Florida International University (FIU)

KeyEvent

The class provides the following constants:

The inherited method getId() returns the specific type of event denoted by one of the constants.

The class also provides the following methods:

MouseEvent

In addition to the inherited method getId(), the class provides the following methods:

PaintEvent

WindowEvent

In addition to the inherited method getId(), the class provides the following method:

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The Event Delegation Model

• The source of an event cane be identified by calling the method getSource() inherited from java.util.EventObject.
• The value of the event type can be determined by calling the method getId() from the respective event class.
• Several sources can generate the same type of events. For example, an ActionEvent can be generated by objects of:
  • Button/JButton
  • List/JList
  • Menuitem/JMenuItem, and
  • TextFiled/JTextField
• In this model:
  1. An event source informs listeners about events when they occur, and
  2. Supply the necessary information about these events.
  3. An event source can generate more than one types of events, and maintains a list of event listeners for each type. For instance:
     • A Button object can generate an EventAction when the button is clicked.
     • Since the button is a component, it can generate all the events for theComponent class as well. That is,it can generate:
       1. ComponentEvent
       2. FocusEvent
       3. KeyEvent
       4. MouseEvent
  4. Subclasses of a source can generate the same events that a source can.
• A source is associated with a listener by:
  • A registration method (adding a listener, i.e. addxxxListener(EventTypeObject),or
  • De-registration method (removing a listener, i.e, removexxxListener(EventTypeObject)
  • Each listener implements a certain interface. See table below.

Semantic Event Handling

Given the following interface:


     public interface constants
     {
         String button[] = {"Write", "Draw", "Paint", "Print"},
     }

     import javax.swing.JFrame;
     import javax.swing.JButton;
     import javax.swing.ImageIcon;
     import java.awt.Container;
     import java.awt.GridBagLayout;
     import java.awt.GridBagConstraints;

     public class myJButton extends JButton implements constants
     {
        myJFrame f;

        JButton source;

        myJButton(myJFrame f)
        {
           this.f = f;
        }

        public void addButtons()
        {
            // 1. Get the contentPane for the frame myJFrame
            Container content = f.getContentPane();

           // 2. Create an object of the class GridBaglayout
           GridBagLayout gbag = new GridBagLayout();

           // 3. Set the layout manager for the container
            content.setLayout(gbag);

           // 4. Create an object of the class GridBagConstraints
           GridBagConstraints constraints = new GridBagConstraints();

           // 5. Layout each component

           constraints.fill = GridBagConstraints.NONE;

           for (int i = 0; i < button.length; i++)
           {

               switch(i)
               {
                   case 0:
                   {
                         source = new JButton(button[i]);
                        constraints.gridx = 0;
                        constraints.gridy = 1;
                        constraints.weightx = 0.5;
                        constraints.gridwidth = 1;
                        constraints.gridheight = 1;
                        break;
                  }
                  case 1:
                  {
                    // Set constraints
                      break;
                  }
                  case 2:
                  {
                      source = new JButton(button[i], new ImageIcon("Firewks1.jpg"));
                    // Set constraints
                     break;
                  }
                  case 3:
                  {
                      source = new JButton(button[i]);
                     // Set constraints
                  }
             }
             gbag.setConstraints(b, constraints);
             content.add(b);

             // Register each button object
              source. addActionListener(new myActionListener(i));
        }
    }
}

Semantic Event Listeners and their Methods

// 3. ActionListener - Typical usage:


import java.awt.event.ActionEvent;
import java.awt.event.ActionListener; // Contains actionPerformed(...)

public class myActionListener implements ActionListener, constants
{
     int i;

     public myActionListener(int i)
     {
        this.i = i;
     }

     public void actionPerformed(ActionEvent e)
     {
        if (button[i].equals(e.getActionCommand()))
          System.out.println(button[i] + " was pressed");
    }
}

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Low-level Event Handling

Semantic Event Listeners and their Methods

An event listener interface can contain more than one method. For example, the WindowListener interface has seven (7) methods. If such interface is implemented, then it is imperative that all the methods are coded, even though a source object might not be interested in all of the methods. The alternate way to handle this siutation is to provide empty body to those methods that are not of interest to the object. Luckly Java has provided these method in classes called Adapter classes. So, instead of implementing the interfaces you would extend the respective adapter class, and you would simply define the body for the method of interest.

	    myJFrame source = new myJFrame(frameTitle);
	    source.addWindowListener(new myWindowEvents());
	    source.addKeyListener(new myKeyEvent());

WindowAdapter -  Typical usage

import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;

class myWindowEvents extends WindowAdapter implements constants
{
     public void windowClosing(WindowEvent e)
     {
         // Flush any buffer
         // Close any opened file(s)
         // Do necessary clean up activity, then
         System.exit(0);
     }

     public void windowClosed(WindowEvent e)
     {
         System.out.println("windowClosed: " + e);
     }

     public void windowOpened(WindowEvent e)
     {
        // Do any necesary initialization tasks before any further activity
       System.out.println("windowOpened: " + e);
     }

     public void windowActivated(WindowEvent e)
     {
          System.out.println("windowActivated: " + e);
     }
}

KeyAdapter - typical usage

import java.awt.event.KeyEvent;
import java.awt.event.KeyAdapter;
     
public class myKeyEvent extends KeyAdapter 
{
     myKeyEvent(){}

     public void keyTyped(KeyEvent e)   //  Handle the key typed event
     {
         displayInfo(e, "KEY TYPED: ");
     }

     public void keyPressed(KeyEvent e)  	// Handle the key pressed event
     {
         displayInfo(e, "KEY PRESSED: ");
     }

     public void keyReleased(KeyEvent e)  // Handle the key released event
     {
         displayInfo(e, "KEY RELEASED: ");
     }

     private void displayInfo(KeyEvent e, String s)
     {
         String charString, keyCodeString;

         char ch = e.getKeyChar();
         int keyCode = e.getKeyCode();


         if (Character.isISOControl(ch))
	         charString = "key character = (an unprintable control character)";
         else
	         charString = "\n" + "key character = '" + ch + "'";

          keyCodeString = "key code = " + keyCode + " (" + KeyEvent.getKeyText(keyCode) + ")";

          System.out.print(s +  "    " +  keyCodeString  + charString + "\n");
     }
}