public Vector multiply(Matrix matrix) {
    int size = matrix.getRows();
    if (size != getSize()) throw new IllegalArgumentException("size mismatch rows");
    double[] result = new double[matrix.getColumns()];
    for (int i = 0; i < result.length; i++) result[i] = multiply(matrix.getColumn(i));
    return new Vector(result);
  }
public Vector multiply(Matrix matrix)