Applet not working

Question

I have the following program to draw some curves on an applet. the applet window loads but at the same time i get the following error message:
please help me out.
java.lang.NumberFormatException: null
     at java.lang.Integer.parseInt(Integer.java:415)
     at java.lang.Integer.parseInt(Integer.java:497)
     at Lissajous.<init>(Lissajous.java:30)
     at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)     at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
     at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
     at java.lang.reflect.Constructor.newInstance(Constructor.java:494)
     at java.lang.Class.newInstance0(Class.java:340)
     at java.lang.Class.newInstance(Class.java:293)
     at sun.applet.AppletPanel.createApplet(AppletPanel.java:710)
     at sun.applet.AppletPanel.runLoader(AppletPanel.java:639)
     at sun.applet.AppletPanel.run(AppletPanel.java:319)
     at java.lang.Thread.run(Thread.java:595)
this is my program here:
import javax.swing.JOptionPane;
import java.applet.*;
import java.awt.*;
import java.awt.geom.Line2D;
public class Lissajous extends Applet{
String a;
String b;
//Get user input for n and c.
public void init() {
     a = JOptionPane.showInputDialog(null, "Value of n?");
     b = JOptionPane.showInputDialog(null, "Value of c?");
double w = getWidth();
double h = getHeight();
int n = Integer.parseInt(a);
int c = Integer.parseInt(b);
public void paint (Graphics g0)
     Graphics2D g1 = (Graphics2D) g0;
     double t = 0;
     //scaling and getting
     //the first point of the curve     
     double x0 = ((w-1)/2)*(Math.sin(n*t+c)+1);
     double y0 = (-(h-1)/2)*(Math.sin(t)-1);
     //Get the next point and draw a line to connect the first point with it
     for (t = 0.10; t < w*1000; t+=0.10){
     double x1 = ((w-1)/2)*(Math.sin(n*t+c)+1);
     double y1 = (-(h-1)/2)*(Math.sin(t)-1);
     //the curve
     Line2D.Double tangent = new Line2D.Double(x0,y0,x1,y1);
     g1.draw(tangent);
     x0 = y1;
     y0 = y1;
} // Lissajous

Answer 1

u have written
String a;
String b;
Before execution of init() method,
u r trying to execute these statements.
int n = Integer.parseInt(a);
int c = Integer.parseInt(b);
Since u have not initialized String variables, it gives error.
so better u initialize a & b to some default values, like String a=0, b=0; (Depending upon ur requirements)
or shift these 2 statements into init() method.