1. Chapter 11—Arrays and ArrayLists
The Art and Science of
An Introduction
to Computer Science
ERIC S. ROBERTS
Java
C H A P T E R 1 1
Arrays and ArrayLists
Little boxes, on a hillside, little boxes made of ticky-tacky
Little boxes, little boxes, little boxes, all the same
There’s a green one and a pink one and a blue one and a yellow one
And they're all made out of ticky-tacky and they all look just the same
—Malvina Reynolds, “Little Boxes,” 1962
Lecture Slides, Part III
Özyeğin University
Slides are adapted from the originals available at
Chapter 11—Arrays and ArrayLists

2. Example: SimpleSnake
In this problem we will implement a simple version of the snake game in which a snake bounces from the walls of the screen.

3. SimpleSnake
Each part of the snake body will have a size of BODY_PART_WIDTH.
There are SNAKE_LENGTH many body parts.
Each body part is a GOval object. All the body parts are kept in a GOval array.
Corresponding to each body part, we keep the velocity in the x direction and in the y direction.
Food for thought: Why don’t we keep a single velocity value?
private GOval[] snakeBody = new GOval[SNAKE_LENGTH];
private double[] xVelocities = new double[SNAKE_LENGTH];
private double[] yVelocities = new double[SNAKE_LENGTH];

4. SimpleSnake public void run() { initializeSnakeBody();
initializeVelocities();
while(true) {
moveSnake();
checkBounds();
pause(20); }

5. SimpleSnake (initializing body parts)
BODY_PART_WIDTH
BODY_PART_WIDTH
BODY_PART_WIDTH / sqrt(2)

6. SimpleSnake public void initializeSnakeBody() {
int xCenter = getWidth() / 2;
int yCenter = getHeight() / 2;
for(int i = 0; i < snakeBody.length; i++) {
snakeBody[i] = createSnakeBodyPartCenteredAt(xCenter, yCenter);
xCenter -= (BODY_PART_WIDTH/Math.sqrt(2));
yCenter -= (BODY_PART_WIDTH/Math.sqrt(2));
add(snakeBody[i]); }
public GOval createSnakeBodyPartCenteredAt(int xCenter, int yCenter) {
GOval circle = new GOval(xCenter - BODY_PART_WIDTH/2,
yCenter - BODY_PART_WIDTH/2,
BODY_PART_WIDTH, BODY_PART_WIDTH);
circle.setFilled(true);
circle.setFillColor(Color.BLUE);
return circle;

7. SimpleSnake public void initializeVelocities() {
for(int i = 0; i < xVelocities.length; i++) {
xVelocities[i] = 3; }
for(int i = 0; i < yVelocities.length; i++) {
yVelocities[i] = 3;
public void moveSnake() {
for(int i = 0; i < snakeBody.length; i++) {
snakeBody[i].move(xVelocities[i], yVelocities[i]);
public void checkBounds() {
double x = snakeBody[i].getX();
double y = snakeBody[i].getY();
if(x < 0 || x > getWidth() - BODY_PART_WIDTH) {
xVelocities[i] *= -1;
if(y < 0 || y > getHeight() - BODY_PART_WIDTH) {
yVelocities[i] *= -1;

8. SimpleSnake v.2
In this problem we will implement yet another simple version of the snake game.
P.S. You may need to click on the window to request focus so that key events will be captured.

9. SimpleSnake v.2
In this version, there is a single, constant SPEED value. The head of the snake has a direction, which can be one of NORTH, EAST, SOUTH, WEST.
After each move, a body part replaces the part in the front.
snake
direction

10. SimpleSnake v.2
In this version, there is a single, constant SPEED value. The head of the snake has a direction, which can be one of NORTH, EAST, SOUTH, WEST.
After each move, a body part replaces the part in the front.
snake
direction

11. SimpleSnake v.2 Moving the snake parts:
Each part element gets the location of the element in front of it.
Head element is moved according to its direction.

12. SimpleSnake public class SimpleSnakeV2 extends GraphicsProgram {
private final int BODY_PART_WIDTH = 20;
private final int SNAKE_LENGTH = 12;
private final int SPEED = BODY_PART_WIDTH;
private GOval[] snakeBody = new GOval[SNAKE_LENGTH];
private final int NORTH = 1;
private final int EAST = 2;
private final int SOUTH = 3;
private final int WEST = 4;
// Direction of the head is one of {N, W, S, E}
private int direction = EAST;
// Cont’d on the next slide
// . . .

13. public void run() {
addGrid();
initializeSnakeBody();
while(true) {
moveSnake();
pause(200); }
public void addGrid() {
for(int i = 0; i < getWidth(); i += BODY_PART_WIDTH) {
add(new GLine(i, 0, i, getHeight()));
for(int i = 0; i < getHeight(); i += BODY_PART_WIDTH) {
add(new GLine(0, i, getWidth(), i));

14. public void initializeSnakeBody() {
int x = SNAKE_LENGTH * BODY_PART_WIDTH;
int y = getHeight() / 2;
for(int i = 0; i < snakeBody.length; i++) {
snakeBody[i] = createSnakeBodyAt(x, y);
x -= BODY_PART_WIDTH;
add(snakeBody[i]); }
public GOval createSnakeBodyAt(int x, int y) {
GOval circle = new GOval(x, y,
BODY_PART_WIDTH, BODY_PART_WIDTH);
circle.setFilled(true);
circle.setFillColor(Color.BLUE);
return circle;

15. public void moveSnake() {
// Each body part replaces the one in front of it
for(int i = snakeBody.length - 1; i >= 1; i--) {
GOval prevPart = snakeBody[i-1];
snakeBody[i].setLocation(prevPart.getX(), prevPart.getY()); }
// Move the head
if(direction == EAST) {
snakeBody[0].move(SPEED, 0);
} else if(direction == WEST) {
snakeBody[0].move(-SPEED, 0);
} else if(direction == NORTH) {
snakeBody[0].move(0, -SPEED);
} else if(direction == SOUTH) {
snakeBody[0].move(0, SPEED);

16. // The method below is invoked whenever the user presses a key
// You do not need to spend too much time understanding this part
public void keyPressed(KeyEvent keyEvent) {
// VK_UP: key code for up arrow key
// VK_RIGHT: key code for right arrow key
// VK_DOWN: key code for down arrow key
// VK_LEFT: key code for left arrow key
if(keyEvent.getKeyCode() == KeyEvent.VK_UP) {
direction = NORTH;
} else if(keyEvent.getKeyCode() == KeyEvent.VK_RIGHT) {
direction = EAST;
} else if(keyEvent.getKeyCode() == KeyEvent.VK_DOWN) {
direction = SOUTH;
} else if(keyEvent.getKeyCode() == KeyEvent.VK_LEFT) {
direction = WEST; }
public void init() {
setSize(600, 400);
addKeyListeners(); // required for key interaction