What is the purpose of a listener in Java?

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I looked for this online, but couldn't find an adequate explanation to what it exactly does. What I saw was a Java Interface and it was passed as a parameter in another class as a "Listener". People added various listeners to a list and called them all through a single method.

I'm not sure why I would use it. Can someone care to explain?

This is my original help post where someone told me to use listeners.

Link


In the code example that you linked the KillMonsterEventListener

public interface KillMonsterEventListener {
    void onKillMonster ();
}

provides a way for users of your API to tell you something like this:

Here is a piece of code. When a monster is killed, call it back. I will decide what to do.

This is a way for me to plug in my code at a specific point in your execution stream (specifically, at the point when a monster is killed). I can do something like this:

yourClass.addKillMonsterEventListener(
    new KillMonsterEventListener() {
        public onKillMonster() {
            System.out.println("A good monster is a dead monster!");
        }
    }
);

Somewhere else I could add another listener:

yourClass.addKillMonsterEventListener(
    new KillMonsterEventListener() {
        public onKillMonster() {
            monsterCount--;
        }
    }
);

When your code goes through the list of listeners on killing a monster, i.e.

for (KillMonsterEventListener listener : listeners) {
    listener.onKillMonster()
}

both my code snippets (i.e. the monsterCount-- and the printout) get executed. The nice thing about it is that your code is completely decoupled from mine: it has no idea what I am printing, what variable I am decrementing, and so on.

What is the purpose of a listener in Java?, In the code example that you linked the KillMonsterEventListener public interface KillMonsterEventListener { void onKillMonster (); }. provides a  What I saw was a Java Interface and it was passed as a parameter in another class as a “Listener”. People added various listeners to a list and called them all through a single method. I’m not sure why I would use it.


Introduction to Event Listeners (The Java™ Tutorials > Creating a , This Swing Java Tutorial describes developing graphical user interfaces (GUIs) for applications and applets using Swing components. What is listener in java The main purpose of Servlet Listener is to read the life cycle actions of ServletRequest object, ServletContext object and HttpSession object. These are simple Java classes which implement one of the two following interfaces


This is part of a programming paradigm called event-driven programming. Objects send messages to other objects on certain occasions, for example when they change. This is used often in GUI programming. Each GUI widget is implemented by a class. When you want to handle e.g. mouse clicks from the user, you add a listener (also called event handler) to GUI widget. When the user clicks on the widget, the widget sends the event to the registered listener(s) so that the application can respond to the mouse click. This seperates the framework (the GUI widget class) and the application code. (In some GUI frameworks, such as Swing, you can add an arbitrary number of listeners to an object; in others, you can specify only one.)

Also in other areas event-driven programming is useful. You might want to observe an object (see Observer pattern). For example, a collection which supports this, might send an event if its contents change. If you need to perform some processing if this occurs, you can add yourself as a listener to this class. The alternative would be to call the post-processing every time you add an item to the collection, but this error-prone.

SWING - Event Listeners, SWING - Event Listeners - Event listeners represent the interfaces responsible to handle events. Java provides various Event listener classes, however, only  The Servlet API provides a number of listener interfaces we can implement in order to react to these events.


Servlet Listener is used for listening to events in a web container, such as when you create a session or place an attribute in a session or if you passivate and activate in another container, to subscribe to these events you can configure listener in web.xml, for example, HttpSessionListener.

Listeners get triggered for an actual physical request that can be attached to events in your app server .With listeners, you can track application-level, session-level, life-cycle changes, attribute changes etc.

You can monitor and react to events in a servlet's life cycle by defining listener objects whose methods get invoked when lifecycle events occur.

Here is the blog post for Servlet Listener http://array151.blogspot.in/2016/12/servlet-listener.html

Events and listeners, Internally, the component may store a listener however it chooses; usually, however, components store listeners in a java.util.Vector or javax.swing.event. Import the package java.awt.event.* if you are dealing with events. The listener object could be defined in a class other than the frame class. In our example, the class that holds the component is also the listener for its events.


Use a listener to let other code inform you of "conditions"/"events". For instance a "mouse listener" could be called if the mouse would have been moved/clicked/dragged. It depends on your application why it provides for listeners.

Java Event Listeners Process User Activity in a GUI, An event listener in Java is designed to process some kind of event — it "listens" for an event, such as a user's mouse click or a key press, and  The event listener, the object that "listens" for events and processes them when they occur. There are several types of events and listeners in Java: each type of event is tied to a corresponding listener.


Java Events and How They Work With Event Listeners, A Java Event Represents a GUI Action in Java's Swing GUI API. Java events are always paired with equivalent listeners. Share  The listener is programmed to react to an input or signal by calling the event's handler. The term event listener is often specific to Java and JavaScript . In other languages, a subroutine that performs a similar function is referred to as an event handler .


Event Handling in Java, Event and Listener (Java Event Handling). Changing the state of an object is known as an event. For example, click on button, dragging mouse etc. The java  Listener is a process that resides on the server whose responsibility is to listen for incoming client connection requests and manage the traffic to the server. Every time a client requests a network session with a server, a listener receives the actual request. If the client information matches the listener information, then the listener grants…


How to Write an Action Listener (The Java™ Tutorials > Creating a , This results in the invocation of the action listener's actionPerformed method (the only method in the ActionListener interface). The single Method, Purpose. You implement an action listener to define what should be done when an user performs certain operation. An action event occurs, whenever an action is performed by the user. Examples: When the user clicks a button , chooses a menu item , presses Enter in a text field .