        }        <br>    }<br>}

In this example, somethingHappened is used to mark whether the event occurs.

The class that wishes to receive event notification must implement the InterestingEvent interface and pass its own reference to the event notifier.

public class CallMe implements InterestingEvent {
 private EventNotifier en;
 
 public CallMe() {
  // 新建一个事件通知者对象，并把自己传递给它
  en = new EventNotifier(this);
 }
 
 // 实现事件发生时，实际处理事件的方法
 public void interestingEvent() {
  // 这个事件发生了，进行处理
 }
}

The above is a very simple example to illustrate the implementation of callbacks in Java.

Of course, you can also register multiple objects interested in this event through registration in the event management or event notifier class.

1. Define an interface InterestingEvent, and the callback method ninterestingEvent(String event) simply receives a String parameter.

interface InterestingEvent {
 public void interestingEvent(String event);
}

2. Implement the InterestingEvent interface, event processing class

class CallMe implements InterestingEvent {
 private String name;
 public CallMe(String name){
  this.name = name;
 }
 public void interestingEvent(String event) {
  System.out.println(name + ":[" +event + "] happened");
 }
}

3. Event manager, or event notification Author

class EventNotifier {
 private List<CallMe> callMes = new ArrayList<CallMe>();
  
 public void regist(CallMe callMe){
  callMes.add(callMe);
 }
  
 public void doWork(){
  for(CallMe callMe: callMes) {
   callMe.interestingEvent("sample event");
  }
 }
}

4. Test

public class CallMeTest {
 public static void main(String[] args) {
  EventNotifier ren = new EventNotifier();
  CallMe a = new CallMe("CallMe A");
  CallMe b = new CallMe("CallMe B");
 
  // regiest
  ren.regist(a);
  ren.regist(b);
   
  // test
  ren.doWork(); 
 }
}

The above is an introduction to the Java callback mechanism , students in need can refer to it.

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