Learning Java, 4th Edition

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, you’ll often 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 ...