Layout Managers

The following program illustrates the use of Layout Managers. To run the program uncomment the section on a particular Layout Manager and comment out those that you do not want. Compile the program and run it.

import java.awt.*;

public class LayoutManagerDemo 
{
  public static void main ( String [] args ) 
  {
    //  Create the frame
    Frame frame = new Frame ( "Layout Manager Demo" );
    
    /*  Demonstrate Flow Layout  
    CheckboxGroup size = new CheckboxGroup ();
	
    Checkbox cb1 = new Checkbox ("Large", size, false);
    Checkbox cb2 = new Checkbox ("Medium", size, false);
    Checkbox cb3 = new Checkbox ("Small", size, false);
		
    frame.setLayout ( new FlowLayout () );
    frame.add (cb1); frame.add (cb2); frame.add (cb3);		
    frame.pack ();
    frame.setVisible ( true ); 
    */
		
    /*  Demonstrate Grid Layout  
    Label xLabel = new Label ("X Coordinate");
    Label yLabel = new Label ("Y Coordinate");
    TextField xInput = new TextField (5);
    TextField yInput = new TextField (5);
		
    frame.setLayout (new GridLayout(2,2,20,20));
    frame.add(xLabel);  frame.add(xInput);
    frame.add(yLabel);  frame.add(yInput);
    frame.pack ();
    frame.setVisible ( true );
    */
		
    /*  Demonstrate Border Layout 
    TextField header = new TextField ("Title");
    header.setEditable (false);
		
    TextField footer = new TextField ("Footnote");
    footer.setEditable (false);
		
    Scrollbar sb1 = new Scrollbar (Scrollbar.VERTICAL, 0, 1, 0, 255);
    Scrollbar sb2 = new Scrollbar (Scrollbar.VERTICAL, 0, 1, 0, 255);
		
    Canvas drawRegion = new Canvas();
    drawRegion.setSize (150,150);
    drawRegion.setBackground (Color.red);
		
    frame.setLayout (new BorderLayout());
    frame.add (header, BorderLayout.NORTH);
    frame.add (footer, BorderLayout.SOUTH);
    frame.add (sb1, BorderLayout.WEST);
    frame.add (sb2, BorderLayout.EAST);
    frame.add (drawRegion, BorderLayout.CENTER);
    frame.pack ();
    frame.setVisible ( true );
    */
		
    /*  Demonstrate Grid Bag Layout  */ 
    String colors[] = {"Violet", "Indigo", "Blue", "Green", 
		       "Yellow", "Orange", "Red"};
    List colorList = new List (colors.length-2, false);
    for (int i = 0; i < colors.length; i++)
	colorList.add (colors[i]);
			
    Label shape = new Label ("Shape");
    shape.setFont (new Font("Times", Font.BOLD, 14));
		
    Choice shapeChoice = new Choice();
    shapeChoice.add("Triangle");  shapeChoice.add("Square");
    shapeChoice.add("Rectangle");  shapeChoice.add("Polygon");
		
    Checkbox cbFill = new Checkbox ("Fill", false);
    cbFill.setFont (new Font("Times", Font.BOLD, 14));
		
    frame.setLayout (new GridBagLayout());
		
    GridBagConstraints gbc = new GridBagConstraints();
		
    gbc.fill = GridBagConstraints.NONE;
    gbc.anchor = GridBagConstraints.NORTHWEST;
    gbc.weightx = 1;  gbc.weighty = 0;

    addUsingGBL (frame, colorList, gbc, 0, 0, 1, 3);
    addUsingGBL (frame, shape, gbc, 1, 0, 1, 2);
    addUsingGBL (frame, shapeChoice, gbc, 2, 0, 1, 2);
    addUsingGBL (frame, cbFill, gbc, 1, 2, 2, 1);

    frame.pack ();
    frame.setVisible ( true );
    }
	
    public static void addUsingGBL (Frame frame, Component component, 
				    GridBagConstraints gbc, int x, 
				    int y, int w, int h) 
    {
      gbc.gridx = x;
      gbc.gridy = y;
      gbc.gridwidth = w;
      gbc.gridheight = h;
      frame.add (component, gbc);
    }

}

Event Handling

• Event classes encapsulate information about different types of user interaction.
• Event source objects inform event listeners about events.
• Event listener objects take appropriate actions.

• Setting up the propagation of events from event sources to event listeners.
• Providing the appropriate actions in event listeners to deal with the events received.

Semantic Events

ActionEvent - This event is generated by an action performed on a GUI component.

• Button is clicked
• List item is double-clicked
• MenuItem is selected
• ENTER key is hit in a TextField

AdjustmentEvent - This event is generated when adjustments are made to an adjustable component like a scrollbar.

ItemEvent - This event is generated when an item is selected or de-selected in a component.

• Checkbox - when the state of a check box changes.
• CheckboxMenuItem - when the state of a checkbox associated with a menu item changes.
• Choice - when an item is selected or de-selected in a choice list.
• List - when an item is selected or de-selected from a list.

TextEvent - This event is generated whenever the content of a text component is changed.

Low-level Events

• ComponentEvent - This event is generated when a component is hidden, shown, moved, or re-sized.
• ContainerEvent - This event is generated when a component is added or removed from a container.
• FocusEvent - This event is generated when a component gains or loses focus. Having the focus means that the component can receive keystrokes.
• KeyEvent - This event is generated when the user presses or releases a key or does both (i.e. types a character).
• MouseEvent - This class is generated when the user moves the mouse or presses a mouse button.
• PaintEvent - This event is generated when a component should have its paint()/update() methods invoked.
• WindowEvent - This event is generated when an important operation is performed on a window like opening, closing, etc.

