Integral function-why it can not run

Question

Hi all,
I am trying to write a code to draw a graph and calculate an area under the graph of sin function. When I run the code, it is working, but when I try to displace the GUI, there are two errors.
- the graph did not appear in the GUI. It should appeare as I already add GPanel into bottomPanel and then add bottomPanel into contentPane. Why?
-when I click button areaButton to execute the calculation. Nothing appear at all. And in the DOS comand, it indicates that java.lang.NumberFormatExecption : empty String. Why?
Can anyone help me to correct the code please. Below is my code.
import java.awt.*;
import javax.swing.*;
import java.text.*;
import java.awt.event.*;
import java.lang.*;
public class IntegralF extends JFrame {  
               // This class is to form a frame
//     double x1, double x2, double a,double  k,double  h,double  fx,double s1,double s2;
     JTextField xText;
     JTextField aText;
     JTextField kText;
     JLabel outputLabel;
     MyPanel GPanel;
        JPanel contentPane;
     //This method is called from within the constructor to initialize the form.
     public IntegralF()     {
          initComponents();
    private void initComponents() {
        MyPanel GPanel= new MyPanel();
        JPanel topPanel = new JPanel();
        JPanel bottomPanel = new JPanel();
        JPanel titlePanel = new JPanel();
        JPanel explainPanel = new JPanel();
        JPanel kPanel = new JPanel();
        JPanel aPanel = new JPanel();
        JPanel xPanel = new JPanel();
        JPanel controlPanel = new JPanel();
        JPanel outputPanel = new JPanel();
        JLabel titleLabel = new JLabel();
        JLabel explainLabel = new JLabel();
        JLabel kLabel = new JLabel();
        JLabel aLabel = new JLabel();
        JLabel xLabel = new JLabel();
        outputLabel = new JLabel();
        kText = new JTextField();
        aText = new JTextField();
        xText = new JTextField();
        JButton areaButton = new JButton();
        JButton exitButton = new JButton();
        contentPane = (JPanel) this.getContentPane();
        setTitle("Simpson's Rule for Definite Integral of Function");
        addWindowListener(new WindowAdapter() {
            public void windowClosing(WindowEvent evt) {
                exitForm(evt);
     // Set bottom Panel
        bottomPanel.setLayout(new GridLayout(10, 0));
     GPanel.setSize(200,200);
        bottomPanel.add(GPanel,BorderLayout.SOUTH);
        getContentPane().add(bottomPanel, BorderLayout.SOUTH);
        //Set Top Panel
        topPanel.setLayout(new GridLayout(8, 0));
        titleLabel.setText("This program is used to calculate an integral of function f(x) = kxsin(x-a)" );
//        Label1.setLayout(new GridLayout(3,0));
        titlePanel.add(titleLabel, BorderLayout.NORTH);
        topPanel.add(titlePanel, null);
        explainLabel.setText("Enter values of k, a, and x (x is in radian)" );
        explainPanel.setLayout(new GridLayout(3,0));
        explainPanel.add(explainLabel, null);
        topPanel.add(explainPanel, null);
        kLabel.setText("k");
        kText.setColumns(3);
        kPanel.add(kLabel, BorderLayout.EAST);
        kPanel.add(kText, BorderLayout.CENTER);
        topPanel.add(kPanel, null);
        aLabel.setText("a");
        aText.setColumns(3);
        aPanel.add(aLabel, BorderLayout.EAST);
        aPanel.add(aText, BorderLayout.CENTER);
        topPanel.add(aPanel, null);
        xLabel.setText("x");
        xText.setColumns(3);
        xPanel.add(xLabel, BorderLayout.WEST);
        xPanel.add(xText, BorderLayout.CENTER);
        topPanel.add(xPanel, null);
        outputPanel.setBorder(new javax.swing.border.EtchedBorder(null, Color.blue));
        outputPanel.setBackground(Color.white);
        outputPanel.add(outputLabel, null);
        topPanel.add(outputPanel, null);
        areaButton.setText("Area");
     areaButton.addActionListener(new MyActionListener());
        controlPanel.add(areaButton, null);
        exitButton.setText("Exit");
        exitButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent evt) {
                exitButtonActionPerformed(evt);
        controlPanel.add(exitButton, null);
        topPanel.add(controlPanel, null);
