Interfaces and Polymorphism - Java Concepts - page 569

S ELF C HECK

15. Why would an inner class method want to access a variable from a
surrounding scope?

16. If an inner class accesses a local variable from a surrounding scope,
what special rule applies?

9.8 Building Applications with Buttons

In this section, you will learn how to structure a graphical application that contains
buttons. We will put a button to work in our simple investment viewer program.
Whenever the button is clicked, interest is added to a bank account, and the new
balance is displayed (see Figure 4 ).

First, we construct an object of the JButton class. Pass the button label to the
constructor:

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

We also need a user interface component that displays a message, namely the current
bank balance. Such a component is called a label. You pass the initial message string
to the JLabel constructor, like this:

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

Figure 4

An Application with a Button

414
415

The frame of our application contains both the button and the label. However, we
cannot simply add both components directly to the frameȌthey would be placed on