How to create custom Listeners in java?

Question

I want to know about how to set our own Listeners in java.For example I have a function that increments number from 1 to 100. i want to set a listener when the value reaches 50. How can i do that? Pls suggest me any tutorial.

• Lucio: Related: Create a custom event in Java.
• WolfArya: there is a good article for you to referrnce: stackoverflow.com/questions/6270132/…

Answer 1

https://stackoverflow.com/a/6270150/3675925

You probably want to look into the observer pattern.

Here's some sample code to get you started:

import java.util.*;
// An interface to be implemented by everyone interested in "Hello" events
interface HelloListener {
 void someoneSaidHello();
}
// Someone who says "Hello"
class Initiater {
 private List<HelloListener> listeners = new ArrayList<HelloListener>();
 public void addListener(HelloListener toAdd) {
 listeners.add(toAdd);
 }
 public void sayHello() {
 System.out.println("Hello!!");
 // Notify everybody that may be interested.
 for (HelloListener hl : listeners)
 hl.someoneSaidHello();
 }
}
// Someone interested in "Hello" events
class Responder implements HelloListener {
 @Override
 public void someoneSaidHello() {
 System.out.println("Hello there...");
 }
}

class Test {
 public static void main(String[] args) {
 Initiater initiater = new Initiater();
 Responder responder = new Responder();
 initiater.addListener(responder);
 initiater.sayHello(); // Prints "Hello!!!" and "Hello there..."
 }
}

• Eren: copy paste from aioobe's answer: stackoverflow.com/questions/6270132/…

Answer 2

Have a look at the source of any class that uses listeners. In fact it's quite easy:

• create an interface for your listener, e.g. MyListener
• maintain a list of MyListener
• upon each event that the listeners should listen to, iterate over the list and call the appropriate method with some event parameter(s)

As for the observer pattern along with some Java code have a look at wikipedia.

Answer 3

There is no built-in mechanism that would allow you to attach listeners to all variables. The object you want to watch needs to provide the support for that by itself. For example it could become Observable and fire off onChange events to its Observers (which you also have to ensure are being tracked).

Answer 4

I would recommend using EventBus for your use case. It has a nice API design and is easy to use. Have a look at their Getting Started section to see how it works.

• randmin: in 2020, the link is github.com/greenrobot/EventBus

Answer 5

You can use the Signals library for it. It will look like that:

interface FiftySignal{
 void on50();
}
class FiftyMaker{
 FiftySignal fiftySignal = Signals.signal(FiftySignal.class).dispatcher;
 void doIt(){
 for(int i = 0; i < 100; i++){
 if(i == 50){
 fiftySignal.on50(); // dispatching the event
 }
 } 
 }
}
class Boo{
 Signal<FiftySignal> fiftySignal = Signals.signal(FiftySignal.class);
 void bar(){
 fiftySignal.addListener(()-> System.out.println("It's 50!")); // listener
 }
}

Disclaimer: I am the author of Signals.