Tutorials : Java by Example : Section 7 :

To rotate things, we need to convert polar to cartesian coordinates:
x = r * Math.cos(theta); and y = r * Math.sin(theta);
Polar coordinates (r, theta) are just another way of specifying points in a 2D plane, where r represents the distance of a point from the center and theta the angle from the horizontal axis (x-axis) through the center. The following applet shows this principle in a minimized form, it rotates a hand around the center and displays the angle accordingly.

//Sourcecode import java.awt.*; import java.applet.*; public class Project40 extends Applet implements Runnable { Thread runner; Image Buffer; Graphics gBuffer; int angle; public void init() { //create graphics buffer, the size of the applet Buffer=createImage(size().width,size().height); gBuffer=Buffer.getGraphics(); } public void start() { if (runner == null) { runner = new Thread (this); runner.start(); } } public void stop() { if (runner != null) { runner.stop(); runner = null; } } public void run() { while(true) { try {runner.sleep(250);} catch (Exception e) { } repaint(); } } void drawStuff() { //paint background black gBuffer.setColor(Color.black); gBuffer.fillRect(0,0,size().width,size().height); //get the center of the applet Point p=new Point(size().width/2, size().height/2); //define the hand length int LENGTH=70; //calculate the vector, using the constant Pi double vector = angle * Math.PI*2 / 360.0; int vx=(int)(p.x+LENGTH*Math.sin(vector)); int vy=(int)(p.y-LENGTH*Math.cos(vector)); gBuffer.setColor(Color.yellow); //the hand gBuffer.drawLine(p.x, p.y, vx, vy); //the round "axis" gBuffer.drawOval(p.x-6,p.y-6,12,12); //display the angle in degrees gBuffer.setFont(new Font("Helvetica",Font.PLAIN,13)); gBuffer.drawString("Angle="+angle+"°",10,20); //increase the angle, to move the hand if(angle<360) angle++; else angle=0; } public void update(Graphics g) { paint(g); } public void paint(Graphics g) { drawStuff(); g.drawImage (Buffer,0,0, this); } }