CircleTest.java

import java.awt.Canvas;
import java.awt.Color;
import java.awt.Graphics2D;

import javax.swing.JFrame;
import javax.swing.WindowConstants;

/**
 * @author Roi Atalla
 */
public class CircleTest {
	public static void main(String[] args) {
		JFrame frame = new JFrame("Circle Test");
		frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
		
		Canvas canvas = new Canvas();
		canvas.setSize(800, 600);
		frame.add(canvas);
		
		frame.pack();
		frame.setVisible(true);
		frame.setLocationRelativeTo(null);
		
		
		Graphics2D graphics = (Graphics2D)canvas.getGraphics();
		int width = canvas.getWidth();
		int height = canvas.getHeight();
		
		Circle staticCircle = new Circle(100, 300, 50);
		Circle movingCircle = new Circle(400, 200, 60);
		
		double velX = -75;
		double velY = 40;
		
		long lastTime = System.nanoTime();
		long lastNow = System.nanoTime();
		int frameCount = 0;
		
		while(true) {
			long now = System.nanoTime();
			if(now - lastTime >= 1e9) {
				lastTime += 1e9;
				System.out.println(frameCount);
				frameCount = 0;
			}
			
			frameCount++;
			
			graphics.clearRect(0, 0, width, height);
			
			double delta = (now - lastNow) / 1e9;
			
			double deltaX = velX * delta;
			double deltaY = velY * delta;
			
			movingCircle.x += deltaX;
			movingCircle.y += deltaY;
			
			double radiuses = movingCircle.radius + staticCircle.radius;
			
			if(movingCircle.distanceFrom(staticCircle) < radiuses) {
				Vector2 staticToMoving = new Vector2(movingCircle.x - staticCircle.x, movingCircle.y - staticCircle.y);
				staticToMoving.normalize();
				
				movingCircle.x = staticCircle.x + staticToMoving.x * radiuses;
				movingCircle.y = staticCircle.y + staticToMoving.y * radiuses;
			}
			
			graphics.setColor(new Color(1, 0, 0, 0.5f));
			graphics.fillOval((int)(staticCircle.x - staticCircle.radius), (int)(staticCircle.y - staticCircle.radius), 
			                  (int)staticCircle.radius * 2, (int)(staticCircle.radius * 2));
			
			graphics.setColor(new Color(0, 0, 1, 0.5f));
			graphics.fillOval((int)(movingCircle.x - movingCircle.radius), (int)(movingCircle.y - movingCircle.radius),
			                  (int)movingCircle.radius * 2, (int)(movingCircle.radius * 2));
			
			long diff = System.nanoTime() - now;
			
			try {
				long sleep = 10 * 1000000 - diff;
				
				if(sleep > 0) {
					long millis = sleep / 1000000;
					int nanos = (int)(sleep % 1000000);
					Thread.sleep(millis, nanos);
				}
			} catch(Exception exc) {
				exc.printStackTrace();
			}
			
			lastNow = now;
		}
	}
	
	static class Circle {
		double x, y, radius;
		
		Circle(double x, double y, double radius) {
			this.x = x;
			this.y = y;
			this.radius = radius;
		}
		
		double distanceFrom(Circle other) {
			double diffX = this.x - other.x;
			double diffY = this.y - other.y;
			return Math.sqrt(diffX * diffX + diffY * diffY);
		}
	}
	
	static class Vector2 {
		double x, y;
		
		Vector2(double x, double y) {
			this.x = x;
			this.y = y;
		}
		
		void normalize() {
			double length = length();
			x /= length;
			y /= length;
		}
		
		double length() {
			return Math.sqrt(x * x + y * y);
		}
	}
}