GRAPHICAL USER INTERFACE (GUI) AND OBJECT-ORIENTED DESIGN (OOD) - Java Programming: From Problem Analysis to Program Design - page 351

This class has one method; Line 2 is the first statement of that method. Let us look at the
statement in Line 4:

length = Double.parseDouble(lengthTF.getText());
The length of the rectangle is stored in the text field lengthTF . We use the method
getText to retrieve the string from this text field, specifying the length. Now the value
of the expression lengthTF.getText() is the length, but it is in a string form. So we
need to use the method parseDouble to convert the length string into an equivalent
decimal number. The length is then stored in the variable length . The statement in Line
5 works similarly for the width.
The statements in Lines 6 and 7 compute the area and the perimeter, respectively. The
statement in Line 8 uses the method setText of the class JTextField to display the area.
Because setText requires that the argument be a string, you need to convert the value of the
variable area into a string. The easiest way to do this is to concatenate the value of area to
an empty string. Similar conventions apply for the statement in Line 9.
It follows that the method actionPerformed displays the area and perimeter in the
corresponding JTextFields .
Before creating an action listener for the JButton exitB , let us summarize what we've
done so far to create and register an action event listener:
1. Created a class that implements the interface ActionListener . For
example, for the JButton calculateB we created the class
CalculateButtonHandler .
2. Provided the definition of the method actionPerformed within the
class that you created in Step 1. The method actionPerformed con-
tains the code that the program executes when a specific event is
generated. For example, when you click the JButton calculateB ,
the program should calculate and display the area and perimeter of the
rectangle.
3. Created and instantiated an object of the class type created in Step 1. For
example,

6

for the JButton calculateB we created the object

cbHandler .
4. Registered the event handler created in Step 3 with the object that
generates an action event using the method addActionListener . For
example, for JButton calculateB the following statement registers the
object cbHandler to listen and register the action event:

calculateB.addActionListener(cbHandler);