Graphical User Interfaces - Building Java Programs: A Back to Basics Approach - page 843

5 import javax.swing.*;
6
7 public class BmiGui1 {
8 public static void main(String[] args) {
9 // set up components
10 JTextField heightField = new JTextField(5);
11 JTextField weightField = new JTextField(5);
12 JLabel bmiLabel = new JLabel(
13 "Type your height and weight");
14 JButton computeButton = new JButton("Compute");
15
16 // layout
17 JPanel north = new JPanel( new GridLayout(2, 2));
18 north.add( new JLabel("Height: "));
19 north.add(heightField);
20 north.add( new JLabel("Weight: "));
21 north.add(weightField);
22
23 // overall frame
24 JFrame frame = new JFrame("BMI");
25 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
26 frame.setLayout( new BorderLayout());
27 frame.add(north, BorderLayout.NORTH);
28 frame.add(bmiLabel, BorderLayout.CENTER);
29 frame.add(computeButton, BorderLayout.SOUTH);
30 frame.pack();
31 frame.setVisible( true );
32 }
33 }
The program currently does nothing when the user presses the Compute button.
Object-Oriented GUIs
Let's think about how to make our BMI GUI respond to clicks on the Compute but-
ton. Clicking a button causes an action event, so we might try putting an
ActionListener on the button. But the code for the listener's actionPerformed
method would need to read the text from the height and weight text fields and use
that information to compute the BMI and display it as the text of the central label.
This is a problem because the GUIs we've written so far were not built to allow so
much interaction between components.
To enable listeners to access each of the GUI components, we need to make our GUI
itself into an object. We'll declare the onscreen components as fields inside that object
and initialize them in the GUI's constructor. In our BmiGui1 example, we can convert
much of the code that is currently in the main method into the constructor for the GUI.