Types of events in Java

An event is one of the most important concepts in Java. The change in the state of an object or behavior by performing actions is referred to as an Event in Java. Actions include button click, keypress, page scrolling, or cursor movement.

Java provides a package java.awt.event that contains several event classes.

We can classify the events in the following two categories:

Foreground Events
Background Events

Foreground Events

Foreground events are those events that require user interaction to generate. In order to generate these foreground events, the user interacts with components in GUI. When a user clicks on a button, moves the cursor, and scrolls the scrollbar, an event will be fired.

Background Events

Background events don't require any user interaction. These events automatically generate in the background. OS failure, OS interrupts, operation completion, etc., are examples of background events.

Delegation Event Model

A mechanism for controlling the events and deciding what should happen after an event occur is referred to as event handling. Java follows the Delegation Event Model for handling the events.

The Delegation Event Model consists of Source and Listener.

Source

Buttons, checkboxes, list, menu-item, choice, scrollbar, etc., are the sources from which events are generated.

Listeners

The events which are generated from the source are handled by the listeners. Each and every listener represents interfaces that are responsible for handling events.

To learn more about Delegation Event Model, go through the following link:

https://www.javatpoint.com/delegation-event-model-in-java

We need to register the source with the listener for handling events. Different types of classes provide different registration methods.

The syntax of registering the source with the listener is as follows:

For example, if we need to register Key and Action events, we use the addActionListener() and addKeyListener() methods.

These are some of the most used Event classes:

S.No.	Event Class	Listener Interface	Methods	Descriptions
1.	ActionEvent	ActionListener	actionPerformed()	ActionEvent indicates that a component-defined action occurred.
2.	AdjustmentEvent	AdjustmentListener	adjustmentValueChanged()	Adjustment events occur by an adjustable object like a scrollbar.
3.	ComponentEvent	ComponentListener	componentResized(), componentMoved(), componentShown() and componentHidden()	An event occurs when a component moved, changed its visibility or the size changed.
4.	ContainerEvent	ContainerListener	componentRemoved() and componentAdded()	The event is fired when a component is added or removed from a container.
5.	FocusEvent	FocusListener	focusLost() and focusGained()	Focus events include focus, focusout, focusin, and blur.
6.	ItemEvent	ItemListener	itemStateChanged()	Item event occurs when an item is selected.
7.	KeyEvent	KeyListener	keyPressed(), keyReleased(), and keyTyped().	A key event occurs when the user presses a key on the keyboard.
8.	MouseEvent	MouseListener and MouseMotionListener	mouseClicked(), mousePressed(), mouseEntered(), mouseExited() and mouseReleased() are the mouseListener methods. mouseDregged() and mouseMoved() are the MouseMotionListener() methods.	A mouse event occurs when the user interacts with the mouse.
9.	MouseWheelEvent	MouseWheelListener	mouseWheelMoved().	MouseWheelEvent occurs when the mouse wheel rotates in a component.
10.	TextEvent	TextListener	textChanged()	TextEvent occurs when an object's text change.
11.	WindowEvent	WindowListener	windowActivated(), windowDeactivated(), windowOpened(), windowClosed(), windowClosing(), windowIconfied() and windowDeiconified().	Window events occur when a window's status is changed.

Let's take an example to understand how we can work with the events and listeners:

EventHandlingExample1.java

// import required classes and package, if any 
package JavaTpoint.Examples; 
 
import java.awt.*; 
import java.awt.event.*; 
 
//create class EventHandlingExample1 to perform event handling within the class 
public class EventHandlingExample1 { 
    
    // create variables for Frame, Label and Panel 
    private Frame frame; 
    private Label header; 
    private Label status; 
    private Panel panel; 
    
    // default constructor 
    public EventHandlingExample1(){ 
        makeGUI(); 
    } 
    
    // main() method start 
    public static void main(String[] args){ 
        
        // create an instance of EventHandlingExample1 
        EventHandlingExample1  obj = new EventHandlingExample1(); 
        obj.addButtonsAndLabel(); 
    } 
    
    // create makeGUI() method to create a UI to perform user interaction 
    private void makeGUI(){ 
        
        // initialize Frame 
        frame = new Frame("Event Handling Example"); 
        
        // set frame size by using setSize() method 
        frame.setSize(400,400); 
        
        //set frame layout by using setLayout() method 
        frame.setLayout(new GridLayout(3, 1)); 
        
        // add window listener to frame 
        frame.addWindowListener(new WindowAdapter() { 
            public void windowClosing(WindowEvent windowEvent){ 
                System.exit(0); 
            }         
        });  
        
        // initialize header and set alignment to center 
        header = new Label(); 
        header.setAlignment(Label.CENTER); 
        
        //initialize status and set its alignment and size 
        status = new Label();         
        status.setAlignment(Label.CENTER); 
        status.setSize(350,100); 
        
        // initialize panel 
        panel = new Panel(); 
        
        // set flow layout to panel 
        panel.setLayout(new FlowLayout()); 
        
        // add header, status and panel to frame 
        frame.add(header); 
        frame.add(panel); 
        frame.add(status); 
        frame.setVisible(true);   
    } 
    
    // create addButtonsAndLabel() method   
    private void addButtonsAndLabel(){ 
        
        // set label test 
        header.setText("Click a button: ");  
        
        // create ok, submit and cancel buttons 
        Button okBtn = new Button("OK"); 
        Button submitBtn = new Button("Submit"); 
        Button cancelBtn = new Button("Cancel"); 
        
        // use setActionCommand() method to set action for ok, submit and cancel buttons 
        okBtn.setActionCommand("OK"); 
        submitBtn.setActionCommand("Submit"); 
        cancelBtn.setActionCommand("Cancel"); 
        
        // add event listener to buttons using addActionListener() and ButtonClickListener() 
        okBtn.addActionListener(new ButtonClickListener());  
        submitBtn.addActionListener(new ButtonClickListener());  
        cancelBtn.addActionListener(new ButtonClickListener());  
        
        // add buttons to panel 
        panel.add(okBtn); 
        panel.add(submitBtn); 
        panel.add(cancelBtn);        
        
        // make frame visible by using setVisible()mmethod 
        frame.setVisible(true);   
    } 
    
    // implements ActionListener interface 
    private class ButtonClickListener implements ActionListener{ 
        
        // define actionPerformed() method of ActionListener 
        public void actionPerformed(ActionEvent event) { 
            
            // get action command using getActionCommand() method of Event 
            String command = event.getActionCommand();   
            
            // code to check which button is pressed 
            if( command.equals( "OK" ))  { 
                status.setText("Ok Button clicked."); 
            } else if( command.equals( "Submit" ) )  { 
                status.setText("Submit Button clicked.");  
            } else  { 
                status.setText("Cancel Button clicked."); 
            }  	 
        }		 
    } 
}

Output: