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Event Handling and Listener

Event Handling:

In Java, events are handled using the Delegation Event Model. This model clearly defines how an event is generated, delivered, and processed.

An event is an object that represents a change of state or user action. Examples:
  • Clicking a button
  • Pressing a key
  • Moving the mouse
  • Submitting a form
  • Creating a session (in web apps)
Event-driven model in Java
Java uses the Delegation Event Model, which has three main parts:
  • Event Source

    The object where the event occurs (e.g., Button, TextField, ServletContext)

  • Event Object

    Contains information about the event (e.g., ActionEvent, MouseEvent)

  • Event Listener

    An interface that receives and handles the event (e.g., ActionListener, MouseListener)

In Java, listeners are objects that wait for and respond to events. They are a core part of event-driven programming, especially used in GUI applications, web applications, and frameworks like Swing, AWT, JavaFX, and Servlets.

Basic idea of a Listener A listener:
  • Listens for an event (like a button click, mouse movement, or request)
  • Gets notified automatically when that event occurs
  • Executes code in response to the event
In simple words:
Event happens -> Listener is notified -> Listener reacts
Listeners help to:
  • Separate event handling logic from core application logic
  • Make programs more interactive
  • Follow loose coupling (event source doesn't need to know details of handler)

Java uses Delegation Event Model

  • Event handling involves:
    • Event Source
    • Event Object
    • Event Listener
  • Events are handled by registering listeners
  • Listener methods are automatically invoked