Assignment 3: Breakout!

CS106A

Assignment 3: Breakout!
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Your job in this assignment is to write the classic arcade game of Breakout, which was invented by
Steve Wozniak before he founded Apple with Steve Jobs (moment of silence). It is a large assignment,
but entirely manageable as long as you break the problem up into pieces. The decomposition is discussed in this handout, and there are several suggestions for staying on top of things in the “Advice,
Tips, and Tricks” section later in this handout.
Due Monday, February 10 at 3:15PM
Assignment Review Hours: Tuesday, February 4, Braun Auditorium, 7:30PM – 8:30PM

The Breakout Game
In Breakout, the initial configuration of the world appears as shown on the left. The colored rectangles
in the top part of the screen are bricks, and the slightly larger rectangle at the bottom is the paddle. The
paddle is in a fixed position in the vertical dimension, but moves back and forth across the screen along
with the mouse until it reaches the edge of its space.
A complete game consists of three turns. On each turn, a ball is launched from the center of the window toward the bottom of the screen at a random angle. That ball bounces off the paddle and the walls
of the world, in accordance with the physical principle generally expressed as “the angle of incidence
equals the angle of reflection” (which turns out to be very easy to implement as discussed later in this
handout). Thus, after two bounces—one off the paddle and one off the right wall—the ball might have
the trajectory shown in the second diagram. (Note that the dotted line is there to show the ball’s path
and won’t appear on the screen.)
As you can see from the second diagram, the ball is about to collide with one of the bricks on the bottom row. When that happens, the ball bounces just as it does on any other collision, but the brick disappears. The third diagram shows what the game looks like after that collision and after the player has
moved the paddle to put it in line with the oncoming ball.
The play on a turn continues in this way until one of two conditions occurs:
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1. The ball hits the lower wall, which means that the player must have missed it with the paddle.
In this case, the turn ends and the next ball is served if the player has any turns left. If not, the
game ends in a loss for the player.
2. The last brick is eliminated. In this case, the player wins, and the game ends immediately.
After all the bricks in a particular column have been cleared, a path
will open to the top wall. When this situation occurs, the ball will often bounce back and forth several times between the top wall and the
upper line of bricks without the user ever having to worry about hitting the ball with the paddle. This condition is called “breaking out”
and gives meaning to the name of the game. The diagram on the right
shows the situation shortly after the first ball has broken through the
wall. That ball will go on to clear several more bricks before it comes
back down an open channel.
It is important to note that, even though breaking out is a very exciting
part of the player’s experience, you don’t have to do anything special
in your program to make it happen. The game is simply operating by
the same rules it always applies: bouncing off walls, clearing bricks,
and otherwise obeying the laws of physics.
The Starter File
The starter project for this assignment has a little more in it than it has in the past, but none of the important parts of the program. The starting contents of the Breakout.java file appear in Figure 1 (on
the last page of this handout). This file takes care of the following details:
• It includes the imports you will need for writing the game.
• It defines the named constants that control the game parameters, such as the dimensions of the
various objects. Your code should use these constants internally so that changing them in your
file changes the behavior of your program accordingly.
Success in this assignment will depend on breaking up the problem into manageable pieces and getting
each one working before you move on to the next. The next few sections describe a reasonable staged
approach to the problem. While you're free to write Breakout however you'd like, we suggest using our
decomposition, since each piece nicely builds off of the previous pieces.
Set up the Bricks
Before you start playing the game, you have to set up the various pieces. Thus, it probably makes
sense to implement the run method as two method calls: one that sets up the game and one that plays it.
An important part of the setup consists of creating the rows of bricks at the top of the game, which look
like this:
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The number, dimensions, and spacing of the bricks are specified using named constants in the starter
file, as is the distance from the top of the window to the first line of bricks. The only value you need to
compute is the x coordinate of the first column, which should be chosen so that the bricks are centered
in the window, with the leftover space divided equally on the left and right sides. The color of the
bricks remain constant for two rows and run in the following rainbow-like sequence: RED, ORANGE,
YELLOW, GREEN, CYAN.
This part of the assignment is almost exactly like the pyramid problem
from Assignment #2. The parts that are only a touch more difficult are
that you need to fill and color the bricks. This extra complexity is more
than compensated by the fact that there are the same number of bricks in
each row, and you don’t have to change the coordinate calculation from
row to row.
