Array Basics

Arrays are a fundamental data structure in Java, allowing you to store and manage collections of elements of the same type efficiently. Here is a simple and comprehensive guide to everything you need to know about arrays in Java.

What is an Array?

• An array is a container object that holds a fixed number of values of a single type (e.g., int, String, double).

• The size (length) of an array is set when it is created and cannot be changed.

• Arrays use a zero-based index system: the first element is at index 0, the second at index 1, and so on.

Declaring and Initializing Arrays

Declaration Syntax:

int[] numbers;      // Preferred style
// or
int numbers[];      // Also valid

Initialization:

• You must allocate memory for the array using the new keyword or initialize it directly.

numbers = new int[5]; // Creates an array of size 5 with default values (0s)

Declaration + Initialization:

int[] numbers = new int[5]; // Array of 5 integers
String[] names = {"Alice", "Bob", "Charlie"}; // Direct initialization

Default Values:

• Numeric types (int, double, etc.): 0

• boolean: false

• Objects (like String): null

Accessing and Modifying Array Elements

• Access elements using their index:

int x = numbers[0]; // Gets the first element
numbers[1] = 42;    // Sets the second element to 42

• Array indices start at 0 and go up to array.length - 1.

Array Length

• Use the .length property to get the size of the array:

int size = numbers.length;

Types of Arrays in Java

1. Single-Dimensional Arrays

• A simple list of elements.

int[] arr = {1, 2, 3, 4, 5};

2. Multi-Dimensional Arrays

• Arrays of arrays (e.g., tables or matrices).

int[][] matrix = new int[3][3]; // 3x3 grid
int[][] matrix = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

3. Jagged Arrays

• Multi-dimensional arrays where sub-arrays can have different lengths.

int[][] jagged = {
    {1, 2},
    {3, 4, 5},
    {6}
};

Looping Through Arrays

• Use a for loop or enhanced for loop to iterate over elements:

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

Array Utility Methods

Java provides the java.util.Arrays class with many helpful methods:

• Sorting: Arrays.sort(arr);

• Binary Search: Arrays.binarySearch(arr, value);

• Filling: Arrays.fill(arr, value);

• Comparing: Arrays.equals(arr1, arr2);

Key Points and Limitations

• Arrays are fixed in size; you cannot add or remove elements after creation.

• All elements must be of the same data type.

• Arrays can store primitive types or objects (including arrays of objects).

• Trying to access an index outside the array bounds causes an ArrayIndexOutOfBoundsException.

Example: Complete Array Usage

int[] scores = new int[3];      // Declaration and memory allocation
scores[0] = 90;                 // Assign values
scores[1] = 85;
scores[2] = 78;
for (int i = 0; i < scores.length; i++) {
    System.out.println("Score " + i + ": " + scores[i]);
}