Developing with Swing

A Basic Swing Example

To write a simple Swing application we do not have to make too many changes to a standard AWT application. Figure 12.3, “A Simple Swing Application” shows a simple Swing button application, that should provide you with a template for writing your own applications.

Figure 12.3. A Simple Swing Application

A Simple Swing Application

Ths source code for this application is below and in

 3   // A Simple Swing Application
 5   import javax.swing.*;
 6   import java.awt.*;
 7   import java.awt.event.*;
 9   public class SimpleSwingApplication extends JFrame implements ActionListener
10   {
12 	private JButton button1, button2;
13 	private JTextField status;
15 	public SimpleSwingApplication()
16 	{
17 	  // call the parent class constructor - sets the frame title
18 	  super("Simple Swing Example");
20 	  this.status = new JTextField(20);
21 	  this.button1 = new JButton("Button 1");
22 	  this.button2 = new JButton("Button 2");
24 	  this.button1.addActionListener(this);
25 	  this.button2.addActionListener(this);
27 	  // default layout is border layout for frame/jframe
28 	  this.getContentPane().add("North",this.status);
29 	  this.getContentPane().add("Center",this.button1);
30 	  this.getContentPane().add("South",this.button2);
32 	  this.pack();  //set the size of the frame according 
33 			//to the component's preferred sizes
34;	//make the frame visible
35 	}
37 	public void actionPerformed(ActionEvent e)
38 	{
39 	  if (e.getActionCommand().equals("Button 1"))
40 	  {
41 	    this.status.setText("Button 1 Pressed");
42 	  }
43 	  else if (e.getActionCommand().equals("Button 2"))
44 	  {
45 	    status.setText("Button 2 Pressed");
46 	  }
47 	}
49 	public static void main(String args[])
50 	{
51 	   new SimpleSwingApplication();
52 	}
53   }

You will notice that this application does not shutdown correctly when the x button is pressed on the top left side of the application. This is because we have not written the code to handle this. You can shutdown the application by typing "CTRL-C" in the DOS shell window.

Some points about this application:

  • It is a full application, not an applet and so extends the JFrame class, the Swing version of Frame (java.awt.Frame) class.

  • The constructor of this application SimpleSwingApplication class calls its parent class constructor JFrame, calling the JFrame(String title) constructor.

  • T

    he main() method creates an object of the SimpleSwingApplication by calling new SimpleSwingApplication(); , which creates an anonymous object.

  • this.getContentPane() returns the Container object for this JFrame. You cannot add components to the JFrame directly - you must use the getContentPane() method call.

  • this.pack() sets the JFrame object to its components preferred sizes. makes the JFrame object visible on the screen. You usually make this call last as all layout has been carried out and the frame will not be changing sizes and moving after it is displayed.

Example Extra Swing Features

As previously discussed, Swing brings a huge range of components to the Java programming language. Many of the components we discussed previously remain much the same. So for example:

The JButton class replaces the java.awt.Button AWT class and has much the same functionality, with minor differences. For example we can add an image icon to the button surface using the following code:

	Image theImage1 = this.getToolkit().getImage("ok.gif");
	ImageIcon i1 = new ImageIcon(theImage1);
	b1 = new JButton(i1);

Where theImage1 is a regular Image object and the ImageIcon takes an Image object as a constructor parameter to allow of the creation of a component based image display, i.e. one that could be added to a layout or any container etc. Once we have created this ImageIcon object we can pass this to the constructor of JButton to create a button with an image display. This can be seen as in Figure 12.4, “A Swing Image Button Example”. The source code for this example is here -

Figure 12.4. A Swing Image Button Example

A Swing Image Button Example

You will notice that I changed the code in the actionPerformed() method from e.getActionCommand() to e.getSource(). If there is no text on a button, if it was just an image button then there would be no action command (by default) to compare to. It is better in this application to compare to the source of the event, i.e. the object reference. You will however have to insure that your object references are made available to the actionPerformed() method, so should probably be private states of the class.

Core Swing Components

Figure 12.5, “The Swing Component Hierarchy” displays a sample of the Swing Component Hierarchy. This is only a subset of the Swing components that are available for use. You can see in this figure that JApplet, JFrame, JDialog and JComponent are replacements for their equivalent AWT components. JFrame is actually a heavyweight component, it will have a different appearance depending on the Operating System it is running on.

