Array Basics

1. Introduction

An array in Java is a container object that holds a fixed number of values of the same type.
Each value in the array is called an element, and every element is accessed using an index.

Key points:

• Arrays store multiple values in a single variable
• All elements in an array must be of the same data type
• Array size is fixed once created
• Elements are stored in contiguous memory locations

Example:

int[] marks = {85, 90, 78, 92};

Here:

• int[] → type of the array (array of integers)
• marks → array name
• {85, 90, 78, 92} → elements of the array

2. Array Terminology

To understand arrays clearly, remember these terms:

• Array type: data type of elements (int[], double[], String[])
• Array name: variable name pointing to the array (marks, prices)
• Array element: each value inside the array (marks[0], marks[1])
• Index: position number of the element (starts from 0)
• Array length: total number of elements in the array (marks.length)

Example:

int[] marks = {85, 90, 78, 92};

// elements
// marks[0] -> 85
// marks[1] -> 90
// marks[2] -> 78
// marks[3] -> 92

int length = marks.length; // 4

3. Array Declaration

You can declare an array in two ways:

int[] numbers;   // preferred style
int numbers[];   // also valid

Both are correct, but the first style is more readable and commonly used. Declaration only tells Java:

• The type of array
• The name of array

It does not create memory for elements yet.

4. Array Creation (Overview)

Array creation happens using the new keyword:

numbers = new int[5];

This:

• Creates an array that can hold 5 integers
• Index range: 0 to 4
• All elements are initialized with default values (0 for int)

Declaration and creation can be combined:

int[] numbers = new int[5];

We explain creation in detail in creation-of-array.mdx, but here you should remember:

• [] means “array”
• [size] decides how many elements the array can hold

5. Initializing Arrays

You can assign values to array elements in two ways:

5.1 Using Index Assignment

int[] numbers = new int[3];

numbers[0] = 10;
numbers[1] = 20;
numbers[2] = 30;

5.2 Using Array Literal

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

This is shorter and more convenient when values are already known.

6. Accessing Array Elements

You can access elements using their index:

int[] marks = {85, 90, 78};

System.out.println(marks[0]); // 85
System.out.println(marks[1]); // 90
System.out.println(marks[2]); // 78

Important:

• First element index → 0
• Last element index → length - 1

So if marks.length is 3, valid indices are 0, 1, 2.

7. Array Length

Every array in Java has a public length field:

int[] numbers = {1, 2, 3, 4, 5};
System.out.println(numbers.length); // 5

Use length when iterating:

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

This ensures:

• You do not go out of bounds
• Your loop adapts if array size changes

8. Default Values in Arrays

When you create an array using new, its elements get default values:

Example:

boolean[] flags = new boolean[3];

System.out.println(flags[0]); // false
System.out.println(flags[1]); // false
System.out.println(flags[2]); // false

9. Common Errors in Arrays

9.1 ArrayIndexOutOfBoundsException

Occurs when you try to access index outside the valid range.

int[] a = new int[3];

// valid: 0, 1, 2

a[3] = 10; // Error at runtime

Always ensure:

0 <= index < array.length

9.2 Forgetting to Initialize the Array

int[] a;
a[0] = 10; // Error: a is not initialized (NullPointerException)

You must create the array before use:

a = new int[5];

10. Simple Complete Example

public class ArrayBasics {
    public static void main(String[] args) {

        // Declare and create array
        int[] marks = new int[3];

        // Initialize elements
        marks[0] = 85;
        marks[1] = 90;
        marks[2] = 78;

        // Print all elements
        for (int i = 0; i < marks.length; i++) {
            System.out.println("marks[" + i + "] = " + marks[i]);
        }
    }
}

Output:

marks[0] = 85
marks[1] = 90
marks[2] = 78

11. Summary

• An array is a collection of elements of the same type, stored in contiguous memory.
• Each element is accessed by an index starting from 0.
• Arrays have a fixed size determined at creation.
• Use array.length to safely iterate over elements.
• Default values are automatically assigned when using new.

Written By: Shiva Srivastava