Graphical User Interfaces - Building Java Programs: A Back to Basics Approach - page 846

the GUI itself. Because this code is inside the GUI object's constructor, we pass the
GUI as a parameter using the keyword this :
// attach GUI as event listener to Compute button
computeButton.addActionListener(this);
The second version of our BMI GUI used a separate class called RunBmiGui2 as a
client to run the GUI. However, using a second class for such a minimal client pro-
gram is a bit of a waste. It is actually legal to place the main method in the BMI GUI
class itself and not use the RunBmiGui2 class. If we do this, the class becomes its
own client, and we can just compile and run the GUI class to execute the program.
(Static methods like main are generally placed above any fields, constructors, and
instance methods in the same file.)
After we implement the listener code and incorporate the main method, the BMI
GUI program is complete:
1 // A GUI to compute a person's body mass index (BMI).
2 // Final version with event handling.
3
4 import java.awt.*;
5 import java.awt.event.*;
6 import javax.swing.*;
7
8 public class BmiGui3 implements ActionListener {
9 // BmiGui3 is its own runnable client program
10 public static void main(String[] args) {
11 BmiGui3 gui = new BmiGui3();
12 }
13
14 // onscreen components stored as fields
15 private JFrame frame;
16 private JTextField heightField;
17 private JTextField weightField;
18 private JLabel bmiLabel;
19 private JButton computeButton;
20
21 public BmiGui3() {
22 // set up components
23 heightField = new JTextField(5);
24 weightField = new JTextField(5);
25 bmiLabel = new JLabel("Type your height and weight");
26 computeButton = new JButton("Compute");
27
28 // attach GUI as event listener to Compute button