05 - Arrays

Arrays let you store multiple values of the same type in a single variable. Think of an array as a row of boxes, each holding one item.

Creating Arrays

Method 1: Declare and Initialize Separately

int[] numbers;           // Declare
numbers = new int[5];    // Create array with 5 spaces

Method 2: All at Once

int[] numbers = new int[5];

Method 3: With Initial Values

int[] numbers = {10, 20, 30, 40, 50};

Array Size

Once you create an array, its size is fixed. You can't add or remove elements later!

int[] inventory = new int[9];  // Always has exactly 9 slots

Accessing Array Elements

Arrays use index numbers starting from 0:

String[] players = {"Alice", "Bob", "Charlie"};

System.out.println(players[0]); // "Alice"
System.out.println(players[1]); // "Bob"
System.out.println(players[2]); // "Charlie"

Visual Representation

Index	0	1	2
Value	"Alice"	"Bob"	"Charlie"

Modifying Array Values

int[] health = {100, 80, 90};

health[1] = 60;  // Change Bob's health to 60

System.out.println(health[0]); // 100
System.out.println(health[1]); // 60
System.out.println(health[2]); // 90

Array Length

Use .length to get the size of an array:

int[] scores = {45, 67, 89, 92, 55};
System.out.println(scores.length); // 5

Length is Not a Method

Notice there are no parentheses! It's array.length, not array.length().

Looping Through Arrays

Using For Loop

String[] items = {"Sword", "Shield", "Potion"};

for (int i = 0; i < items.length; i++) {
    System.out.println(items[i]);
}

Using Enhanced For Loop (For-Each)

String[] items = {"Sword", "Shield", "Potion"};

for (String item : items) {
    System.out.println(item);
}

Enhanced For Loop

The enhanced for loop is simpler and safer, but you can't modify the array or know the current index.

// When you need the index
for (int i = 0; i < items.length; i++) {
    System.out.println(i + ": " + items[i]);
}

// When you just need the values
for (String item : items) {
    System.out.println(item);
}

Multidimensional Arrays

Arrays can hold other arrays, creating a grid:

int[][] grid = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

System.out.println(grid[0][0]); // 1
System.out.println(grid[1][2]); // 6
System.out.println(grid[2][1]); // 8

Visual Representation

	0	1	2
0	1	2	3
1	4	5	6
2	7	8	9

Looping Through 2D Arrays

int[][] grid = {
    {1, 2, 3},
    {4, 5, 6}
};

for (int row = 0; row < grid.length; row++) {
    for (int col = 0; col < grid[row].length; col++) {
        System.out.print(grid[row][col] + " ");
    }
    System.out.println();
}

Practical Examples

Player Inventory System

String[] inventory = new String[9]; // 9 hotbar slots

// Add items
inventory[0] = "Diamond Sword";
inventory[1] = "Shield";
inventory[8] = "Food";

// Display inventory
for (int i = 0; i < inventory.length; i++) {
    if (inventory[i] != null) {
        System.out.println("Slot " + i + ": " + inventory[i]);
    } else {
        System.out.println("Slot " + i + ": Empty");
    }
}

Find Highest Score

int[] scores = {45, 92, 67, 88, 55, 71};
int highest = scores[0];

for (int i = 1; i < scores.length; i++) {
    if (scores[i] > highest) {
        highest = scores[i];
    }
}

System.out.println("Highest score: " + highest);

Calculate Average

double[] temperatures = {23.5, 25.0, 22.8, 24.3, 26.1};
double sum = 0;

for (double temp : temperatures) {
    sum += temp;
}

double average = sum / temperatures.length;
System.out.println("Average temperature: " + average);

Block Grid (2D Array)

String[][] terrain = new String[5][5];

// Fill with grass
for (int x = 0; x < 5; x++) {
    for (int z = 0; z < 5; z++) {
        terrain[x][z] = "grass";
    }
}

// Place some stone
terrain[2][2] = "stone";
terrain[1][3] = "stone";

// Display grid
for (int x = 0; x < terrain.length; x++) {
    for (int z = 0; z < terrain[x].length; z++) {
        System.out.print(terrain[x][z] + " ");
    }
    System.out.println();
}

Common Array Operations

Copy an Array

int[] original = {1, 2, 3, 4, 5};
int[] copy = original.clone();

// Or manually
int[] copy2 = new int[original.length];
for (int i = 0; i < original.length; i++) {
    copy2[i] = original[i];
}

Search for a Value

String[] items = {"Sword", "Shield", "Potion", "Bow"};
String target = "Potion";
int foundIndex = -1;

for (int i = 0; i < items.length; i++) {
    if (items[i].equals(target)) {
        foundIndex = i;
        break;
    }
}

if (foundIndex != -1) {
    System.out.println("Found at index: " + foundIndex);
} else {
    System.out.println("Not found");
}

Count Occurrences

String[] blocks = {"stone", "dirt", "stone", "grass", "stone"};
String searchFor = "stone";
int count = 0;

for (String block : blocks) {
    if (block.equals(searchFor)) {
        count++;
    }
}

System.out.println(searchFor + " appears " + count + " times");

Array Limitations

Fixed Size

Arrays can't grow or shrink. If you need flexibility, you'll learn about ArrayList later!

int[] numbers = new int[5];
// Can't add a 6th element!

// If you need more space, create a new array
int[] bigger = new int[10];
for (int i = 0; i < numbers.length; i++) {
    bigger[i] = numbers[i];
}

05 - Arrays

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