GUI with ActionListener

  import javax.swing.*;
import java.awt.*;

import java.awt.event.*;

public class ScaleConverter2 extends JFrame
                   implements ActionListener{
                

  private JLabel lblValue, lblResult;
  private JButton btnCompute, btnCancel;
  private JRadioButton rbKm, rbMetre, rbCm;
  private JRadioButton rbToKm, rbToMetre, rbToCm;
  private JPanel convertFromPnl, convertToPnl, calculatePnl;
  private JTextField txtValue, txtResult;

  public ScaleConverter2() {
    initConvertFromPnl();
    initConvertToPnl();
    initCalculatePnl();

    getContentPane().setLayout(new GridLayout(3, 1));
    getContentPane().add(convertFromPnl);
    getContentPane().add(convertToPnl);
    getContentPane().add(calculatePnl);

  } //end constructor

  public void initConvertFromPnl() {
    convertFromPnl = new JPanel(new GridLayout(3, 1));
    rbKm = new JRadioButton("Km");
    rbMetre = new JRadioButton("Metre");
    rbCm = new JRadioButton("Cm");
    convertFromPnl.add(rbKm);
    convertFromPnl.add(rbMetre);
    convertFromPnl.add(rbCm);
    convertFromPnl.setBorder(BorderFactory.createTitledBorder("Convert from"));
    
    // Set KM as the default
    rbKm.setSelected(true);
  
  }

  public void initConvertToPnl() {
    convertToPnl = new JPanel(new GridLayout(3, 1));
    rbToKm = new JRadioButton("Km");
    rbToMetre = new JRadioButton("Metre");
    rbToCm = new JRadioButton("Cm");
    convertToPnl.add(rbToKm);
    convertToPnl.add(rbToMetre);
    convertToPnl.add(rbToCm);
    convertToPnl.setBorder(BorderFactory.createTitledBorder("Convert from"));
  
    rbToMetre.setSelected(true);
  }

  public void initCalculatePnl() {
    calculatePnl = new JPanel(new GridLayout(3, 2,5,5));
    lblValue = new JLabel("Value");
    lblResult = new JLabel("Result");
    btnCompute = new JButton("Compute");
    btnCancel = new JButton("Cancel");
    txtValue = new JTextField(10);
    txtResult = new JTextField(10);

    calculatePnl.add(lblValue);
    calculatePnl.add(txtValue);
    calculatePnl.add(lblResult);
    calculatePnl.add(txtResult);
    calculatePnl.add(btnCompute);
    calculatePnl.add(btnCancel);

    calculatePnl.setBorder(BorderFactory.createTitledBorder("Convert from"));
  
    btnCompute.addActionListener(this);
    
  } //end init
  
  
  public void actionPerformed(ActionEvent ev){
    if (ev.getSource() == btnCompute){
      String input = txtValue.getText();//retrieve from textfield
      double value = Double.parseDouble(input);
      double result = 0;
      
      // if the user wants to convert km to m
      if (rbKm.isSelected() && 
          rbToMetre.isSelected()){
          result = value*1000;
      }
      
      txtResult.setText(String.valueOf(result));
    }
  } //end actionPerformed

  public static void main(String args[]) {
    ScaleConverter2 f = new ScaleConverter2();
    f.setSize(400, 400);
    f.setVisible(true);
    f.setTitle("Scale Converter");
  } //end main

} //end class