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Event Handling in Java

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  • Difficulty Level : Medium
  • Last Updated : 18 Feb, 2022

An event can be defined as changing the state of an object or behavior by performing actions. Actions can be a button click, cursor movement, keypress through keyboard or page scrolling, etc. 

The java.awt.event package can be used to provide various event classes. 

Classification of Events

  • Foreground Events
  • Background Events

Types of Events

1. Foreground Events

Foreground events are the events that require user interaction to generate, i.e., foreground events are generated due to interaction by the user on components in Graphic User Interface (GUI). Interactions are nothing but clicking on a button, scrolling the scroll bar, cursor moments, etc.

2. Background Events

Events that don’t require interactions of users to generate are known as background events. Examples of these events are operating system failures/interrupts, operation completion, etc.

Event Handling

It is a mechanism to control the events and to decide what should happen after an event occur. To handle the events, Java follows the Delegation Event model.

Delegation Event model

  • It has Sources and Listeners.

Delegation Event Model

  • Source: Events are generated from the source. There are various sources like buttons, checkboxes, list, menu-item, choice, scrollbar, text components, windows, etc., to generate events.
  • Listeners: Listeners are used for handling the events generated from the source. Each of these listeners represents interfaces that are responsible for handling events.

To perform Event Handling, we need to register the source with the listener.

Registering the Source With Listener

Different Classes provide different registration methods.

Syntax:

addTypeListener()

where Type represents the type of event.

Example 1: For KeyEvent we use addKeyListener() to register.

Example 2:that For ActionEvent we use addActionListener() to register.

Event Classes in Java

Event Class

Listener Interface

Description

ActionEvent

ActionListener

An event that indicates that a component-defined action occurred like a button click or selecting an item from the menu-item list.

AdjustmentEvent

AdjustmentListener

The adjustment event is emitted by an Adjustable object like Scrollbar.

ComponentEvent

ComponentListener

An event that indicates that a component moved, the size changed or changed its visibility.

ContainerEvent

ContainerListener

When a component is added to a container (or) removed from it, then this event is generated by a container object.

FocusEvent

FocusListener

These are focus-related events, which include focus, focusin, focusout, and blur.

ItemEvent

ItemListener

An event that indicates whether an item was selected or not.

KeyEvent

KeyListener

An event that occurs due to a sequence of keypresses on the keyboard.

MouseEvent

MouseListener & MouseMotionListener

The events that occur due to the user interaction with the mouse (Pointing Device).

MouseWheelEvent

MouseWheelListener

An event that specifies that the mouse wheel was rotated in a component. 

TextEvent

TextListener

An event that occurs when an object’s text changes.

WindowEvent

WindowListener

An event which indicates whether a window has changed its status or not.

Note: As Interfaces contains abstract methods which need to implemented by the registered class to handle events.

Different interfaces consists of different methods which are specified below.

Listener Interface

Methods

ActionListener

  • actionPerformed()

AdjustmentListener

  • adjustmentValueChanged()

ComponentListener

  • componentResized()
  • componentShown()
  • componentMoved()
  • componentHidden()

ContainerListener

  • componentAdded()
  • componentRemoved()

FocusListener

  • focusGained()
  • focusLost()

ItemListener

  • itemStateChanged()

KeyListener

  • keyTyped()
  • keyPressed()
  • keyReleased()

MouseListener

  • mousePressed()
  • mouseClicked()
  • mouseEntered()
  • mouseExited()
  • mouseReleased()

MouseMotionListener

  • mouseMoved()
  • mouseDragged()

MouseWheelListener

  • mouseWheelMoved()

TextListener

  • textChanged()

WindowListener

  • windowActivated()
  • windowDeactivated()
  • windowOpened()
  • windowClosed()
  • windowClosing()
  • windowIconified()
  • windowDeiconified()

Flow of Event Handling

  1. User Interaction with a component is required to generate an event.
  2. The object of the respective event class is created automatically after event generation, and it holds all information of the event source.
  3. The newly created object is passed to the methods of the registered listener.
  4. The method executes and returns the result.

Code-Approaches

The three approaches for performing event handling are by placing the event handling code in one of the below-specified places.

  1. Within Class
  2. Other Class
  3. Anonymous Class

Note: Use any IDE or install JDK to run the code, Online compiler may throw errors due to the unavailability of some packages.

Event Handling Within Class

Java




// Java program to demonstrate the
// event handling within the class
 
import java.awt.*;
import java.awt.event.*;
 
class GFG extends Frame implements ActionListener {
 
    TextField textField;
 
    GFGTop()
    {
        // Component Creation
        textField = new TextField();
 
        // setBounds method is used to provide
        // position and size of the component
        textField.setBounds(60, 50, 180, 25);
        Button button = new Button("click Here");
        button.setBounds(100, 120, 80, 30);
 
        // Registering component with listener
        // this refers to current instance
        button.addActionListener(this);
 
        // add Components
        add(textField);
        add(button);
 
        // set visibility
        setVisible(true);
    }
   
    // implementing method of actionListener
    public void actionPerformed(ActionEvent e)
    {
        // Setting text to field
        textField.setText("GFG!");
    }
   
    public static void main(String[] args)
    {
      new GFGTop();
    }
}

 
 

Output

After Clicking, the text field value is set to GFG!

Explanation

  1. Firstly extend the class with the applet and implement the respective listener.
  2. Create Text-Field and Button components.
  3. Registered the button component with respective event. i.e. ActionEvent by addActionListener().
  4. In the end, implement the abstract method.

Event Handling by Other Class

 

Java




// Java program to demonstrate the
// event handling by the other class
 
import java.awt.*;
import java.awt.event.*;
 
class GFG1 extends Frame {
   
    TextField textField;
   
    GFG2()
    {
        // Component Creation
        textField = new TextField();
       
        // setBounds method is used to provide
        // position and size of component
        textField.setBounds(60, 50, 180, 25);
        Button button = new Button("click Here");
        button.setBounds(100, 120, 80, 30);
       
        Other other = new Other(this);
       
        // Registering component with listener
        // Passing other class as reference
        button.addActionListener(other);
       
        // add Components
        add(textField);
        add(button);
       
        // set visibility
        setVisible(true);
    }
   
    public static void main(String[] args)
    {
      new GFG2();
    }
}

 

 

Java




/// import necessary packages
import java.awt.event.*;
 
// implements the listener interface
class Other implements ActionListener {
                          
    GFG2 gfgObj;
   
    Other(GFG1 gfgObj)
    {
      this.gfgObj = gfgObj;
    }
   
    public void actionPerformed(ActionEvent e)
    {
        // setting text from different class
        gfgObj.textField.setText("Using Different Classes");
    }
}

 
 

Output

Handling event from different class

Event Handling By Anonymous Class

 

Java




// Java program to demonstrate the
// event handling by the anonymous class
 
import java.awt.*;
import java.awt.event.*;
 
class GFG3 extends Frame {
   
    TextField textField;
   
    GFG3()
    {
        // Component Creation
        textField = new TextField();
       
        // setBounds method is used to provide
        // position and size of component
        textField.setBounds(60, 50, 180, 25);
        Button button = new Button("click Here");
        button.setBounds(100, 120, 80, 30);
       
        // Registering component with listener anonymously
        button.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e)
            {
                // Setting text to field
                textField.setText("Anonymous");
            }
        });
       
        // add Components
        add(textField);
        add(button);
       
        // set visibility
        setVisible(true);
    }
   
    public static void main(String[] args)
    {
        new GFG3();
    }
}

 
 

Output

Handling anonymously

 


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