Java Arrays

Fixed-size, ordered collection of elements of the same type.

Declaring an array

Declaration

// Empty array of size 5
int[] numbers = new int[5];

// Array initialized with values
int[] primes = {2, 3, 5, 7, 11};

// String array
String[] names = {"Raman", "Aman", "Priya"};

Accessing elements

Arrays are zero-indexed. The first element is at index 0.

Index access

String[] names = {"Raman", "Aman", "Priya"};

System.out.println(names[0]);  // "Raman"
System.out.println(names[2]);  // "Priya"

names[1] = "Sara";            // modify element

ArrayIndexOutOfBoundsException Accessing names[5] on an array of size 3 throws ArrayIndexOutOfBoundsException at runtime. Always check i < array.length.

Length

Use .length (it's a field, not a method — no parentheses):

length

int[] nums = {10, 20, 30, 40};
System.out.println(nums.length);  // 4

Iterating an array

Two ways

int[] nums = {10, 20, 30};

// Classic for loop — when you need the index
for (int i = 0; i < nums.length; i++) {
    System.out.println(i + ": " + nums[i]);
}

// for-each — when you only need the value
for (int n : nums) {
    System.out.println(n);
}

Default values

When you create an array with new int[5], all elements get the default for that type: 0 for numbers, false for boolean, null for objects.

Multi-dimensional arrays

An array of arrays — useful for grids, matrices, tables.

2D array

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

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

for (int[] row : matrix) {
    for (int v : row) {
        System.out.print(v + " ");
    }
    System.out.println();
}

Useful array utilities

The java.util.Arrays class has helpful static methods:

Arrays utility

import java.util.Arrays;

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

Arrays.sort(nums);                     // {1, 2, 5, 8, 9}
System.out.println(Arrays.toString(nums));  // [1, 2, 5, 8, 9]

int[] copy = Arrays.copyOf(nums, 3);   // {1, 2, 5}
boolean equal = Arrays.equals(nums, copy);

Limitation: fixed size

Once an array is created, its size is fixed forever. To grow or shrink dynamically, use ArrayList (covered in the Collections section).

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