How to send output to a specific field?

Question

Can someone point me in the right direction to get an output from a caculation to go into a specific Jtext field? What I want it to do is gather the info from what the user puts in the fields, then the program does the calculation, then out put the calculation to another text field.
thanks!
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.ItemEvent;
import java.awt.event.ItemListener;
public class Mortgage5 extends JFrame implements ItemListener, ActionListener {
    // set up row 1
    JPanel row1 = new JPanel();
    JLabel principleLabel = new JLabel("Please enter your Principle with no commas: ", JLabel.RIGHT);
    JTextField principle = new JTextField(10);
    //set up row 2
    JPanel row2 = new JPanel();
    JLabel termLabel = new JLabel("Please enter the term of the loan in years: ", JLabel.RIGHT);
    JTextField term = new JTextField(10);
    //set up row 3
    JPanel row3 = new JPanel();
    JLabel aprLabel = new JLabel("Please enter your interest rate then press the enter key: ", JLabel.RIGHT);
    JTextField apr = new JTextField(10);
    //set up row 4 the monthly payment output
    JPanel row4 = new JPanel();
    JLabel paymentLabel = new JLabel("Here is your monthly payment: ", JLabel.RIGHT);
    JTextField payment = new JTextField(10);
    //setup row 5 clear or exit
    JPanel row5 = new JPanel();
    JButton calculate = new JButton("Caculate Monthly Payments");
    JButton clear = new JButton("Clear Fields");
    JButton quit = new JButton("Quit");
    public Mortgage5() {
        super("Mortgage Calculator");
        setSize(600, 500);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        GridLayout layout = new GridLayout(5, 1, 10, 5);
        Container pane = getContentPane();
        pane.setLayout(layout);
        FlowLayout layout1 = new FlowLayout(FlowLayout.LEFT, 10, 10);
        row1.setLayout(layout1);
        row1.add(principleLabel);
        row1.add(principle);
        pane.add(row1);
        FlowLayout layout2 = new FlowLayout(FlowLayout.LEFT, 10, 10);
        row2.setLayout(layout2);
        row2.add(termLabel);
        row2.add(term);
        pane.add(row2);
        FlowLayout layout3 = new FlowLayout(FlowLayout.LEFT, 10, 10);
        row3.setLayout(layout3);
        row3.add(aprLabel);
        row3.add(apr);
        pane.add(row3);
        FlowLayout layout4 = new FlowLayout(FlowLayout.LEFT, 10, 10);
        row4.setLayout(layout4);
        row4.add(paymentLabel);
        row4.add(payment);
        pane.add(row4);
        FlowLayout layout5 = new FlowLayout(FlowLayout.CENTER, 10, 10);
        row5.setLayout(layout5);
        row5.add(calculate);
        row5.add(clear);
        row5.add(quit);
        Mortgage5.this.setVisible(false);
        pane.add(row5);
        setVisible(true);
        //Add Listeners
        calculate.addActionListener(this);
        clear.addActionListener(this);
        quit.addActionListener(this);
        principle.addActionListener(this);
        term.addActionListener(this);
        apr.addActionListener(this);     
    public static void main(String[] arguments) {
        Mortgage5 frame = new Mortgage5();
    public void actionPerformed(ActionEvent event) {
        if (event.getSource().equals(clear)) {
            principle.setText(null);
            term.setText(null);
            apr.setText(null);
            payment.setText(null);
        if (event.getSource().equals(quit)) {
            System.exit(0);
        if (event.getSource().equals(calculate)) {    
            double interestRate = Integer.parseInt(apr.getText());
            int loanYears = Integer.parseInt(term.getText());
            double prin = Integer.parseInt(principle.getText());
            //double principle;
            double month_payment;
            double monthlyinterest;
            int months;
            //allows for currency format
            //NumberFormat formatter = NumberFormat.getCurrencyInstance(Locale.US);
            monthlyinterest = (interestRate / 12)/100;          //monthly interest
            months = (loanYears * 12);
            //monthly payment calculation
            month_payment = (prin * monthlyinterest) / (1-Math.pow(1 + monthlyinterest, - months));      
    public void itemStateChanged(ItemEvent event) {
}

Answer 1

payment.setText(""+month_payment);
or
payment.setText(Double.toString(month_payment));
or
payment.setText(new DecimalFormat("###,###.##").format(month_payment));