Tutorials : Java by Example : Section 7 :

Now let's rotate some polygons! This is not quite easy, we will need a few helper functions to do the conversions for us. They can handle any polygon you define and rotate it with the specified angle. Everything should become clear from the sourcecode Only your imagination is the limit now for your own creations!

//Sourcecode import java.awt.*; import java.applet.*; public class Project42 extends Applet implements Runnable { Thread runner; Image Buffer; Graphics gBuffer; //cx, cy = center of rotation int angle, cx, cy; //variables for rotation double radians = 0.0; double cos = 1.0; double sin = 0.0; public void init() { //create graphics buffer, the size of the applet Buffer=createImage(size().width,size().height); gBuffer=Buffer.getGraphics(); cx=size().width/2; cy=size().height/2; } 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(25);} catch (Exception e) { } repaint(); } } int rotate_x(int x, int y) { return ((int) (cx + x * cos - y * sin)); } int rotate_y(int x, int y) { return ((int) (cy + y * cos + x * sin)); } void SetAngle(double a) { radians = (a * 2 * Math.PI) / 360; cos = Math.cos(radians); sin = Math.sin(radians); } void RotatePolygon(int x[], int y[], int n) { int new_x[] = new int [n]; int new_y[] = new int [n]; for(int i=0; i<n; i++) { new_x[i]=rotate_x(x[i], y[i]); new_y[i]=rotate_y(x[i], y[i]); } gBuffer.fillPolygon(new_x, new_y, n); } void drawStuff() { //paint background black gBuffer.setColor(Color.black); gBuffer.fillRect(0,0,size().width,size().height); int radius; int x[]; int y[]; //the triangle radius=140; x= new int[3]; y= new int[3]; x[0]=-20; x[1]=0; x[2]=20; y[0]=-radius+80; y[1]=-radius; y[2]=-radius+80; gBuffer.setColor(Color.red); RotatePolygon(x, y, 3); //the square radius=60; x= new int[4]; y= new int[4]; x[0]=radius; x[1]=radius+50; x[2]=radius+50; x[3]=radius; y[0]=-25; y[1]=-25; y[2]=25; y[3]=25; gBuffer.setColor(Color.blue); RotatePolygon(x, y, 4); //the top-down triangle radius=60; x= new int[3]; y= new int[3]; x[0]=-50; x[1]=0; x[2]=50; y[0]=radius+50; y[1]=radius; y[2]=radius+50; gBuffer.setColor(Color.green); RotatePolygon(x, y, 3); //the hexagon radius=120; x= new int[6]; y= new int[6]; x[0]=-radius; x[1]=-radius+30; x[2]=-radius+60; x[3]=-radius+60; x[4]=-radius+30; x[5]=-radius; y[0]=-20; y[1]=-35; y[2]=-20; y[3]=20; y[4]=35; y[5]=20; gBuffer.setColor(Color.magenta); RotatePolygon(x, y, 6); //display the angle in degrees gBuffer.setColor(Color.yellow); gBuffer.setFont(new Font("Helvetica",Font.PLAIN,13)); gBuffer.drawString("Angle="+angle+"°",10,20); //the center mark gBuffer.setColor(Color.white); gBuffer.drawOval(cx-6,cy-6,12,12); gBuffer.drawLine(cx-12,cy,cx+12,cy); gBuffer.drawLine(cx,cy-12,cx,cy+12); if(angle<360) angle++; else angle=0; SetAngle(angle); } public void update(Graphics g) { paint(g); } public void paint(Graphics g) { drawStuff(); g.drawImage (Buffer,0,0, this); } }