add data validation

I have the following code for a future value calculator:

I need to add data validation so that the user must enter data in all three fields:  can someone please get me started adding the validation code?


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



public class FutureValueApp
{
    public static void main(String[] args)
    {
        JFrame frame = new FutureValueFrame();
        frame.setVisible(true);
    }
}

class FutureValueFrame extends JFrame
{
    public FutureValueFrame()
    {
        setTitle("Future Value Calculator");
        centerWindow(this);
        setSize(267, 200);
        setResizable(false);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        JPanel panel = new FutureValuePanel();
        this.add(panel);
    }

    private void centerWindow(Window w)
    {
        Toolkit tk = Toolkit.getDefaultToolkit();
        Dimension d = tk.getScreenSize();
        setLocation((d.width-w.getWidth())/2, (d.height-w.getHeight())/2);
    }
}

class FutureValuePanel extends JPanel implements ActionListener
{
    private JTextField  paymentTextField,
                        rateTextField,
                        yearsTextField,
                        futureValueTextField;
    private JLabel      paymentLabel,
                        rateLabel,
                        yearsLabel,
                        futureValueLabel;
    private JButton     calculateButton,
                        exitButton;

    public FutureValuePanel()
    {
        // display panel
        JPanel displayPanel = new JPanel();
        displayPanel.setLayout(new FlowLayout(FlowLayout.RIGHT));

        // payment label
        paymentLabel = new JLabel("Monthly Payment:");
        displayPanel.add(paymentLabel);

        // payment text field
        paymentTextField = new JTextField(10);
        displayPanel.add(paymentTextField);

        // rate label
        rateLabel = new JLabel("Yearly Interest Rate:");
        displayPanel.add(rateLabel);

        // rate text field
        rateTextField = new JTextField(10);
        displayPanel.add(rateTextField);

        // years label
        yearsLabel = new JLabel("Number of Years:");
        displayPanel.add(yearsLabel);

        // years text field
        yearsTextField = new JTextField(10);
        displayPanel.add(yearsTextField);

        // future value label
        futureValueLabel = new JLabel("Future Value:");
        displayPanel.add(futureValueLabel);

        // future value text field
        futureValueTextField = new JTextField(10);
        futureValueTextField.setEditable(false);
        futureValueTextField.setFocusable(false);
        displayPanel.add(futureValueTextField);

        // button panel
        JPanel buttonPanel = new JPanel();
        buttonPanel.setLayout(new FlowLayout(FlowLayout.RIGHT));

        // calculate button
        calculateButton = new JButton("Calculate");
        calculateButton.addActionListener(this);
        buttonPanel.add(calculateButton);

        // exit button
        exitButton = new JButton("Exit");
        exitButton.addActionListener(this);
        buttonPanel.add(exitButton);

        // add panels to main panel
        this.setLayout(new BorderLayout());
        this.add(displayPanel, BorderLayout.CENTER);
        this.add(buttonPanel, BorderLayout.SOUTH);
    }


    public void actionPerformed(ActionEvent e)
    {
        Object source = e.getSource();
        if (source == exitButton)
            System.exit(0);
        else if (source == calculateButton)
        {
            double payment = Double.parseDouble(paymentTextField.getText());
            double rate = Double.parseDouble(rateTextField.getText());
            int years = Integer.parseInt(yearsTextField.getText());
            double futureValue = FinancialCalculations.calculateFutureValue(
                payment, rate, years);
            NumberFormat currency = NumberFormat.getCurrencyInstance();
            futureValueTextField.setText(currency.format(futureValue));
        }


            }

             }