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Using JButton

A JButton generally represents a button that, when clicked by the user, carries out a particular action. A JButton has a so-called ActionListener attached to it, which in effect defines the task to be performed when the button is clicked. There are generally two patterns for using a JButton. The first is to create an individual action listener for each button in a given window, dialog box etc. The second is to use a single action listener for all the buttons— possibly even making the window/dialog itself be the ActionListener. The first pattern usually looks as follows:

JButton butt = new JButton("Click me!");
butt.addActionListener(new ActionListener() {
  public void actionPerformed(ActionEvent evt) {
    // ... called when button clicked
  }
});

Using this first pattern has the advantage that the grubby details of the button click code is completely hidden from outside callers. At its simplest, the second pattern looks something like this:

public class MyFrame extends JFrame implements ActionListener {
  private JButton button1 = new JButton("Click me!");
  private JButton button2 = new JButton("Click me too!");

  public MyFrame() {
    button1.addActionListener(this);
    button2.addActionListener(this);
    ... add buttons to frame ...
  }

  public void actionPerformed(ActionEvent evt) {
    Object src = evt.getSource();
    if (src == button1) {
      ... perform action for button 1
    } else if (src == button2) {
      ... perform action for button 2
    }
  }
}

This pattern can obviously be a lot more succinct when you have several buttons in your window that each perform simple tasks.

Action commands

In some cases, you might have several buttons that you want to perform the same action. A facility called action commands can occasionally make this easier. In this pattern, you attach a string to each button that defines the type of action that that button is to perform when clicked. The pattern looks as follows:

public class MyFrame extends JFrame implements ActionListener {
  private JButton button1 = new JButton("Exit");
  private JButton button2 = new JButton("Cancel");
  private JButton button3 = new JButton("Also Exit");

  public MyFrame() {
    button1.setActionCommand("EXIT");
    button2.setActionCommand("CANCEL");
    button3.setActionCommand("EXIT");
    button1.addActionListener(this);
    button2.addActionListener(this);
    button3.addActionListener(this);
    ... add buttons to frame ...
  }

  public void actionPerformed(ActionEvent evt) {
    String cmd = evt.getActionCommand();
    if (cmd == "EXIT") {
      ... exit
    } else if (src == "CANCEL") {
      ... cancel
    }
  }
}

In this case, then, buttons 1 and 3 will perform the same action. Notice in the actionPerformed() method that this is one of the few cases when we can break the golden rule about string comparison. Normally, you should compare strings with equals() (or equalsIgnoreCase() etc). In this particular instance, it so happens that we know that Swing will give us back the exact same String object that we passed to setActionCommand(). And we know that that object is a pooled String constant. (If you're not familiar with this concept, then it basically works like this: whenever you write a constant string in your program, all of those constant strings with the same content will actually end up being the selfsame String object. So although we write "EXIT" twice, there's actually only one String object created with the text EXIT.)


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