Array

An array is a data structure that can hold multiple values of the same data type in a single variable, instead of declaring separate variables for each value.

Arrays are fixed size → cannot be resized after creation.

Example of array:

String[] strs = new String[]{"hello", "world"};
int[] numbers = {10, 20, 30, 40, 50};

Declaring an Array

Arrays in Java are objects, and they require both:

• Type of elements (e.g., int, String)
• Size or initialization

Two ways to declare:

1. Declaration and then initialization

int[] arr = new int[5];  // creates an array with 5 elements (default value = 0)
arr[0] = 10;
arr[1] = 20;

2. Declaration with values

int[] arr = new int[]{10, 20, 30, 40, 50};
int[] arr2 = {1, 2, 3};  // new int[] can be infer

Accessing Elements

Use index numbers starting from 0.

int[] arr = {1, 2, 3, 4, 5}
System.out.println(arr[0]);  // First element 1
System.out.println(arr[4]);  // Fifth element 5
System.out.println(arr[5]);  // ArrayIndexOutOfBoundsException

Array Length

The length property gives the total number of elements (no bracket):

Array's length cannot change, if you want to change, use List instead.

int[] arr = {1, 2, 4};
System.out.println(arr.length);  // Output: 3

Looping Through an Array

For loop and for each loop

int[] arr = {1, 4, 7};
for (int i = 0; i < arr.length; i++) {
    System.out.println(arr[i]);
}
for (int i: arr) {
    System.out.println(arr[i]);
}

Default Values in Arrays

When you create an array without assigning values, Java fills it with default values:

• Numeric types → 0
• boolean → false
• Object references → null

Example:

String[] names = new String[3];
System.out.println(names[0]);  // null

Multidimensional Arrays

Arrays can be array of arrays (like a matrix).

Example (2D array):

int[][] matrix = {
    {1, 2, 3},
    {4, 5, 6, 7}
};
System.out.println(matrix[0][1]);  // Output: 2

int[][] grid = new int[2][3];  // 2 rows, 3 columns
grid[0][0] = 10;

Array Utility Class (java.util.Arrays)

The Arrays class provides helpful methods for working with arrays:

import java.util.Arrays;

int[] nums = {5, 2, 8};
Arrays.sort(nums);                 // Quick Sort
System.out.println(Arrays.toString(nums)); // [2, 5, 8]
Arrays.sort(nums, Collections.reverseOrder()); // reverse

int index = Arrays.binarySearch(nums, 5); // Search element, return -1 if not found
System.out.println(index); // 1

int[] a = {1, 2, 3};
int[] b = {1, 2, 3};
System.out.println(a == b); // false
System.out.println(Arrays.equals(a, b));  // true


int[] nums = {1, 2, 3};
int[] copy = Arrays.copyOf(nums, 5);  // [1, 2, 3, 0, 0]
int[] part = Arrays.copyOfRange(nums, 1, 2);  // [2, 3]

Arrays.fill(nums, 7);  // [7, 7, 7]
Arrays.setAll(nums, i -> i + 1);  / [8, 8, 8];

Summary

• Array stores multiple values of the same type.
• Index starts at 0.
• Use .length for size.
• Can be 1D or multidimensional.
• Size is fixed after creation.