Building Applications with Buttons

• Example: investment viewer program; whenever button is clicked, interest is added, and new balance is displayed

• Construct an object of the JButton class:

JButton button = new JButton("Add Interest");

• We need a user interface component that displays a message

JLabel label = new JLabel("balance: " + account.getBalance());

• Use a JPanel container to group multiple user interface components together:

JPanel panel = new JPanel();
panel.add(button);
panel.add(label);
frame.add(panel);

• Listener class adds interest and displays the new balance:

class AddInterestListener implements ActionListener
 {
    public void actionPerformed(ActionEvent event)
    {
       double interest = account.getBalance() * INTEREST_RATE / 100;
       account.deposit(interest);
       label.setText("balance=" + account.getBalance());
    }
 }

• Add AddInterestListener as inner class so it can have access
   to surrounding final variables (account and label)

Click here to see the example.

How do you place the "balance: . . ." message to the left of the "Add Interest" button?

   Answer: First add label to the panel, then add button

Why was it not necessary to declare the button variable as final?

   Answer: The actionPerformed method does not access that variable.