TheAlgorithms/Java · error · IllegalArgumentException

Input matrices cannot be null

Error message

Input matrices cannot be null

What it means

Thrown by MatrixMultiplication.multiply when matrixA or matrixB is null. The null check runs first, before any indexing, so a null operand produces a clear message rather than a NullPointerException on matrixA.length / matrixB.length.

Source

Thrown at src/main/java/com/thealgorithms/matrix/MatrixMultiplication.java:39

 *
 */

public final class MatrixMultiplication {
    private MatrixMultiplication() {
    }

    /**
     * Multiplies two matrices.
     *
     * @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];

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Null-check both operands at the call site and fail or default to a valid matrix.
2. Use Objects.requireNonNull(matrixA, "matrixA") to fail earlier with context.
3. Ensure deserializers return empty arrays instead of null for absent fields.

Example fix

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

// after
Objects.requireNonNull(a, "matrixA");
Objects.requireNonNull(b, "matrixB");
double[][] r = MatrixMultiplication.multiply(a, b);

Objects.requireNonNull(matrixA, "matrixA");
Objects.requireNonNull(matrixB, "matrixB");
double[][] r = MatrixMultiplication.multiply(matrixA, matrixB);

When it happens

Trigger: Call multiply(null, b), multiply(a, null), or pass a matrix reference that was never assigned (e.g., a field left null after a failed initialization).

Common situations: An optional matrix parameter that arrived empty, a deserialization that produced null for an absent JSON field, or a code path that constructs the second operand conditionally and skipped it.

Related errors

• Maze must not be null or empty.
• Input matrices must not be empty
• Matrices cannot be multiplied: incompatible dimensions.
• The input matrix cannot be null
• The input matrix cannot be null

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/97754e3ef597b539. Report an issue: GitHub.