Here’s a suggestion: Why don’t you get this part of the program working
by Monday, February 3rd so that you can produce just the diagram at the
right? The display has most of what you see on the final screen and will
give you considerable confidence that you can get the rest done, and
you’ll be well on your way before time gets short.
Create the Paddle
The next step is to create the paddle. At one level, this is considerably easier than the bricks. There is
only one paddle, which is a filled GRect. You even know its position relative to the bottom of the window.
The challenge in creating the paddle is to make it track the mouse. Mouse tracking makes use of the
event-driven model discussed in Chapter 9 of the textbook. Here, however, you only have to pay attention to the x coordinate of the mouse because the y position of the paddle is fixed. The only additional
wrinkle is that you should not let the paddle move off the edge of the window. Thus, you’ll have to
check to see whether the x coordinate of the mouse would make the paddle extend beyond the boundary and change it if necessary to ensure that the entire paddle is visible in the window.
Here’s a soon-to-become-boring suggestion: Why don’t you get this part of the program working by
Wednesday, February 5th, so that you can follow the mouse with the paddle? This entire part of the
program takes fewer than 10 lines of code, so it shouldn’t take so long. The hard part lies in reading
the Graphics chapter and understanding what you need to do.
Create a Ball and Get it to Bounce off the Walls
At one level, creating the ball is easy, given that it’s just a filled GOval. The interesting part lies in getting it to move and bounce appropriately. You are now past the “setup” phase and into the “play”
phase of the game. To start, create a ball and put it in the center of the window. As you do so, keep in
mind that the coordinates of the GOval do not specify the location of the center of the ball but rather its
upper left corner. The mathematics is not any more difficult, but may be a bit less intuitive.
The program needs to keep track of the velocity of the ball, which consists of two separate components,
which you will presumably declare as instance variables like this:
private double vx, vy;
The velocity components represent the change in position that occurs on each time step. Initially, the
ball should be heading downward, and you might try a starting velocity of +3.0 for vy (remember that
y values in Java increase as you move down the screen). The game would be boring if every ball took
the same course, so you should choose the vx component randomly.
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In line with our discussion of generating random numbers from this week, you should initialize vx as
follows:
1. Declare a local variable of type RandomGenerator by writing
RandomGenerator rgen = RandomGenerator.getInstance();
2. Initialize the vx variable as follows:
vx = rgen.nextDouble(1.0, 3.0);
if (rgen.nextBoolean(0.5)) vx = -vx;
This code sets vx to be a random double in the range 1.0 to 3.0 and then makes it negative half
the time. This strategy works much better for Breakout than calling
rgen.nextDouble(-3.0, +3.0)
which might generate a ball going more or less straight down. That would make life far too
easy for the player.
Once you’ve done that, your next challenge is to get the ball to bounce around the world, ignoring entirely the paddle and the bricks. To do so, you need to check to see if the coordinates of the ball have
gone beyond the boundary, taking into account that the ball has a nonzero size. Thus, to see if the ball
has bounced off the right wall, you need to see whether the coordinate of the right edge of the ball has
become greater than the width of the window; the other three directions are treated similarly. For now,
have the ball bounce off the bottom wall so that you can watch it make its path around the world. You
can change that test later so that hitting the bottom wall signifies the end of a turn.
Computing what happens after a bounce is extremely simple. If a ball bounces off the top or bottom
wall, all you need to do is reverse the sign of vy. Symmetrically, bounces off the side walls simply reverse the sign of vx.
Checking for Collisions
Now comes the interesting part. In order to make Breakout into a real game, you have to be able to tell
whether the ball is colliding with another object in the window. As scientists often do, it helps to begin
by making a simplifying assumption and then relaxing that assumption later. Suppose the ball were a
single point rather than a circle. In that case, how could you tell whether it had collided with another
object?
If you look in Chapter 9 (page 299) at the methods that are defined at the GraphicsProgram level, you
will discover that there is a method
public GObject getElementAt(double x, double y)
that takes a position in the window and returns the graphical object at that location, if any. (Note that
this method already exists; we're not asking you to implement it yourself). If there are no graphical objects that cover that position, getElementAt returns the special constant null. If there is more than
one, getElementAt always chooses the one closest to the top of the stack, which is the one that appears
to be in front on the display.
What happens if you call getElementAt(x, y), where x and y are the coordinates of the ball? If the
point (x, y) is underneath an object, this call returns the graphical object with which the ball has collided. If there are no objects at the point (x, y), you’ll get the value null.
