Java Program to Simulate Motion of Simple Pendulum

This is a java program to simulate motion of simple pendulum using java graphics. This was a problem given in computer graphics lab, which i was supposed to do using graphics.h header file in C. This is the java version of the same. If you are looking for a C Program, here it is: C Program to Simulate Simple Pendulum Motion - Computer Graphics Lab.

Output is as follows:

import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JFrame;


public class JavaApplication1 extends JFrame{

    int pivotx,pivoty;
    double thetamax,theta;;
    double len=260;
    int x,y,ymax,xmax;
    int bobradius=30;
    int xsign=-1,ysign=1;
    double omega;
    
    public JavaApplication1()
    {
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setSize(600,400);
        thetamax=60*Math.PI/180;
        pivotx=getWidth()/2;
        pivoty=40;
        ymax=(int) (pivoty+len*Math.cos(thetamax));
        xmax=(int) (pivotx+len*Math.sin(thetamax));
        x=xmax;
        y=ymax;
        theta=thetamax;
        System.out.println("pivotx:"+pivotx+" pivoty:"+pivoty);
        System.out.println("starting x:"+x+ "y:"+y);
        System.out.println("starting theta="+theta);
        t.start();
    }

    @Override
    public void paint(Graphics g) {
        System.out.println("called");        
        g.setColor(Color.WHITE);
        g.fillRect(0, 0,getWidth(),getHeight());
        g.setColor(Color.red);
        g.fillOval(pivotx-4,pivoty-4,8,8);
        g.drawLine(pivotx,pivoty, x,y);
        g.fillOval(x-bobradius/2,y-bobradius/2,bobradius,bobradius);
        
    }
    
    Thread t=new Thread()
    {
      public void run()
      {
    try {
      while(true)
      {
      if(x>=pivotx+Math.abs(len*Math.sin(thetamax)))
        {
        System.out.println("right extreme");
        xsign=-1;
        ysign*=-1;
        x=xmax-1;
        Thread.sleep(40);
        }
      else if(x<=pivotx-Math.abs(len*Math.sin(thetamax)))
      {
          System.out.println("left extreme");
          ysign*=-1;
          xsign=1;
          x=(int) (pivotx-Math.abs(len*Math.sin(thetamax))+2);
          Thread.sleep(40);
      }
      else if(y>=pivoty+len)
        {
              ysign*=-1;
              System.out.println("mean position");
        }

      omega=y/60*Math.PI/180;
      double decr=xsign*omega;
          System.out.println("decrement:"+decr);
      theta=theta+decr;
      x=(int) (pivotx+len*Math.sin(theta));
      y=(int) (pivoty+len*Math.cos(theta));
          System.out.println("new theta:"+theta);
      repaint();
      Thread.sleep(40);
    
      }
            } catch (InterruptedException ex) {}
      }
    };
    
    
    public static void main(String[] args) {
        new JavaApplication1().setVisible(true);
    }
    
}