AWT events are represented by the subclasses of the java.awt.AWTEvent class. The source of an event can be identified by calling the getSource() method, and the event's type value determined by calling the getID() method. In the event delegation model, an event source informs event listeners about events when these occur.

How is the association between the source and the listener established? Each event source defines methods for registering (addXListener()) and removing (removeXListener()) listeners which implement a particular listener interface (Xlistener). Each Xlistener interface defines methods that accept specific event type as argument.

Semantic Event Handling

Semantic Event Listener Interfaces and Their Methods

Low Level Event Handling

Low Level Event Listener Interfaces and Their Methods

import java.awt.*;
import java.awt.event.*;

public class SimpleFrame extends Frame
{
  public Button toggle;
  public ButtonListener buttonListener;

  public SimpleFrame ()
  {
    // Create a window
    super ( "Simple Frame" );

    // Create one button
    toggle = new Button ( "ON" );

    // Set a flow layout manager
    setLayout ( new FlowLayout ( FlowLayout.CENTER ) );

    // Add the button
    add ( toggle );

    // Create and add the listener to the button
    buttonListener = new ButtonListener ( this );
    toggle.addActionListener ( buttonListener );

    // Pack the window and make it visible
    pack();
    setVisible ( true );
  }

  public static void main ( String [] args )
  {
    new SimpleFrame ();
  }
}

class ButtonListener implements ActionListener
{
  private SimpleFrame theFrame;

  public ButtonListener ( SimpleFrame aFrame )
  {
    theFrame = aFrame;
  }

  public void actionPerformed ( ActionEvent evt )
  {
    if ( evt.getSource() == theFrame.toggle )
    {
      String str = theFrame.toggle.getLabel();
      if ( str.equals ( "ON" ) )
        theFrame.toggle.setLabel ( "OFF" );
      else
	theFrame.toggle.setLabel ( "ON" );
    }
  }
}

import java.awt.*;
import java.awt.event.*;

public class AddTwo extends Frame
{
  public Button sButton;
  public TextField xField;
  public TextField yField;
  public TextField sField;
  public ButtonListener buttonListener;
  
  public AddTwo ()
  {
     // Create a window
     super ( "Add Two Numbers" );
     
     // Create the controls
     Label xLabel = new Label ( "Enter x: " );
     Label yLabel = new Label ( "Enter y: " );
     sButton = new Button ( " Sum " );
     
     xField = new TextField ( 20 );
     xField.setEditable ( true );
     yField = new TextField ( 20 );
     yField.setEditable ( true );     
     sField = new TextField ( 20 );
     sField.setEditable ( false );
     
     // Set the Grid Layout Manager
     setLayout ( new GridLayout ( 3, 2 ) );
     
     // Add the controls
     add ( xLabel );   add ( xField );
     add ( yLabel );   add ( yField );
     add ( sButton );  add ( sField );
     
     // Create and add the listener to the button
     buttonListener = new ButtonListener ( this );
     sButton.addActionListener ( buttonListener );
     
     // Pack the window and make it visible
     pack();
     setVisible ( true );
  }
  
  public static void main ( String [] args )
  {
    new AddTwo ();  
  }
}

class ButtonListener implements ActionListener
{
  private AddTwo aFrame;
  
  public ButtonListener ( AddTwo someFrame )
  {
    aFrame = someFrame;
  }
  public void actionPerformed ( ActionEvent evt )
  {
      if ( evt.getSource() == aFrame.sButton )
      {
        String xStr = aFrame.xField.getText();
        double x = Double.parseDouble ( xStr );
        
        String yStr = aFrame.yField.getText();
        double y = Double.parseDouble ( yStr );
        
        aFrame.sField.setText ( String.valueOf ( x + y ) );
      }
  }
}

import java.awt.*;
import java.awt.event.*;

public class SimpleWindow extends Frame
{
  public Button qButton;
  public QuitHandler quitHandler;

  public SimpleWindow ()
  {
    // Create a window
    super ( "Simple Window" );

    // Create one button
    qButton = new Button ( "Quit" );

    // Set a flow layout manager
    setLayout ( new FlowLayout ( FlowLayout.CENTER ) );

    // Add the button
    add ( qButton );

    // Create and add the listener to the button
    quitHandler = new QuitHandler ( this );
    qButton.addActionListener ( quitHandler );
    
    // Add the listener to the window
    addWindowListener ( quitHandler );

    // Pack the window and make it visible
    pack();
    setVisible ( true );
  }

  public static void main ( String [] args )
  {
    new SimpleWindow ();
  }
}

class QuitHandler implements ActionListener, WindowListener
{
  private SimpleWindow theWindow;

  public QuitHandler ( SimpleWindow aWindow )
  {
    theWindow = aWindow;
  }

  // Terminate the application
  private void terminate ()
  {
    System.out.println ( "Quitting the application" );
    theWindow.dispose();
    System.exit (0);
  }

  // Invoked when the user clicks the quit button
  public void actionPerformed ( ActionEvent evt )
  {
    if ( evt.getSource() == theWindow.qButton )
      terminate ();
  }
  
  // Invoked when the user clicks the close-box
  public void windowClosing ( WindowEvent evt )
  {
    terminate ();
  }
  
  // Unused methods of the WindowListener interface
  public void windowOpened ( WindowEvent evt ) {}
  public void windowIconified ( WindowEvent evt ) {}
  public void windowDeiconified ( WindowEvent evt ) {}
  public void windowDeactivated ( WindowEvent evt ) {}
  public void windowClosed ( WindowEvent evt ) {}
  public void windowActivated ( WindowEvent evt ) {}
}