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So far, so good. But, unfortunately, the ball is not a single point. It occupies physical area and therefore may collide with something on the screen even though its center does not. The easiest thing to do
—which is in fact typical of the simplifying assumptions made in real computer games—is to check a
few carefully chosen points on the outside of the ball and see whether any of those points has collided
with anything. As soon as you find something at one of those points, you can declare that the ball has
collided with that object.
In your implementation, the easiest thing to do is to check the four corner points on the square in which
the ball is inscribed. Remember that a GOval is defined in terms of its bounding rectangle, so that if the
upper left corner of the ball is at the point (x, y), the other corners will be at the locations shown in this
diagram:
(x, y) (x + 2r, y)
(x, y + 2r) (x + 2r, y + 2r)
These points have the advantage of being outside the ball—which means that getElementAt can’t return the ball itself—but nonetheless close enough to make it appear that collisions have occurred.
Thus, for each of these four points, you need to:
1. Call getElementAt on that location to see whether anything is there.
2. If the value you get back is not null, then you need look no farther and can take that value as
the GObject with which the collision occurred.
3. If getElementAt returns null for a particular corner, go on and try the next corner.
4. If you get through all four corners without finding a collision, then no collision exists.
It would be very useful to write this section of code as a separate method
private GObject getCollidingObject()
that returns the object involved in the collision, if any, and null otherwise. You could then use it in a
declaration like
GObject collider = getCollidingObject();
which assigns that value to a variable called collider.
From here, the only remaining thing you need to do is decide what to do when a collision occurs.
There are only two possibilities. First, the object you get back might be the paddle, which you can test
by checking
if (collider == paddle) . . .
If it is the paddle, you need to bounce the ball so that it starts traveling up. If it isn’t the paddle, the
only other thing it might be is a brick, since those are the only other objects in the world. Once again,
you need to cause a bounce in the vertical direction, but you also need to take the brick away. To do
so, all you need to do is remove it from the screen by calling the remove method.
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Finishing Up
If you’ve gotten to here, you’ve done all the hard parts. There are, however, a few more details you
need to take into account:
• You’ve got to take care of the case when the ball hits the bottom wall. In the prototype you’ve
been building, the ball just bounces off this wall like all the others, but that makes the game
pretty hard to lose. You’ve got to modify your loop structure so that it tests for hitting the bottom wall as one of its terminating conditions.
• You’ve got to give the player three chances to break all the bricks. We recommend that before
each round, you wait for the user to click the mouse by calling the waitForClick() method,
which pauses until the user clicks the mouse. Once the user clicks, you can serve the ball.
• You’ve got to check for the other terminating condition, which is hitting the last brick. How do
you know when you’ve done so? Although there are other ways to do it, one of the easiest is to
have your program keep track of the number of bricks remaining. Every time you hit one, subtract one from that counter. When the count reaches zero, you must be done. In terms of the requirements of the assignment, you can simply stop at that point, but it would be nice to give the
player a little feedback that at least indicates whether the game was won or lost.
• You’ve got to experiment with the settings that control the speed of your program. How long
should you pause in the loop that updates the ball? Do you need to change the velocity values
to get better play action?
• You’ve got to test your program to see that it works. Play for a while and make sure that as
many parts of it as you can check are working. If you think everything is working, here is
something to try: Just before the ball is going to pass the paddle level, move the paddle quickly
so that the paddle collides with the ball rather than vice-versa. Does everything still work, or
does your ball seem to get “glued” to the paddle? If you get this error, try to understand why it
occurs and how you might fix it.
Advice, Tricks, and Tips
Here are some survival hints for this assignment:
• Start as soon as possible. This assignment is due in just over a week, which will be here before
you know it. If you wait until the day before this assignment is due, you will have a very hard
time getting it all together.
• Implement the program in stages, as described in this handout. Don’t try to get everything
working all at once. Implement the various pieces of the project one at a time and make sure
that each one is working before you move on to the next phase.
• Set up a milestone schedule. In the handout, I’ve suggested that you get the brick display up
and running by this coming Monday and the paddle moving by Wednesday. If you do, you’ll
have lots of time for the more interesting parts of the assignment and for implementing extensions.
• Don’t try to extend the program until you get the basic functionality working. The following
section describes several ways in which you could extend the implementation. Several of those
are lots of fun. Don’t start them, however, until the basic assignment is working. If you add
extensions too early, you’ll find that the debugging process gets really difficult.