     getContentPane().add(topPanel, BorderLayout.NORTH);
        pack();
        private void exitButtonActionPerformed(ActionEvent evt) {
        System.exit(0);
          private void exitForm(WindowEvent evt) {
        System.exit(0);
        public static void main(String args[]) {
        new IntegralF().setVisible(true);
   /**Overriding so that it exits when the window is closed.*/
    protected void processWindowEvent(WindowEvent e) {
         super.processWindowEvent(e);
         if (e.getID() == WindowEvent.WINDOW_CLOSING) {
               System.exit(0);
     class MyActionListener implements ActionListener     {
          public void actionPerformed(ActionEvent e) {
            double  x2 = Double.parseDouble(xText.getText());
            double  k = Double.parseDouble(kText.getText());
            double  a = Double.parseDouble(aText.getText());
            GPanel.drawGraph(a,k,x2);
            GPanel.repaint();
                  double fx = Function(a, k, x2);
            outputLabel.setText(Double.toString(fx));
     double Function(double a,double k,double x2){
     // VARIABLE DECLARATION
     double s=0;
     double s1=0;
     double s2=0;
     double xEven=0;
     double xOdd=0;
     double h=0;
     double sumOdd=0;
     double sumEven=0;
      h= (double) x2/1000;
      System.out.println("h =" + h);
       // ODD NUMBERS OPERATION
      if ((k!=0)&&(x2!=0)){
         for(int i=1; i < 1000/2 +1 ; i++){
             xOdd =  ((2 * i) -1) * h;
             s1 = 4 * k * xOdd * Math.cos(xOdd-a);
             sumOdd = sumOdd + s1;
          // EVEN NUMBERS OPERATION
        for (int j=1; j < (1000/2)  ; j++){
          xEven= 2 * j * h;
          s2 = 2 * k * xEven * Math.cos(xEven - a);
          sumEven = sumEven + s2;
          // ADDITION TO OBTAIN FINAL SIMPSON'S
        s=(sumEven + sumOdd + (x2 * k * Math.cos(x2-a))) * (h/3);
      else if (k==0){
        s=0;     
      else if (x2==0){
        s=0;
      return s;
class MyPanel extends JPanel {
   double a, k, x2;
   public static final int STEPS = 200;
   void GPanel(double aC, double kC, double x2C) {
     a = aC;
     k = kC;
     x2 = x2C;
     this.setBackground(Color.yellow);
     this.setLayout(null);
    public void paint(Graphics g) {
      super.paint(g);
      g.setColor(Color.blue);
      for (int i = 0; i <= STEPS; i++) {
           //THE STARTING POINT
         double oldX = i * x2 /((double) STEPS);
         double oldY = k * oldX * Math.cos(oldX - a);
         double[] oldXY = {oldX, oldY};
         //THE ENDING POINT
         double newX = (i+1) * x2 /((double) STEPS);
         double newY = k * newX * Math.cos(newX - a);
         double[] newXY = {newX, newY};
         g.drawLine(pxlXY(oldXY)[0], pxlXY(oldXY)[1], pxlXY(newXY)[0],pxlXY(newXY)[1]);
      //DRAW A Y AXIS, TEXT TABLES ETC..
      g.drawLine(5, 130, 465, 130);
      g.drawString(Integer.toString((int)(k*x2)), 5, 10);
      g.drawString(Integer.toString((int)(k*x2*(-1))), 5, 255);
      g.drawString(Integer.toString(0), 5, 130);
      g.drawString(Integer.toString((int)x2), 450, 130);
   private int[] pxlXY(double[] xy) {
       int pxlX = (int) (xy[0] * 460 / x2) + 5;
       int pxlY = (int) (xy[1] * 125 / (k * x2) ) + 125 + 5;
       int[] points = {pxlX, pxlY};
       return points;
   public void drawGraph(double aC, double kC, double x2C) {
     a = aC;
     k = kC;
     x2 = x2C;
     this.repaint();
}And this is a code to display GUI
public class Application {
  /*  Constructor */
  public Application() {
    IntegralF frame = new IntegralF();
    frame.setVisible(true);
  /* Main method */
  public static void main(String[] args) {
    new Application();

Answer 1

first bug: in initComponents() you say
MyPanel GPanel = new MyPanel();Since GPanel is a member of the IntegralF class you probably mean:
GPanel = new MyPanel();Just a hint, use a lower case letter for variables:
JPanel p1 = null;
GPanel p2 = null; // looks like another class name !
MyPanel gPanel = null; // gPanel looks more like a variable than a classYou get and store contentPane, but then further on you say
getContentPane().add(...). Do it one way or the other.
Don't "mix n match"
You set a grid layout on bottomPanel, then try to add GPanel to the
SOUTH of bottomPanel - that doesn't make sense !