Figure 12.5. The Swing Component Hierarchy

The Swing Component Hierarchy

The Swing equivalent components for the AWT components that we have discussed previously are:

  • JButton - as discussed replaces the AWT Button and has a few changes. For one as in Figure 12.5, “The Swing Component Hierarchy” the parent class is AbstractButton which defines a common behaviour for all buttons, for example there is a two-state JToggleButton class also inheriting from AbstractButton. Button objects can also have an image icon added to the button area.

  • JTextField - replaces the TextField class and remains esentially the same. There is a slight difference in the event structure and we have the added benefit of some derived subclasses, such as JPasswordField which allows a password to be entered using a character mask, and JFormattedTextField to allow for the entry of formatted text, such as a date of birth.

  • JTextArea - replaces the TextArea class and is slightly different. For one thing, the JTextArea does not handle scrolling of the text to allow you to have more control over its functionality. To add a scrolling capability to a JTextArea component you can add it to a JScrollPane component. By default, the line wrapping property is set to false. You can enable line wrapping by using the setWrapStyleWord() method that requires a boolean value, where true means wrap at word boundaries and false means wrap at character boundaries (could be mid-word).

  • JPanel - replaces the Panel class and remains much the same

  • JRadioButton - replaces the RadioButton component and has the added facility of adding an ImageIcon to it. Again you use this with a ButtonGroup object to create a group of buttons where only one may be selected at a time.

  • JCheckBox - replaces the CheckBox component. It is actually a child of the AbstractButton class (as is JButton). As such, you can add an ImageIcon object. Other operation remains the same.

  • JComboBox - is the Swing implementation of ComboBox, a combination of a text field and drop-down list that lets the user either type in a value or select it from a list that is displayed when the user asks for it. The editing capability can also be disabled so that the JComboBox acts only as a drop down list.

  • JList - is the replacement for List. Simiarly to the JTextArea you have to add the component to a JScrollPane to enable scrolling on this component. The items in the list can be changed dynamically at run-time and the component has a special listener swing.event.ListDataListener.

Exercise - Swing Components

Task: Write the application as shown in Figure 12.6, “A Swing Exercise for You!”, where:

Figure 12.6. A Swing Exercise for You!

A Swing Exercise for You!

Hint: Remember to use the API documentation for method calls etc. You will probably have to use the setForeground(Color c) method to change the text colour of the TextArea, unless you can work out a better way.

Solution: My solution is here, but please don't look at it until you have had a good attempt at it -

Other Properties added to Swing Components

ToolTips can be added to components to allow a further description of the functionality of the component. You can use the setToolTipText() method of the JComponent class. When you float over the JButton object the ToolTip will appear as in Figure 12.7, “An Example of Swing ToolTips”.

  this.button1 = new JButton("Image Button 1",icon);
  this.button1.setToolTipText("Press this Button to Display Button 1 Information");
  this.button2 = new JButton("Image Button 2",icon);
  this.button1.setToolTipText("Press this Button to Display Button 2 Information");

Figure 12.7. An Example of Swing ToolTips

An Example of Swing ToolTips

Swing also provides borders for JComponent components. Border is an interface that has many default implementations that you can use, or even create your own. Figure 12.8, “An Example of Swing Borders” shows three different border types that you can use. The JTextField component is surrounded by a MatteBorder with a colour of black, the first JButton component is surrounded by an EmptyBorder and the last JButton component is surrounded by a LineBorder with a line width of 5 pixels, a colour of black and rounded corners set to true. You must import the javax.swing.border.* package to use these pre-defined borders. The code used for the example shown in Figure 12.8, “An Example of Swing Borders” is as below:

 3   EmptyBorder eborder = new EmptyBorder(5,5,5,5);	
 4   MatteBorder mborder = new MatteBorder(10,10,10,10,;
 5   LineBorder lborder = new LineBorder(, 5, true);
 7   this.status = new JTextField(20);
 8   this.status.setBorder(mborder);         
10   this.button1 = new JButton("Image Button 1",icon);
11   this.button1.setToolTipText("Press this Button to Display Button 1 Information");
12   this.button1.setBorder(eborder);
14   this.button2 = new JButton("Image Button 2",icon);
15   this.button2.setToolTipText("Press this Button to Display Button 2 Information");
16   this.button2.setBorder(lborder);

Figure 12.8. An Example of Swing Borders

An Example of Swing Borders

The code for both of these examples can be seen in