This is my Solution on CS106A - Section 20 - Problem 3 - Random Circles from the Computer Science Course CS106A of Prof. Mehran Sahami at the STANFORD University.

Question: On some runs of this program you might not see ten circles. Why?

My Answer: Since there is no special controlling on where to place new circles on the canvas it is possible that it puts a new larger circle on top of an older smaller circle, covering the older circle completly. So its possible to have LESS THEN 10 circles visible on the canvas.

RandomCircles.java

/*
 * File: RandomCircles.java
 * -------------------
 * Name: Axel Werner [mail@awerner.myhome-server.de]
 * Section Leader:
 * 
 * CS106A - Section 20 - Problem 3 - Random Circles
 * 
 * Write a GraphicsProgram that draws a set of ten circles 
 * with different sizes, positions, and colors. 
 * Each circle should have a randomly chosen color, 
 * a randomly chosen radius between 5 and 50 pixels, and a 
 * randomly chosen position on the canvas, subject to the
 * condition that the entire circle must fit inside the 
 * canvas without extending past the edge.
 * 
 */
 
import acm.graphics.*;
import acm.program.*;
import acm.util.*;
 
import java.awt.*;
 
/**
 * Program to display random colored and random size circles
 * on the canvas. Number of circles and valid radiuses are 
 * configurable by static finals (class variables).
 * 
 * @author Axel Werner [mail@awerner.myhome-server.de]
 *
 */
public class RandomCircles extends GraphicsProgram {
 
	private static final int MAX_CIRCLES = 10;
 
	private static final int MIN_RADIUS = 5;
	private static final int MAX_RADIUS  = 50;
 
/**
 * Main Program	
 */
	public void run(){
		final int CANVAS_WIDTH = getWidth();
		final int CANVAS_HEIGHT = getHeight();
 
		for(int circ=MAX_CIRCLES; circ > 0 ; circ--){
 
			int r = getRand(MIN_RADIUS,MAX_RADIUS);
 
			/* Note:
			 * Formular "CANVAS_WIDTH-2*r" makes sure that the
			 * Object stays completly within the Canvas surface
			 * and does not exceed edges. 
			 */
			int xPos = getRand(0,CANVAS_WIDTH-2*r); 
			int yPos = getRand(0,CANVAS_HEIGHT-2*r);
 
			addCircle(r, xPos, yPos);
		}
	}
 
	/**
	 * Method to put Circle GObjects of random Color on the Canvas.
	 * 
	 * @param r a integer Radius value
	 * @param xPos a integer X position on the Canvas
	 * @param yPos a integer Y position on the Canvas
	 */
	private void addCircle(int r, int xPos, int yPos) {
		GOval newCircle = new GOval(xPos, yPos, r*2, r*2);
		newCircle.setFilled(true);
		Color rndColor = getRandColor();
		newCircle.setColor( rndColor );
		newCircle.setFillColor( rndColor );
		add(newCircle);
	}
 
	/**
	 * Method to return a random Color
	 * 
	 * @return a random Color.
	 */
	private Color getRandColor() {
		RandomGenerator rnd = RandomGenerator.getInstance();
		return rnd.nextColor();
	}
 
	/**
	 * Method to return a random Number between
	 * the given min + max Parameters.
	 * 
	 * @param min minimal integer value to pick a random number from.
	 * @param max maximum integer value to pick a random number from.
	 * @return a random integer number
	 */
	private int getRand(int min, int max) {
		RandomGenerator rnd = RandomGenerator.getInstance();
		return rnd.nextInt(min, max);
	}
}

— Axel Werner 2012-04-03 22:49