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Arrays in Java

An array is how you store a list of values under one name. Instead of creating 100 separate variables for 100 exam scores, you create one array. Simple idea, but it unlocks a ton of power.

Single-Dimensional Arrays

An array holds multiple values of the same type, accessed by index. The index always starts at 0 — so the first element is at index 0, not 1. This trips up beginners constantly. Think of it like seats in a row at a cinema — seat 0, seat 1, seat 2... The row has a fixed number of seats, and you access each by its number.

Java

// Declare and initialize
int[] scores = {85, 92, 78, 95, 88};

// Access elements — remember: 0-indexed!
System.out.println(scores[0]);       // 85 — first element
System.out.println(scores[4]);       // 88 — last element
System.out.println(scores.length);   // 5 — total elements

// Common mistake: accessing scores[5] on a 5-element array
// → ArrayIndexOutOfBoundsException (the most common Java crash for beginners)

// Iterate with for-each
for (int score : scores) {
    System.out.println(score);
}

2D Arrays — Tables and Grids

A 2D array is an array of arrays — think of it as a spreadsheet grid, a chessboard, or a seating chart. You need two indices to access a value: row and column.

Java

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

// matrix[row][column]
System.out.println(matrix[1][2]);  // 6 — row 1, col 2

// Print the entire grid
for (int i = 0; i < matrix.length; i++) {
    for (int j = 0; j < matrix[i].length; j++) {
        System.out.print(matrix[i][j] + " ");
    }
    System.out.println();
}
// Output:
// 1 2 3
// 4 5 6
// 7 8 9

Arrays Utility Class — Don't Reinvent the Wheel

Java's java.util.Arrays class gives you built-in methods for sorting, searching, copying, and printing arrays. There's almost never a reason to write your own sort from scratch.

Java

import java.util.Arrays;

int[] arr = {5, 2, 8, 1, 9, 3};

Arrays.sort(arr);
System.out.println(Arrays.toString(arr));  // [1, 2, 3, 5, 8, 9]

// Binary search — only works on sorted arrays!
int idx = Arrays.binarySearch(arr, 8);    // returns 4 (index of 8)

// Copy
int[] firstFour = Arrays.copyOf(arr, 4);  // [1, 2, 3, 5]

// Fill an array with a value
int[] zeros = new int[5];
Arrays.fill(zeros, 0);  // [0, 0, 0, 0, 0]

Arrays in Java

Single-Dimensional Arrays

2D Arrays — Tables and Grids

Arrays Utility Class — Don't Reinvent the Wheel

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