Physics class:


package balls;

import java.awt.Color;
import java.awt.Component;
import java.awt.Rectangle;
import java.util.ArrayList;

public class Physics extends Thread {

	private ArrayList<Ball> balls;
	private Component parent;
	private Component boundary;
	private long lastTime;
	private long sleepTime;
	private boolean done;

	public Physics(Component parent, Component boundary, int framesPerSecond) {
		this.parent = parent;
		this.boundary = boundary;
		sleepTime = 1000 / framesPerSecond;
		lastTime = System.currentTimeMillis();
		balls = new ArrayList<Ball>();
		done = false;
		start();
	}

	public synchronized void stopIt() {
		done = true;
	}

	private synchronized boolean getDone() {
		return done;
	}

	public synchronized void addBall(Ball ball) {
		balls.add(ball);
	}

	private synchronized void handleBalls() {
		long currentTime;
		currentTime = System.currentTimeMillis();
		for (int i = 0; i < balls.size(); i++)
			setBallPosition(currentTime - lastTime, balls.get(i));
		for (int i = 0; i < balls.size(); i++)
			for (int j = i + 1; j < balls.size(); j++)
				if (balls.get(i).intersectsBall(balls.get(j))) {
					balls.get(i).setCollided();
					balls.get(j).setCollided();
				}
		lastTime = currentTime;
	}

	private void setBallPosition(long timeDiff, Ball ball) {
		double deltaTime = timeDiff / 1000.0;
		double oldX = ball.getX();
		double oldY = ball.getY();
		double radius = ball.getRadius();
		double velocityX = ball.getVelocityX();
		double velocityY = ball.getVelocityY();
		double newX = oldX + velocityX * deltaTime;
		double newY = oldY + velocityY * deltaTime;
		Rectangle bounds = boundary.getBounds();
		ball.setX(newX);
		ball.setY(newY);
		if (newX - radius <= 0) {
			newX = 2 * radius - newX;
			ball.setVelocityX(Math.abs(velocityX));
		}
		if (newX + radius >= bounds.width) {
			newX = 2 * (bounds.width - radius) - newX;
			ball.setVelocityX(-Math.abs(velocityX));
		}
		if (newY - radius <= 0) {
			newY = 2 * radius - newY;
			ball.setVelocityY(Math.abs(velocityY));
		}
		if (newY + radius >= bounds.height) {
			newY = 2 * (bounds.height - radius) - newY;
			ball.setVelocityY(-Math.abs(velocityY));
		}
	}

	public void run() {
		while (!getDone()) {
			handleBalls();
			parent.repaint();
			try {
				sleep(sleepTime);
			} catch (InterruptedException e) {
			}
		}
	}




}



Ball class:

package balls;

import java.awt.Color;
import java.awt.Graphics;

public class Ball {

	private double x, y; // position
	private final double radius;

	private Color color;

	private double velocityX, velocityY;

	public Ball(final double x, final double y, final double radius,
			final Color color, final double velocityX, final double velocityY) { //added velx and vely... i'll remove if wrong
		this.x = x;
		this.y = y;
		this.radius = radius;
		this.color = color;

		// this.velocityX = this.velocityY = 0;
		this.velocityX = 10;
		this.velocityY = 10;
	} //Ball object

	public void draw(final Graphics g) {
		final double xAdjust = x - radius;
		final double yAdjust = y - radius;
		final double diameter = 2 * radius;

		g.setColor(color);
		g
				.fillOval((int) xAdjust, (int) yAdjust, (int) diameter,
						(int) diameter);
	} //draw()

	public double getX() {
		return x;
	} //getX()

	public double getY() {
		return y;
	} //getY()

	public double getRadius() {
		return radius;
	} //getX()

	public Color getColor() {
		return color;
	} //getColor()

	public double getVelocityX() {
		return velocityX;
	} //getVelocityX()

	public double getVelocityY() {
		return velocityY;
	} //getVelocityY()

	public void setX(double x) {
		this.x = x;
	} //setX()

	public void setY(double y) {
		this.y = y;
	} //setY()

	public void setColor(Color color) {
		this.color = color;
	} //setColor()

	public void setVelocityX(double velocityX) {
		this.velocityX = velocityX;
	} //setVelocityX()

	public void setVelocityY(double velocityY) {
		this.velocityY = velocityY;
	} //setVelocityY()

	public boolean intersectsBall(Ball b) {
		double x1 = b.getX();
		double y1 = b.getY();
		double rad1 = b.getRadius();
		double x2 = x;
		double y2 = y;
		double rad2= radius;

		double distance = Math.sqrt(((y2-y1)*(y2-y1))-(((x2-x1)*x2-x1)));
		if (distance ==  ( rad1+(rad2)) )
			return true;
		else return false;
	} //intersectsBall()

	public void setCollided() {
		this.setColor(Color.red);

	} //setCollided()

} //Ball class


BallsApplet class:

// Brock Haun
// HELP! Nothing is working for part 3. I did everything, but no actual balls are produced on the applet.

package balls;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Rectangle;
import java.util.Random;

import javax.swing.JApplet;

public class BallsApplet extends JApplet {

	private int numBalls = 20; //just for private usage. makes code more readable.
	private Ball balls[] = new Ball[numBalls];

	private boolean stop = false; //private usage
	private Physics physics = new Physics(this,this,10);

	@Override
	public void init() {
		setSize(400, 500);
		//ball = new Ball(30, 30, 10, Color.GREEN);



		for (int i=0;i<numBalls;i++) {
			balls[i]= makeRandomBall();
			physics.addBall(balls[i]);
		} //for





		//physics.addBall(ball);
	} //init()

	@Override
	public void paint(Graphics g) {

		Rectangle bounds = getBounds();
		g.setColor(Color.lightGray);
		g.fillRect(0, 0, bounds.width, bounds.height);

		int redBalls = 0;
		int greenBalls = 0;

		for (int i=0;i<numBalls;i++) {
		 balls[i].draw(g);
		 if (balls[i].getColor().equals(Color.red))
			 redBalls++;
		 else greenBalls++;
		} //for


		//string output
		String greenCount = "Green balls: " + greenBalls;
		String redCount = "Red balls: " + redBalls;

		g.drawString(greenCount, 20, 20);
		g.drawString(redCount, 20, 30);
		g.drawString("Brock Haun" , 300, 10);

		if (greenBalls == 0)
			physics.stopIt();

	} //paint()

	private int randomRange(int low, int high) {
		Random r = new Random();
		int randomInt = r.nextInt(high)+ low;

		return randomInt;
	} //randomRange()

	private Ball makeRandomBall() {
		Random r = new Random();


		int randX = r.nextInt(180)+21; //instructions say between 20 and 200, not inclusive so ceiling=179,floor=21. rand starts at 0.
		int randY = r.nextInt(180)+21;
		int randRad = r.nextInt(7)+3; //same as before,  but BETWEEN 2 and 10, so 3-9

		//making random values for x and y is a 2 step process since we can't declare a negative range.
		//first we make a random int (0 or 1); then we take -1 and square it by the random int (0 or 1). if it is 0,
		//the sign won't change (stay positive). if it is 1, the sign will become negative.

		int sign = r.nextInt(2);

		double randVelX =  (Math.pow(-1, sign)*(r.nextInt(41)));
		double randVelY =  (Math.pow(-1, sign)*(r.nextInt(41)));

		Ball randomBall = new Ball(randX,randY,randRad,Color.green, randVelX, randVelY);

		return randomBall;
	} //makeRandomBall()

} //BallsApplet class