Event Handling Java

Event Handling Java


In AWT components, we came to know every component (except Panel and Label) generates events when interacted by the user like clicking over a button or pressing enter key in a text field etc. Listeners handle the events. Let us know the style (or design pattern) Java follows to handle the events.

The event handling Java involves four types of classes.

1. Event Sources
2. Event classes
3. Event Listeners
4. Event Adapters

1. Event Sources

Event sources are components, subclasses of java.awt.Component, capable to generate events. The event source can be a button, TextField or a Frame etc.

2. Event classes

Almost every event source generates an event and is named by some Java class. For example, the event generated by button is known as ActionEvent and that of Checkbox is known as ItemEvent. All the events are listed in java.awt.event package. Following list gives a few components and their corresponding listeners.

Component Event it generates
Button, TextField, List, Menu ActionEvent
Frame WindowEvent
Checkbox, Choice, List ItemEvent
Scrollbar AdjustmentEvent
Mouse (hardware) MouseEvent
Keyboard (hardware) KeyEvent

The events generated by hardware components (like MouseEvent and KeyEvent) are known as low-level events and the events generated by software components (like Button, List) are known as semantic events.

3. Event Listeners

The events generated by the GUI components are handled by a special group of interfaces known as "listeners". Note, Listener is an interface. Every component has its own listener, say, AdjustmentListener handles the events of scrollbar Some listeners handle the events of multiple components. For example, ActionListener handles the events of Button, TextField, List and Menus. Listeners are from java.awt.event package.

More description on listeners and list of listeners is available at Java AWT Listeners.

4. Event Adapters

When a listener includes many abstract methods to override, the coding becomes heavy to the programmer. For example, to close the frame, you override seven abstract methods of WindowListener, in which, infact you are using only one method. To avoid this heavy coding, the designers come with another group of classes known as "adapters". Adapters are abstract classes defined in java.awt.event package. Every listener that has more than one abstract method has got a corresponding adapter class.

More description on adapters and list of adapters is available at Java AWT Adapters.

What happens internally at a button click?

We know the events are handled by listeners and ActionListener handles the events of a button. Observe the following skeleton code.

The above program contains a small part of event handling Java code of a big program, and is used here just for the explanation of internal event handling Java mechanism.

A button object btn is created for which events are yet to be linked. For this, the first step is implementing ActionListener to the class ButtonDemo. In the second step, we link or register the button btn with the ActionListener. For this addActionListener() method of Button class is used. The parameter "this" refers the ActionListener.

btn.addActionListener(this);

With the above statement, btn is linked with the ActionListener.

1. When the button btn is clicked, the button generates an event called ActionEvent. It is the nature of the button taken care by JVM.

2. This ActionEvent reaches the ActionListener because we registered the button with ActionListener earlier.

3. Now, the question is, what ActionListener does with the ActionEvent object it received?

The ActionListener simply calls actionPerformed() method and passes the ActionEvent object to the parameter as follows.

public void actionPerformed(ActionEvent e)

The parameter for the above method comes from ActionListener.

4. Finally the ActionEvent object generated by the button btn reaches the e object of ActionEvent. All this is done by JVM implicitly. For this reason, the getActionCommand() method of ActionEvent class knows the label of the button btn.

String str = e.getActionCommand();

The e represents an object of ActionEvent and the value for the e is coming from button btn. str is nothing but the label of the button OK.

Scroll down and click over Page 2 to go for next page.

26 thoughts on “Event Handling Java

  1. Goldi

    Easiest way of explanation..thanks alot sir ..sir if i start going through to your all the contents of java on this site..Is it enough to be a good java programmer ??

  2. Y.Sowmya

    Sir,How to write a program so that a new window is opened by pressing F2 key. I have tried but it is not working.I have used window and key events.

  3. Marse

    Nice tutorial sir. I’s wondering if you could recommend us some of your particular favourite books of java.

    Thanks again for the tuto. I really appreciated it.

    1. S. Nageswara Rao, Corporate Trainer Post author

      Now-a-days, Internet is the best source of knowledge. So, don’t on depend on books. If you purchase one book today, tomorrow it will be obsolete. For example, if you purchase to day a Java text book, next month September, Java 8 is getting released. Then you purchase another book also.

      Tell your friends also to derive knowledge from this web site.

  4. ananth

    sir

    i have inserted 3 labels in rectangle using paint method. my intention is particular label have to be moved within a rectangle when i drag it by using mouse in java

  5. Pishu

    Hi Sir,

    Your way of teaching is very good. it would be better for us , if you upload some your video tutorials on Java.
    Apache POI is missing in your website. Please provide some good understanding of java Apache POI . Thanks in advance.

    BR//
    Pishu

  6. ishu singhal

    sir i want to know that…. is it possible to have different actionlistener code for two or more buttons in a class.

    1. S. Nageswara Rao, Corporate Trainer Post author

      See this code.

      import java.awt.* ; import java.awt.event.* ; import javax.swing.* ;
      public class ProgressiveTest {
      public static void main ( String args[ ] ) {

      JFrame frame = new JFrame( “Swing’s ProgressMonitor” ) ;
      JButton button = new JButton( “Start Iteration” ) ;
      frame.getContentPane().add( button, BorderLayout.CENTER );

      int min = 0 ; int max = 100 ;
      String[ ] message = new String[ 2 ] ;
      message[ 0 ] = “Performing Iterations.” ;
      message[ 1 ] = “Wait for completion…….” ;
      final ProgressMonitor monitor = new ProgressMonitor(frame, message, “Iteration”,
      min, max ) ;
      final Runnable runnable = new Runnable( ) {
      public void run( ) {
      int sleepTime = 500 ;
      for( int i = 1 ; i < 100 ; i++ ) { try { monitor.setNote( "Iteration " + i ) ; monitor.setProgress( i ) ; if ( monitor.isCanceled( ) ) { monitor.setProgress( 100 ) ; break ; } Thread.sleep( sleepTime ) ; } catch ( InterruptedException e ) { } } monitor.close( ) ; } } ; button.addActionListener( new ActionListener( ) { public void actionPerformed( ActionEvent event ) { // run the iterations in a separate thread Thread thread = new Thread( runnable ); thread .start( ) ; } } ) ;

  7. Josef

    Dear Sir
    I’m doing project for school.
    I have two sets of 4 buttonsa1a,2a,3a,4a and 1b,2b,3b,4b. I’m trying to write if script for condition: if 1a .clicked && 2b .clicked go to page2 in my site.

    Any suggestions??
    Thank you

Leave a Reply

Your email address will not be published. Required fields are marked *