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Possible extensions
This assignment is perfect for those of you who are looking for + or (dare I say it) ++ scores, because
there are so many possible extensions. If you are going to create a version of your program with
extensions, you should submit two versions of the assignment: the basic version that meets all the
assignment requirements and the extended version. Here are a few ideas of for possible extensions (of
course, we encourage you to use your imagination to come up with other ideas as well):
• Add sounds. Make the ball make a sound when it bounces off of something! The starter project
contains an audio clip file called bounce.au that contains that sound. You can load the sound
by writing
AudioClip bounceClip = MediaTools.loadAudioClip("bounce.au");
and later play it by calling
bounceClip.play();
• Add messages. Our demo version of the assignment (available online at the CS106A website)
waits for the user to click the mouse before serving each ball and announces whether the player
has won or lost at the end of the game. These are just GLabel objects that you can add and remove at the appropriate time.
• Improve the user control over bounces. The program gets rather boring if the only thing the
player has to do is hit the ball. It is far more interesting, if the player can control the ball by hitting it at different parts of the paddle. The way the old arcade game worked was that the ball
would bounce in both the x and y directions if you hit it on the edge of the paddle from which
the ball was coming.
• Add in the “kicker.” The arcade version of Breakout lured you in by starting off slowly. But, as
soon as you thought you were getting the hang of things, the program sped up, making life just
a bit more exciting.
• Keep score. You could easily keep score, generating points for each brick. In the arcade game,
bricks were more valuable higher up in the array, so that you got more points for red bricks than
cyan bricks. You could display the score underneath the paddle, since it won’t get in the way
there.
• Use your imagination. What else have you always wanted a game like this to do?
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Writeup
As a last step in this assignment, we'd like you to answer three short questions about the programs
you've written. There is a file called writeup.txt included with the starter code for this programming
assignment. Before you submit your final version of the assignment, please answer those questions.
Your section leader can then discuss your answers during your interactive grading session for this assignment to help you improve your programming technique and style.
Your grade for the writeup is based purely on whether or not you actually answer the questions rather
than on the quality of your answers. That said, we'd appreciate it if you answered them honestly so that
your section leader can offer better feedback.
For reference, the questions we'd like you to answer in the writeup.txt file are listed here.
1. When discussing instance variables in lecture, we stressed that instance variables should be a
“weapon of last resort” when communicating data around in a program. It's almost always
preferable to use parameter passing and return values to move data between different parts of
the program. For each instance variable in your program, explain why you chose to make it an
instance variable rather than a local variable. (Constants don't count as instance variables, so
you don't need to describe the constants in your program.)
2. The Breakout game ends either when the user loses three balls or when all the bricks are broken. How did you test whether the game would end correctly in this second case? Did you make
any changes to the code when testing?
Good luck!
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/*
* File: Breakout.java
* -------------------
* This file will eventually implement the game of Breakout.
*/
import acm.graphics.*;
import acm.program.*;
import acm.util.*;
import java.awt.*;
import java.awt.event.*;
public class Breakout extends GraphicsProgram {
/** Width and height of application window in pixels */
public static final int APPLICATION_WIDTH = 400;
public static final int APPLICATION_HEIGHT = 600;
/** Dimensions of game board (usually the same) */
private static final int WIDTH = APPLICATION_WIDTH;
private static final int HEIGHT = APPLICATION_HEIGHT;
/** Dimensions of the paddle */
private static final int PADDLE_WIDTH = 60;
private static final int PADDLE_HEIGHT = 10;
/** Offset of the paddle up from the bottom */
private static final int PADDLE_Y_OFFSET = 30;
/** Number of bricks per row */
private static final int NBRICKS_PER_ROW = 10;
/** Number of rows of bricks */
private static final int NBRICK_ROWS = 10;
/** Separation between bricks */
private static final int BRICK_SEP = 4;
/** Width of a brick */
private static final int BRICK_WIDTH =
(WIDTH - (NBRICKS_PER_ROW - 1) * BRICK_SEP) / NBRICKS_PER_ROW;
/** Height of a brick */
private static final int BRICK_HEIGHT = 8;
/** Radius of the ball in pixels */
private static final int BALL_RADIUS = 10;
/** Offset of the top brick row from the top */
private static final int BRICK_Y_OFFSET = 70;
/** Number of turns */
private static final int NTURNS = 3;
public void run() {
/* You fill this in, along with any subsidiary methods */
}
}
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