///////    Matrix.java ///////

class Matrix
{   public Matrix(int r, int c)
    {   if ( r > 0 && c > 0 )
             mat = new double[nr=r][nc=c];
        else 
        {   mat = null; nr=nc=0;   }
    }

    public Matrix(double[][] m) 
    {   mat = m; 
        nr = m.length; 
        nc = m[0].length;
    }

    public double getElement(int i, int j)
    {   return mat[i][j];  }

    public void setElement(int i, int j, double e)
    {   mat[i][j] = e;  }

    public int rows() { return nr; }
    public int cols() { return nc; }

    public Matrix times(Matrix b)
    {   if ( nc != b.nr ) return null;  // incompatible dimensions
        Matrix p = new Matrix(nr,b.nc);
        for (int i=0 ; i < nr ; i++)
        {   for (int j=0 ; j < b.nc; j++)
                p.setElement(i,j, 
                  rowTimesCol(i, b.mat, j));
        }
        return(p);
    }

    public String toString()
    {   StringBuffer buf = new StringBuffer();
        for (int i = 0; i < nr; i++)
        {  buf.append("( ");
           for (int j = 0; j < nc-1; j++)
           {   buf.append(mat[i][j]);
               buf.append("  ");
           }
           buf.append(mat[i][nc-1]);
           buf.append(" )\n");
        }
        buf.setLength(buf.length()-1);
        return new String(buf);
    }

    public static void main(String[] args)
    {   double[][] a = { {1.0,-2.0,1.5}, {1.0,2.4,3.1}};
        double[][] b = { {9.0,0.7},{-2.0,30.5},{-9.0,4.0}};
        Matrix m1 = new Matrix(a);
        Matrix m2 = new Matrix(b);
        Matrix prod = m1.times(m2);
        System.out.println(prod);
    }

    private double rowTimesCol(int ri, double[][] b, int ci)
    {   double sum=0.0d;
        double[] row = mat[ri];
        for (int k=0; k < b.length; k++)
            sum += row[k] * b[k][ci];
        return(sum);
    }

    private double[][] mat; // the matrix
    private int nr, nc;     // rows and cols
}