GRAPHICAL USER INTERFACE (GUI) AND OBJECT-ORIENTED DESIGN (OOD) - Java Programming: From Problem Analysis to Program Design - page 356

in the rectangle program earlier in this chapter, we can create the window by making
the application extend the class JFrame . To get access to the container, we will use
a reference variable of the Container type. To create labels, we use objects of type
JLabel ; to create text fields, we use objects of type JTextField . Suppose that we
have the following declarations:

JLabel celsiusLabel; //label Celsius

JLabel fahrenheitLabel; //label Fahrenheit

JTextField celsiusTF; //text field Celsius
JTextField fahrenheitTF; //text field Fahrenheit
When the user enters the temperature in the text field celsiusTF and presses the
Enter key, we want the program to show the equivalent temperature in the text field
fahrenheitTF and vice versa.
Recall that when you click a JButton , it generates an action event. Moreover, the
action event is handled by the method actionPerformed of the interface
ActionListener . Similarly, when you press the Enter key in a text field, it
generates an action event. Therefore, we can register an action event listener with
the text fields celsiusTF and fahrenheitTF to take the appropriate action.
Based on this analysis and the GUI shown in Figure 6-9, you can design an event-
driven algorithm as follows:
1. Have a listener in each text field.
2. Register an event handler with each text field.
3. Let each event handler registered with a text field do the following:
a. Get the data from the text field once the user presses Enter .
b. Apply the corresponding formula to perform the conversion.
c. Set the value of the other text field.
This process of adding an event listener and then registering the event listener to a
text field is similar to the process we used to register an event listener to a JButton
earlier in the chapter. (This process will be described later in this programming
example.)

The input to the program is either the temperature in Celsius or the temperature in
Fahrenheit. If the input is a value for Celsius, then the program calculates the
equivalent temperature in Fahrenheit. Similarly, if the input is a value for Fahrenheit,
then the program calculates the equivalent temperature in Celsius. Therefore, the
program needs the following variables:

VARIABLES,
OBJECTS,
AND NAMED
CONSTANTS

double celsius; //variable to hold Celsius

double fahrenheit; //variable to hold Fahrenheit