How to Convert miles to km and vice versa with 2 JtextFields in Java?

Question

I got a windows prompt box that needs to convert miles to km and vice versa in 2 fields. When the user fills field1, it converts it in Field2. When the user changes the value of a field it recalculates the fields.

I can't get passed the first field, it only calculates the first statement.

This is the action of the Calculate button:

 if (jTextField1.getText() != null ) {
       jTextField2.setText("" + Double.parseDouble(jTextField1.getText()) * 0.62137);
    } 

    if (jTextField2.getText() != null) {

        jTextField1.setText("" + Double.parseDouble(jTextField2.getText()) * 1.62137);
    }

Answer 1

if (jTextField1.getText() != null ) {
   jTextField2.setText("" + Double.parseDouble(jTextField1.getText()) * 0.62137);
} 

if (jTextField2.getText() != null) {

    jTextField1.setText("" + Double.parseDouble(jTextField2.getText()) * 1.62137);
}

The first if statement sets text to jTextField2 which no longer makes it null for next if

Maybe take out the second if and do something like this

if (jTextField1.getText() != null ) {
   double value = Double.parseDouble(jTextField1.getText());
   String text1 = (value * 0.62137) + "";
   String text2 = (value * 1.62137) + "";

   jTextField2.setText(text1);
   jTextField1.setText(text2);
} 

Edit: After much hacking

import java.awt.BorderLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.SwingConstants;
import javax.swing.SwingUtilities;

public class MilesKiloConverter {

private JTextField jtfMiles = new JTextField(15);
private JTextField jtfKilos = new JTextField(15);

private JLabel jlbMiles = new JLabel("miles");
private JLabel jlbKilos = new JLabel("km");

private JButton compute = new JButton("Compute");
private JFrame frame = new JFrame();

double km = 0;
double miles = 0;

public void createAndSHowGui() {
    JPanel p1 = new JPanel(new GridLayout(2, 2, 5, 5));
    p1.add(jtfMiles);
    p1.add(jlbMiles);
    p1.add(jtfKilos);
    p1.add(jlbKilos);

    frame.add(p1, BorderLayout.CENTER);
    frame.add(compute, BorderLayout.SOUTH);

    jtfMiles.setHorizontalAlignment(SwingConstants.RIGHT);
    jtfKilos.setHorizontalAlignment(SwingConstants.RIGHT);

    jtfKilos.setFocusable(true);


    compute.addActionListener(new ActionListener(){
        @Override
        public void actionPerformed(ActionEvent e) {
            double newKm = 0;
            double newMiles = 0;

            if ("".equals(jtfMiles.getText())) {
                newKm = Double.parseDouble(jtfKilos.getText());
                jtfMiles.setText(String.valueOf(newKm * 1.62137));
                km = newKm;
            } else if ("".equals(jtfKilos.getText())) {
                newMiles = Double.parseDouble(jtfMiles.getText());
                jtfKilos.setText(String.valueOf(newMiles * 0.62137));

            } else {
                newMiles = Double.parseDouble(jtfMiles.getText());
                newKm = Double.parseDouble(jtfKilos.getText());

                double milesDiff = miles - newMiles;
                double kiloDiff = km - newKm;

                if (Math.abs(milesDiff) > Math.abs(kiloDiff)) {
                    jtfKilos.setText(String.valueOf(newMiles * 1.62137));
                    km = newKm;
                    miles = newMiles;
                } else {
                    jtfMiles.setText(String.valueOf(newKm * 0.62137));
                    km = newKm;
                    miles = newMiles;
                }
            }
        }
    });

    frame.setTitle("Miles/Kilometers Converter");
    frame.pack();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);

}

public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable(){
        @Override
        public void run(){
            new MilesKiloConverter().createAndSHowGui();
        }
    });
}
}