2D Arrays (Matrices)

A 2D Array in Java is effectively an array of arrays. It represents a grid or a matrix of elements.

Representation

Instead of a single line of elements, imagine a grid with rows and columns.

• Row 0: [1, 2, 3]
• Row 1: [4, 5, 6]
• Row 2: [7, 8, 9]

Element 5 is at index [1][1] (Row 1, Column 1). Element 3 is at [0][2].

Implementation in Java

public class MatrixExample {
    public static void main(String[] args) {
        // Declaring a 3x3 2D array
        int[][] matrix = new int[3][3];

        // Assigning values
        matrix[0][0] = 1;
        matrix[0][1] = 2;
        matrix[0][2] = 3;
        matrix[1][0] = 4;
        matrix[1][1] = 5;
        // ...
        
        // Literal 2D Array Initialization
        int[][] grid = {
            {1, 2, 3}, // Row 0
            {4, 5, 6}, // Row 1
            {7, 8, 9}  // Row 2
        };

        // Traversal
        for (int i = 0; i < grid.length; i++) { // iterate over rows
            for (int j = 0; j < grid[i].length; j++) { // iterate over columns
                System.out.print(grid[i][j] + " ");
            }
            System.out.println(); // new line after each row
        }
    }
}

Jagged Arrays

Since a 2D array in Java is an array of arrays, the inner arrays don't have to be of the same length! These are called Jagged Arrays.

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