Java Arrays: Single-Dimensional, 2D Matrices & Jagged Arrays

An Array is a foundational data structure in Java that stores a fixed-size sequential collection of elements of the exact same data type. In Java, all arrays are dynamically allocated Objects on the Heap.

1. Single-Dimensional Arrays (1D)

Arrays use zero-based indexing, meaning the first element is at index 0 and the last element is at index length - 1:

public class ArrayDemo {
    public static void main(String[] args) {
        // Approach 1: Declare and allocate size (initialized to 0)
        int[] numbers = new int[5];
        numbers[0] = 10;
        numbers[1] = 20;

        // Approach 2: Declare with literal initialization
        String[] fruits = { "Apple", "Mango", "Banana", "Orange" };

        // Reading length and accessing items:
        System.out.println("Array Size: " + fruits.length); // 4
        System.out.println("First Fruit: " + fruits[0]);     // Apple
        System.out.println("Last Fruit: " + fruits[fruits.length - 1]); // Orange
    }
}

2. Two-Dimensional Arrays (Matrices)

A 2D array represents data in a tabular grid of rows and columns:

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

// Accessing element at Row 1, Column 2:
System.out.println("Element [1][2]: " + matrix[1][2]); // Output: 6

3. Jagged Arrays (Arrays of Unequal Row Lengths)

Because Java treats multidimensional arrays as "arrays of arrays", each row can hold a sub-array of differing length:

public class JaggedArrayDemo {
    public static void main(String[] args) {
        // Declare 2D array with 3 rows, unspecified columns
        int[][] jagged = new int[3][];

        jagged[0] = new int[2]; // Row 0 has 2 columns
        jagged[1] = new int[4]; // Row 1 has 4 columns
        jagged[2] = new int[1]; // Row 2 has 1 column

        System.out.println("Row 0 columns: " + jagged[0].length); // 2
        System.out.println("Row 1 columns: " + jagged[1].length); // 4
    }
}