Section 17.1. Buttons and Labels

17.1. Buttons and Labels

We'll start with the simplest components: buttons and labels. Frankly, there isn't much to say about them. If you've seen one button, you've seen them all, and you've already seen buttons in the applications in Chapter 2 (HelloJava3 and HelloJava4). A button generates an ActionEvent when the user presses it. To receive these events, your program registers an ActionListener, which must implement the actionPerformed( ) method. The argument passed to actionPerformed( ) is the event itself.

There's one more thing worth saying about buttons, which applies to any component that generates an action event. Java lets us specify an "action command" string for buttons (and other components, like menu items, that can generate action events). The action command is less interesting than it sounds. It is just a String that serves to identify the component that sent the event. By default, the action command of a JButton is the same as its label; it is included in action events so that you could use it to figure out which button an event came from. However, often you'll know this from the context of your event listener.

To get the action command from an action event, call the event's getActionCommand( ) method. The following code checks whether the user pressed the button labeled Yes:

     public void actionPerformed(ActionEvent e){         if (e.getActionCommand(  ).equals("Yes") {             //the user pressed "Yes"; do something             ...         }     } 

Yes is a string, not a command per se. You can change the action command by calling the button's setActionCommand( ) method. The following code changes button myButton's action command to "confirm":

     myButton.setActionCommand("confirm"); 

It's a good idea to get used to setting action commands explicitly; this helps to prevent your code from breaking when you or some other developer internationalizes it or otherwise changes the button's label. If you rely on the button's label, your code stops working as soon as that label changes; a French user might see the label Oui rather than Yes.

Swing buttons can have an image in addition to a label. The JButton class includes constructors that accept an Icon object, which knows how to draw itself. You can create buttons with captions, images, or both. A handy class called ImageIcon takes care of loading an image for you and can be used to add an image to a button. The following example shows how this works:

     //file: PictureButton.java     import java.awt.*;     import java.awt.event.*;     import javax.swing.*;     public class PictureButton {       public static void main(String[] args)       {         JFrame frame = new JFrame(  );         Icon icon = new ImageIcon("rhino.gif");         JButton button = new JButton(icon);         button.addActionListener( new ActionListener(  ) {           public void actionPerformed(ActionEvent ae) {             System.out.println("Urp!");           }         });         frame.getContentPane(  ).add( button );         frame.pack(  );         frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );         frame.setVisible(true);       }     } 

The example creates an ImageIcon from the rhino.gif file. Then, a JButton is created from the ImageIcon. The whole thing is displayed in a JFrame. This example also shows the idiom of using an anonymous inner class as an ActionListener.

There's even less to be said about JLabel components. They're just text strings or images housed in a component. There aren't any special events associated with labels; about all you can do is specify the text's alignment, which controls the position of the text within the label's display area. As with buttons, JLabels can be created with Icons if you want to create a picture label. The following code creates some labels with different options:

     // default alignment (CENTER)     JLabel label1 = new JLabel("Lions");     // left aligned     JLabel label2 = new JLabel("Tigers", SwingConstants.LEFT);     //label with no text, default alignment     JLabel label3 = new JLabel(  );     // create image icon     Icon icon = new ImageIcon("rhino.gif");     // create image label     JLabel label4 = new JLabel(icon);     // assigning text to label3     label3.setText("and Bears");     // set alignment     label3.setHorizontalAlignment(SwingConstants.RIGHT); 

The alignment constants are defined in the SwingConstants interface.

We've built several labels, using a variety of constructors and several of the class's methods. To display the labels, just add them to a container by calling the container's add( ) method.

You can set other label characteristics, such as changing their font or color, using the methods of the Component class, JLabel's distant ancestor. For example, you can call setFont( ) and setColor( ) on a label, as with any other component.

Given that labels are so simple, why do we need them at all? Why not find a way to draw a text string directly on the container object? Remember that a JLabel is a JComponent. That means that labels have the normal complement of methods for setting fonts and colors that we mentioned earlier as well as the ability to be persistently and sensibly managed by a layout manager. Therefore, they're much more flexible than a text string drawn procedurally at an arbitrary location within a container. Speaking of layoutsif you use the setText( ) method to change the text of your label, the label's preferred size may change. But the label's container automatically lays out its components when this happens, so you don't have to worry about it.