Java Tutorial - Array Chapter

1. Introduction

What is an Array?

An array in Java is a fixed-size collection of elements of the same data type. It lets you store multiple values in a single variable name.

Without Array


int m1 = 78;
int m2 = 85;
int m3 = 92;
int m4 = 67;
int m5 = 88;

With Array


int[] marks = {78, 85, 92, 67, 88};

All elements are of same type (e.g. int).
Size is fixed when array is created.
Index starts from 0 to length - 1.

2. Basics

Declaration and Creation

Declaration


// Recommended
int[] numbers;

// Also valid
int marks[];

Creation (Memory Allocation)


int[] numbers = new int[5];     // 5 integers, default 0
double[] prices = new double[3]; // 3 doubles, default 0.0
String[] names = new String[2];  // 2 references, default null

Valid index range is 0 to length - 1. Access outside this range throws ArrayIndexOutOfBoundsException.

3. Initialization

Initializing Arrays

Direct Initialization (Array Literal)


int[] nums = {10, 20, 30, 40};
String[] cities = {"Delhi", "Mumbai", "Pune"};

Index-wise Initialization


int[] nums = new int[4];

nums[0] = 10;
nums[1] = 20;
nums[2] = 30;
nums[3] = 40;

4. Input

Reading Array from User


import java.util.Scanner;

public class ReadArray {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter size of array: ");
        int n = sc.nextInt();

        int[] arr = new int[n];

        System.out.println("Enter " + n + " elements:");
        for (int i = 0; i < n; i++) {
            arr[i] = sc.nextInt();
        }

        System.out.println("You entered:");
        for (int i = 0; i < n; i++) {
            System.out.println("Index " + i + " = " + arr[i]);
        }

        sc.close();
    }
}

5. Traversing

Traversing an Array

Using `for` loop


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

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

Using enhanced `for-each` loop


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

for (int value : arr) {
    System.out.println(value);
}

6. Programs

Basic Programs on Arrays

1. Sum of elements


int[] arr = {5, 10, 15, 20};
int sum = 0;

for (int x : arr) {
    sum += x;
}

System.out.println("Sum = " + sum);

2. Maximum element


int[] arr = {3, 8, 2, 10, 6};

int max = arr[0];

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

System.out.println("Maximum = " + max);

7. Types

Types of Arrays

1. One-Dimensional Array


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

2. Two-Dimensional Array (Matrix)


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

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

8. Built-in

Built-in Functions (java.util.Arrays)

Utility methods in java.util.Arrays:

Arrays.toString()


import java.util.Arrays;

int[] arr = {10, 20, 30};
System.out.println(Arrays.toString(arr)); // [10, 20, 30]

Arrays.sort()


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

Arrays.sort(arr);

System.out.println(Arrays.toString(arr)); // [10, 20, 30, 40]

Arrays.binarySearch()


int[] arr = {10, 20, 30, 40, 50};
int index = Arrays.binarySearch(arr, 30); // 2

Arrays.equals()


int[] a = {1, 2, 3};
int[] b = {1, 2, 3};

System.out.println(Arrays.equals(a, b)); // true

9. Practice

Practice Problems

Reverse an array without using a second array.
Count how many elements are even and odd.
Find the second largest element without sorting.
Check if an array is sorted in ascending order.
Merge two arrays into a third array.
Count frequency of a given number in an array.