How to Create and Use Custom Events in Java?

Java Custom Event Creation

When attempting to establish a response mechanism where object interactions involve specific events, the Observer Pattern emerges as a viable solution. Utilizing this pattern, objects can subscribe to events of interest, and upon the occurrence of those events, they will be notified to take appropriate actions.

Java Implementation

Consider the following code snippet as an example:

import java.util.*;

// HelloEvent: Represents an event of "Hello"
class HelloEvent {
    private String message;
    
    public HelloEvent(String message) {
        this.message = message;
    }
    
    public String getMessage() {
        return message;
    }
}

// HelloListener: Interface for objects interested in "Hello" events
interface HelloListener {
    void onHello(HelloEvent event);
}

// Initiater: Publishes "Hello" events
class Initiater {
    private List<hellolistener> listeners = new ArrayList();
    
    public void addListener(HelloListener listener) {
        listeners.add(listener);
    }
    
    public void sayHello(String message) {
        HelloEvent event = new HelloEvent(message);
        
        for (HelloListener listener : listeners) {
            listener.onHello(event);
        }
    }
}

// Responder: Subscribes to "Hello" events
class Responder implements HelloListener {
    @Override
    public void onHello(HelloEvent event) {
        System.out.println("Hello there! " + event.getMessage());
    }
}

public class CustomEventExample {
    public static void main(String[] args) {
        Initiater initiater = new Initiater();
        Responder responder = new Responder();
        
        initiater.addListener(responder);
        
        initiater.sayHello("Good morning!");  // Prints "Hello there! Good morning!"
    }
}</hellolistener>

Explanation

In this example, the HelloEvent class encapsulates the event data (in this case, a greeting message). The HelloListener interface defines the method that subscribers must implement to handle event notifications. Initiater represents the object that triggers the event and maintains a list of subscribers. Responder is the subscriber that responds when a "Hello" event occurs.

The CustomEventExample class demonstrates how to create, publish, and subscribe to custom events. When the Initiater calls the sayHello method, it fires a HelloEvent, which is received and processed by Responder.

This approach provides a flexible way to establish event-driven communication between objects in Java applications.

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