In your MouseListener class, you register to "listen to" any of these events so that you are informed when they occur. When the event fires for example, the user clicks the mouse, as per the mouseClicked method above , a relevant MouseEvent object representing that event is created and passed to the MouseListener object registered to receive it.
Event listeners are represented by different interfaces, each of which is designed to process an equivalent event. Note that event listeners are flexible in that a single listener can be registered to "listen" to multiple types of events.
This means that, for a similar set of components that perform the same type of action, one event listener can handle all the events. Here are some of the most common types:. Actively scan device characteristics for identification.
Use precise geolocation data. Select personalised content. Create a personalised content profile. Measure ad performance. Select basic ads. Create a personalised ads profile.
Select personalised ads. Apply market research to generate audience insights. In the above code, both MultiListener and Eavesdropper implement the ActionListener interface and register as action listeners using the JButton addActionListener method.
Both classes' implementations of the actionPerformed method are similar: they simply add the event's action command to a text area. All rights reserved. Hide TOC. Writing Event Listeners. General Information about Writing Event Listeners.
Listeners Supported by Swing Components. Implementing Listeners for Commonly Handled Events. How to Write an Internal Frame Listener. How to Write a List Selection Listener. How to Write a Property Change Listener.
How to Write a Tree Expansion Listener. How to Write a Tree Selection Listener. How to Write an Undoable Edit Listener. Here is the code that implements the event handling for the button: public class Beeper Click the Blah blah blah button.
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. 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. 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. It depends on your application why it provides for listeners. Listeners do some work when an event occurs. They are called as "Event Listeners". Events like click, hover etc.. For Example, we have ActionListener interface in Java.
It calls actionPerformed method when an event occurs. Listeners are used for notify about state changes. You can think about Listeners in most of time as Observers, so every time something interesting happen your listener will be called. Stack Overflow for Teams — Collaborate and share knowledge with a private group.
Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. What is the purpose of a listener in Java? Ask Question. Asked 6 years, 3 months ago.
Active 4 years, 7 months ago. Viewed 50k times. 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.
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