How to Rotate Matrix 90 Degrees in Java - Simple Guide

To rotate a matrix 90 degrees clockwise in Java, create a new matrix and set each element at position rotated[j][n - 1 - i] to the original element at matrix[i][j]. This shifts rows to columns in reverse order, effectively rotating the matrix.

Syntax

To rotate a matrix 90 degrees clockwise, you use a nested loop to copy elements from the original matrix to a new matrix with swapped indices.

• matrix[i][j]: element at row i and column j in the original matrix.
• rotated[j][n - 1 - i]: position in the rotated matrix where the element is placed.
• n: size of the matrix (number of rows or columns).

java

for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
        rotated[j][n - 1 - i] = matrix[i][j];
    }
}

Example

This example shows how to rotate a 3x3 matrix 90 degrees clockwise and print the result.

java

public class RotateMatrix {
    public static void main(String[] args) {
        int[][] matrix = {
            {1, 2, 3},
            {4, 5, 6},
            {7, 8, 9}
        };
        int n = matrix.length;
        int[][] rotated = new int[n][n];

        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                rotated[j][n - 1 - i] = matrix[i][j];
            }
        }

        System.out.println("Original matrix:");
        printMatrix(matrix);
        System.out.println("\nRotated matrix 90 degrees clockwise:");
        printMatrix(rotated);
    }

    private static void printMatrix(int[][] matrix) {
        for (int[] row : matrix) {
            for (int val : row) {
                System.out.print(val + " ");
            }
            System.out.println();
        }
    }
}

Output

Original matrix:
1 2 3 
4 5 6 
7 8 9 

Rotated matrix 90 degrees clockwise:
7 4 1 
8 5 2 
9 6 3 

Common Pitfalls

Common mistakes when rotating a matrix include:

• Modifying the original matrix in place without a proper algorithm, which can overwrite needed values.
• Mixing up the indices, such as using rotated[i][j] instead of rotated[j][n - 1 - i].
• For non-square matrices, this method needs adjustment because the number of rows and columns differ.

java

/* Wrong way: overwriting original matrix without backup */
for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
        matrix[j][n - 1 - i] = matrix[i][j]; // Overwrites original data
    }
}

/* Right way: use a new matrix to store rotated values */
int[][] rotated = new int[n][n];
for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
        rotated[j][n - 1 - i] = matrix[i][j];
    }
}

Quick Reference

To rotate a square matrix 90 degrees clockwise:

• Create a new matrix of the same size.
• Use nested loops to assign rotated[j][n - 1 - i] = matrix[i][j].
• Print or use the rotated matrix as needed.

Step	Action	Code snippet
1	Create new matrix	int[][] rotated = new int[n][n];
2	Loop through original matrix	for (int i = 0; i < n; i++) {...}
3	Assign rotated values	rotated[j][n - 1 - i] = matrix[i][j];
4	Use rotated matrix	printMatrix(rotated);

Key Takeaways

Always use a new matrix to store rotated values to avoid overwriting original data.

Rotate by assigning element at (i, j) to (j, n - 1 - i) in the new matrix.

This method works only for square matrices; non-square matrices need different handling.

Use nested loops to iterate through all elements for rotation.

Print the rotated matrix to verify the result visually.