Unit 2: Even handling

5th Semester

Event Handling

Any program that uses GUI (graphical user interface) such as Java application written for windows, is event driven. Event describes the change in state of any object. For Example : Pressing a button, Entering a character in Textbox, Clicking or Dragging a mouse, etc.

Components of Event Handling

Event handling has three main components,

  • Events : An event is a change in state of an object.
  • Events Source : Event source is an object that generates an event.
  • Listeners : A listener is an object that listens to the event. A listener gets notified when an event occurs.

How Events are handled ?

A source generates an Event and send it to one or more listeners registered with the source. Once event is received by the listener, they process the event and then return. Events are supported by a number of Java packages, like java.utiljava.awt and java.awt.event.

Important Event Classes and Interface

Event Classes Description Listener Interface
ActionEvent generated when button is pressed, menu-item is selected, list-item is double clicked ActionListener
MouseEvent generated when mouse is dragged, moved,clicked,pressed or released and also when it enters or exit a component MouseListener
KeyEvent generated when input is received from keyboard KeyListener
ItemEvent generated when check-box or list item is clicked ItemListener
TextEvent generated when value of textarea or textfield is changed TextListener
MouseWheelEvent generated when mouse wheel is moved MouseWheelListener
WindowEvent generated when window is activated, deactivated, deiconified, iconified, opened or closed WindowListener
ComponentEvent generated when component is hidden, moved, resized or set visible ComponentEventListener
ContainerEvent generated when component is added or removed from container ContainerListener
AdjustmentEvent generated when scroll bar is manipulated AdjustmentListener
FocusEvent generated when component gains or loses keyboard focus FocusListener


Steps to handle events:

  1. Implement appropriate interface in the class.
  2. Register the component with the listener.

Registration Methods

For registering the component with the Listener, many classes provide the registration methods. For example:

  • Button
    • public void addActionListener(ActionListener a){}
  • MenuItem
    • public void addActionListener(ActionListener a){}
  • TextField
    • public void addActionListener(ActionListener a){}
    • public void addTextListener(TextListener a){}
  • TextArea
    • public void addTextListener(TextListener a){}
  • Checkbox
    • public void addItemListener(ItemListener a){}
  • Choice
    • public void addItemListener(ItemListener a){}
  • List
    • public void addActionListener(ActionListener a){}
    • public void addItemListener(ItemListener a){}

Example of Event Handling

HTML code :

2.7 Java Event Handling Code

We can put the event handling code into one of the following places:

  1. Within class
  2. Other class
  3. Anonymous class

Java event handling by implementing ActionListener


public void setBounds(int xaxis, int yaxis, int width, int height); have been used in the above example that sets the position of the component it may be button, textfield etc.
event handling in java

2) Java event handling by outer class


3) Java event handling by anonymous class