TheAlgorithms/Java · error · IllegalArgumentException

Matrices cannot be multiplied: incompatible dimensions.

Error message

Matrices cannot be multiplied: incompatible dimensions.

What it means

Thrown by MatrixMultiplication.multiply when matrixA's column count (matrixA[0].length) does not equal matrixB's row count (matrixB.length). Matrix multiplication A x B is only defined when cols(A) == rows(B); otherwise the inner k-loop would index out of bounds, so the guard rejects the operation with a clear message.

Source

Thrown at src/main/java/com/thealgorithms/matrix/MatrixMultiplication.java:49

     * @param matrixA the first matrix rowsA x colsA
     * @param matrixB the second matrix rowsB x colsB
     * @return the product of the two matrices rowsA x colsB
     * @throws IllegalArgumentException if the matrices cannot be multiplied
     */
    public static double[][] multiply(double[][] matrixA, double[][] matrixB) {
        // Check the input matrices are not null
        if (matrixA == null || matrixB == null) {
            throw new IllegalArgumentException("Input matrices cannot be null");
        }

        // Check for empty matrices
        if (matrixA.length == 0 || matrixB.length == 0 || matrixA[0].length == 0 || matrixB[0].length == 0) {
            throw new IllegalArgumentException("Input matrices must not be empty");
        }

        // Validate the matrix dimensions
        if (matrixA[0].length != matrixB.length) {
            throw new IllegalArgumentException("Matrices cannot be multiplied: incompatible dimensions.");
        }

        int rowsA = matrixA.length;
        int colsA = matrixA[0].length;
        int colsB = matrixB[0].length;

        // Initialize the result matrix with zeros
        double[][] result = new double[rowsA][colsB];

        // Perform matrix multiplication
        for (int i = 0; i < rowsA; i++) {
            for (int j = 0; j < colsB; j++) {
                for (int k = 0; k < colsA; k++) {
                    result[i][j] += matrixA[i][k] * matrixB[k][j];
                }
            }
        }
        return result;

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Verify matrixA[0].length == matrixB.length before calling; transpose one operand if needed.
2. Log both shapes at the call site to diagnose mismatches quickly.
3. Wrap multiplication in a helper that documents the cols(A)==rows(B) contract.

Example fix

// before
double[][] r = MatrixMultiplication.multiply(a, b);

// after
if (a[0].length != b.length) {
    // transpose b or swap operands as appropriate
    b = MatrixTranspose.doubleTranspose(b); // your transpose helper
}
double[][] r = MatrixMultiplication.multiply(a, b);

if (matrixA[0].length != matrixB.length) {
    throw new IllegalArgumentException(
        "cols(A)=" + matrixA[0].length + " must equal rows(B)=" + matrixB.length);
}
double[][] r = MatrixMultiplication.multiply(matrixA, matrixB);

When it happens

Trigger: Call multiply on a 2x3 matrix A and a 2x4 matrix B (cols(A)=3 != rows(B)=2). Common with transposed operands, swapped argument order, or data loaded with rows/columns inverted.

Common situations: Transposing one matrix but not the other, passing (B, A) instead of (A, B), CSV loaded as row-major when the algorithm expects column-major, or a refactor that changed orientation.

Related errors

• Input matrices cannot be null
• Input matrices must not be empty
• Maze must not be null or empty.
• Maze must be a square (n x n) matrix.
• Matrix A cannot be empty.

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/2ee6ba934580aa84. Report an issue: GitHub.