1. Organizing Memory in Java
Arrays
Organizing Memory in Java

2. What is an Array
You can think of an Array like a container, it holds several things
Previously when we worked with variables each variable represented ONE item (number, text, image, etc).
Arrays are unique in that they are stored in a single contiguous block of memory

4. Declaring an Array int[] scores = new int[5];
Square brackets, [], tell Java that this is more than just an integer

5. Elements can be initialized at declaration:
Index/Subscript 84 93 97 78 62
score[1]
score[3]
score[4]
score[0]
score[2]
Elements can be initialized at declaration:
int scores[] = {78,84,62,93,97};

6. Computer Scientists count from 084 93 97 78 62
score[1]
score[3]
score[4]
score[0]
score[2]
The first position is score[0]
For efficiency, an array is the address of the first element.
The number in brackets represents how far from the first item to move.
score[4] means start at score and move over by 4 integers (arriving at the 5th)

7. length In Java, arrays have a handy property called length
You can find out how many items are in the array
scores.length

8. Task #1
We’re going to play more with Greenfoot and use an Array to do it.
Your task will be to bring a character to life with an animation

9. Finding an animation

11. Copy/Paste the URL

12. Getting the frames
The process we are about to do is called “exploding” a gif
There are free online sites that are usually the simplest option

14. Greenfoot Project Folder
Save your images in the images folder
Keeps your work organized
Allows you to refer to images by relative path rather than absolute

15. Other useful tasks

16. Animation with an array
Think of the Array like a filmstrip
Each index holds 1 image in the animation
As the “filmstrip” plays, individual frames make up a movie 1 2 3 4 5

17. Java private final static GreenfootImage[] IDLE = {
new GreenfootImage("MarioIdle (1).gif"),
new GreenfootImage("MarioIdle (2).gif"),
new GreenfootImage("MarioIdle (3).gif"),
new GreenfootImage("MarioIdle (4).gif"),
new GreenfootImage("MarioIdle (5).gif"),
new GreenfootImage("MarioIdle (6).gif") }; 1 2 3 4 5
MarioIdle (1)
MarioIdle (2)
MarioIdle (3)
MarioIdle (4)
MarioIdle (5)
MarioIdle (6)

18. Java private final static GreenfootImage[] IDLE = {
new GreenfootImage("MarioIdle (1).gif"),
new GreenfootImage("MarioIdle (2).gif"),
new GreenfootImage("MarioIdle (3).gif"),
new GreenfootImage("MarioIdle (4).gif"),
new GreenfootImage("MarioIdle (5).gif"),
new GreenfootImage("MarioIdle (6).gif") };
private – hide the array from everyone but Mario (no one else needs it)
final – a constant, don’t allow changes!  NOTICE THE ALL CAPS
static – shared by all the Marios that are put in the game
GreenfootImage[] – tells Java what we are keeping track of, an array of images
IDLE – the name in PROPER style for a constant
{ contents, of, array, separated, by, commas };

19. Control
You’ve probably never though of it before but something has to keep track of which frame to display
We need a variable for Mario to remember 1 2 3 4 5
frame_

20. Java
public class Mario extends Actor { //initialized in the class because it is static – shared by all Marios private final static GreenfootImage[] IDLE = {new GreenfootImage("MarioIdle (1).gif"), new GreenfootImage("MarioIdle (2).gif"), new GreenfootImage("MarioIdle (3).gif"), new GreenfootImage("MarioIdle (4).gif"), new GreenfootImage("MarioIdle (5).gif"), new GreenfootImage("MarioIdle (6).gif")}; private int frame_;//declare the things Mario needs to remember public Mario() frame_ = 0;//initialized in the constructor – belongs to a Mario } …

21. static demo private static int frame_ vs private int frame
Mario has 6 frames in his animation
To help you see the bug of sharing (static) the frame_ variable I’ll put 6 Marios into the World

22. act() Greenfoot is a game engine with a simple rule
Every frame of the movie it asks everything in the game (Worlds/Levels, Actors/Characters, etc.) how it should behave
It does this through the act() method
public void act()
Everyone gets a turn to act before the next frame is rendered

23. act() How should Mario act()? Update frame of animation
Display the current image

24. Update animation frame
Increment the frame number
If you have gone past the end of the animation
move the frame back to the beginning
this will loop the animation
frame_++;
if( frame_ >= IDLE.length ) {
frame_ = 0; }

25. If statements
conditional statements allow you to tell Java when code should (and should not) be executed
Syntax:
if( condition ) {
code to execute when condition is true
(ignored when false) }
Note indenting and braces {}

26. updateAnimation
public void updateAnimation() { setImage( IDLE[ frame_ ] ); frame_++; if( frame_ >= IDLE.length ) frame_ = 0; }

27. act
public void act() { // Add your action code here. updateAnimation(); } The [ ] brackets let you access the pieces in the array

28. [] brackets 1 2 3 4 5
IDLE[0]
IDLE[1]
IDLE[2]
IDLE[3]
IDLE[4]
IDLE[5]

29. Your turn
We will develop a character that can later be built into a video game
To cut down on complexity, try remove gravity from your game (top down – Legend of Zelda, aircraft, space, etc.)
We can negotiate games that require gravity/platforms but you should run it past me first
We will improve their functionality later
Learn to use:
An array
If statements
Declare class variables
Continue to:
Make methods
Declare local variables