Arrays in Java

Introduction

An array in Java is a container object that holds a fixed number of values of a single type. It is one of the most fundamental data structures used to store collections of data efficiently in contiguous memory locations.

Core Concepts

Fixed Size: Once an array is created, its length cannot be changed.
Same Data Type: All elements in an array must be of the exact same type (primitive types like int, double or object references like String, CustomClass).
Contiguous Memory: Arrays store elements in contiguous memory locations, which allows for fast, constant-time $O(1)$ access using indices.
Zero-Indexed: In Java, array indexing starts at 0. If an array has a size of $N$ , the valid indices are $0$ to $N-1$ .

Basic Operations

1. Declaration

You can declare an array by specifying the data type followed by square brackets [].

int[] myNumbers;    // Recommended Java style
String myStrings[]; // Alternative style (C/C++ style), less common in Java

2. Initialization and Allocation

You must define the size of the array when you allocate memory for it using the new keyword, or you can initialize it directly with values.

// Declaration and memory allocation (size 5)
// Elements are initialized to default values (0 for int, null for objects, false for boolean)
int[] myNumbers = new int[5]; 

// Declaration with inline values (Array Literal)
int[] myInitializedArray = {10, 20, 30, 40, 50};

3. Accessing and Modifying Elements

You access elements by placing the index inside the square brackets.

int[] scores = {95, 85, 75, 60, 90};

int firstScore = scores[0]; // Accessing the first element (95)
scores[2] = 100;            // Modifying the third element from 75 to 100

4. Finding Length

Every array has a built-in .length property (not a method, so no parentheses) that returns the number of elements the array can hold.

int size = scores.length; // Returns 5

Common Mistakes & Exceptions

ArrayIndexOutOfBoundsException: Attempting to access an index outside the valid range (e.g., negative index or index $\ge$ length) will throw an exception at runtime.
```
int[] arr = new int[3];
System.out.println(arr[3]); // THROWS ArrayIndexOutOfBoundsException! Valid indices are 0, 1, 2.
```
Fixed Size Limitation: Forgetting that arrays cannot be resized. If you need a collection that can grow or shrink dynamically, use ArrayList from the java.util package instead of a standard array.

Advanced Tip: Multi-dimensional Arrays

Java supports arrays of arrays, commonly used to represent 2D matrices.

// A 2D array with 3 rows and 4 columns
int[][] matrix = new int[3][4];

// Inline initialization
int[][] grid = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};
System.out.println(grid[1][2]); // Output: 6