Multidimensional Arrays — Java

Java Multidimensional Arrays

What is a 2D Array?

A two-dimensional array represents a grid of values — rows and columns — which is useful for things like game boards, matrices, seating charts, or spreadsheets. In Java, a 2D array is really an array of arrays: the outer array holds references to inner arrays, each of which is a regular 1D array.

Declaring a 2D Array

Java

// 3 rows, 4 columns, all values default to 0
int[][] grid = new int[3][4];

// Declare and fill with literal values
int[][] matrix = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

Accessing Elements with grid[i][j]

Java

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

System.out.println(matrix[0][0]); // first row, first column
System.out.println(matrix[1][2]); // second row, third column

matrix[2][0] = 100; // update bottom-left value
System.out.println(matrix[2][0]);

1
6
100

Note

The first index picks the row, the second picks the column within that row: matrix[row][column].

Traversing with Nested Loops

The standard way to visit every element of a 2D array is a nested for loop — the outer loop walks the rows, the inner loop walks the columns of the current row.

Java

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

for (int row = 0; row < matrix.length; row++) {
    for (int col = 0; col < matrix[row].length; col++) {
        System.out.print(matrix[row][col] + " ");
    }
    System.out.println();
}

1 2 3
4 5 6
7 8 9

Tip

matrix.length gives the number of rows. matrix[row].length gives the number of columns IN THAT SPECIFIC row — which matters a lot once you realize Java arrays don't have to be rectangular.

"Arrays of Arrays": Jagged Arrays

Because a Java 2D array is genuinely an array of separate array objects, there's no rule that every row has to be the same length. You can build a JAGGED array, where each row has its own, independent size. This is a real Java-specific flexibility that languages with true fixed-size matrices don't offer.

Java

int[][] triangle = new int[4][];
triangle[0] = new int[] {1};
triangle[1] = new int[] {1, 2};
triangle[2] = new int[] {1, 2, 3};
triangle[3] = new int[] {1, 2, 3, 4};

for (int[] row : triangle) {
    for (int value : row) {
        System.out.print(value + " ");
    }
    System.out.println();
}

Note

new int[4][] creates the outer array with 4 slots but leaves each inner array unallocated (null) until you assign it explicitly. Each row is then free to have any length you choose.

Rectangular vs Jagged

Aspect	Rectangular 2D array	Jagged array
Row lengths	All equal	Can differ per row
Declaration	new int[rows][cols]	new int[rows][], then fill rows individually
Typical use	Matrices, grids, game boards	Triangular data, variable-length records per row

Practice Exercises

Create a 4x4 int matrix, fill it with the row number times the column number, then print it as a grid.
Write a method that sums all the values in a 2D int array using nested loops.
Build a jagged array representing Pascal's triangle for 5 rows.