Java Arrays

There are two main ways to create an array. Use the first when you already know the values. Use the second when you know the size but will fill it later.

// Approach 1: Declare with known values
int[] scores = {85, 92, 78, 95, 88};
String[] days = {"Mon", "Tue", "Wed", "Thu", "Fri"};

// Approach 2: Declare with size (values default to 0, null, or false)
int[] temperatures = new int[7];     // 7 slots, all start at 0
String[] names = new String[100];    // 100 slots, all start at null
boolean[] flags = new boolean[5];    // 5 slots, all start at false

Every element in an array has an index (position number). Java uses 0-based indexing, which means the first element is at index 0, not 1. This trips up almost every beginner, so let's be clear:

int[] scores = {85, 92, 78, 95, 88};
//               [0]  [1]  [2]  [3]  [4]  ← indices

scores[0]   // 85  (first element)
scores[2]   // 78  (third element)
scores[4]   // 88  (last element)

scores.length    // 5 (number of elements — NOT an index!)
scores[scores.length - 1]  // 88 (last element — ALWAYS use this pattern)

// Modify an element
scores[2] = 82;  // Change 78 to 82

// ❌ CRASH: ArrayIndexOutOfBoundsException!
scores[5]   // Index 5 doesn't exist in a 5-element array!
scores[-1]  // Negative indices don't work in Java

The real power of arrays comes from processing every element with a loop. There are two main approaches:

for (int i = 0; i < scores.length; i++) {
    System.out.println("Student " + i + 
        ": " + scores[i]);
}
// Use i < scores.length, NOT <=

for (int score : scores) {
    System.out.println("Score: " + score);
}
// No index variable needed
// Can't modify the array with this

These operations come up constantly in real programming. Master these patterns and you'll solve 80% of array problems:

int[] scores = {85, 92, 78, 95, 88};

// Sum all elements
int sum = 0;
for (int score : scores) sum += score;
System.out.println("Sum: " + sum);  // 438

// Calculate average (cast to double for decimal result!)
double average = (double) sum / scores.length;
System.out.println("Average: " + average);  // 87.6

// Find maximum and minimum
int max = scores[0];
int min = scores[0];
for (int score : scores) {
    if (score > max) max = score;
    if (score < min) min = score;
}
System.out.println("Max: " + max);  // 95
System.out.println("Min: " + min);  // 78

// Linear search — find a value
int target = 92;
int foundAt = -1;  // -1 means "not found"
for (int i = 0; i < scores.length; i++) {
    if (scores[i] == target) {
        foundAt = i;
        break;
    }
}
System.out.println("Found " + target + " at index " + foundAt);

Java provides java.util.Arrays with ready-made methods for common array tasks. Always use these instead of writing your own:

import java.util.Arrays;

int[] data = {42, 17, 93, 8, 55};

// Sort (ascending, in-place — modifies original!)
Arrays.sort(data);
// data is now {8, 17, 42, 55, 93}

// Print readable representation
System.out.println(Arrays.toString(data));
// → "[8, 17, 42, 55, 93]"

// Copy an array
int[] copy = Arrays.copyOf(data, data.length);  // full copy
int[] first3 = Arrays.copyOf(data, 3);          // first 3 elements

// Fill all elements with a value
int[] zeros = new int[10];
Arrays.fill(zeros, -1);  // all elements become -1

// Compare two arrays
boolean same = Arrays.equals(data, copy);  // true (same content)

// Binary search (array MUST be sorted first!)
int index = Arrays.binarySearch(data, 42);  // returns 2

A 2D array is an "array of arrays" — think of it as a table with rows and columns. Each element is accessed with two indices: [row][column]. They're used for game boards, spreadsheets, images, and mathematical matrices.

// Create a 3×4 matrix
int[][] matrix = {
    {1, 2, 3, 4},     // row 0
    {5, 6, 7, 8},     // row 1
    {9, 10, 11, 12}   // row 2
};

// Access: matrix[row][column]
matrix[0][0]   // 1  (top-left)
matrix[1][2]   // 7  (row 1, column 2)
matrix[2][3]   // 12 (bottom-right)

// Dimensions
matrix.length        // 3 (number of rows)
matrix[0].length     // 4 (number of columns)

// Iterate with nested loops
for (int row = 0; row < matrix.length; row++) {
    for (int col = 0; col < matrix[row].length; col++) {
        System.out.print(matrix[row][col] + "\t");
    }
    System.out.println();
}

A common question: when should you use an array vs an ArrayList? Here's the decision guide:

Rule of thumb: Use arrays when the size is known and won't change (days of week, game board). Use ArrayList when you need to add/remove items (shopping cart, to-do list).