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POBasic GUI: Computing for the area and perimeter of a rectangle, code works but "variables are never read"
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copied!<p>I've already come up with a GUI code that computes for the area and perimeter of a rectangle when a user inputs both the length and the width:</p> <pre><code>import java.awt.*; import java.awt.event.*; import javax.swing.*; public class RectangleGUI extends JFrame { JLabel lengthL; JLabel widthL; JLabel areaL; JLabel perimeterL; JTextField lengthTF; JTextField widthTF; JTextField areaTF; JTextField perimeterTF; JButton CalculateB; JButton ExitB; CalculateButtonHandler cbHandler; ExitButtonHandler ebHandler; public RectangleGUI() { setTitle("Area and Perimeter of a Rectangle"); lengthL = new JLabel("Enter the length: ", JLabel.RIGHT); widthL = new JLabel ("Enter the width: ", JLabel.RIGHT); areaL = new JLabel ("Area: ", JLabel.RIGHT); perimeterL = new JLabel ("Perimeter: ", JLabel.RIGHT); lengthTF = new JTextField(10); widthTF = new JTextField (10); areaTF = new JTextField (10); perimeterTF = new JTextField (10); CalculateB = new JButton("Calculate"); cbHandler = new CalculateButtonHandler(); CalculateB.addActionListener(cbHandler); ExitB = new JButton("Exit"); ebHandler = new ExitButtonHandler(); ExitB.addActionListener(ebHandler); Container pane = getContentPane(); pane.setLayout(new GridLayout(5,2)); pane.add(lengthL); pane.add(lengthTF); pane.add(widthL); pane.add(widthTF); pane.add(areaL); pane.add(areaTF); pane.add(perimeterL); pane.add(perimeterTF); pane.add(CalculateB); pane.add(ExitB); setSize(400,300); setVisible(true); } public class CalculateButtonHandler implements ActionListener { public void actionPerformed (ActionEvent e) { double l, w, area, peri; l=Double.parseDouble(lengthTF.getText()); w=Double.parseDouble(widthTF.getText()); area=l*w; peri=2*(l+w); areaTF.setText("" + area); perimeterTF.setText("" + peri); } } public class ExitButtonHandler implements ActionListener { public void actionPerformed (ActionEvent e) { System.exit(0); } } public static void main(String[] args) { RectangleGUI rectObject = new RectangleGUI(); } } </code></pre> <p>This code should run on the console but it bugs me that on Eclipse, <code>RectangleGUI</code> in <code>public class RectangleGUI extends JFrame {</code> is underlined with a yellow squiggly line because "The serializable class RectangleGUI does not declare a static final serialVersionUID field of type long".</p> <p>In the latter part of the code, <code>rectObject</code> is also underlined because the variable is apparently "never read". I know I should use this variable at one point but when I try to, the code messes up and won't work again.</p> <p>I would like to understand why this is happening. I am only a beginner and I want to be able to avoid these mistakes in the future.</p> <p>ALSO: During run-time, I tried not to put a value in the "width" text field and the Eclipse console returned an error message. What can I do that instead of Eclipse alerting me that something is wrong, a GUI prompt will pop-up telling me to put a value in "width"? I've tried experimenting and failed time and time again.</p> <p>This is not homework, but I am discovering GUI for the first time, and a little insight on why the errors appear the way they are would be very much appreciated. Thanks!</p>

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