Multiply

public FractionMatrix multiply(FractionMatrix that)
    {
        Fraction sum = new Fraction( 0, 1 );
        if (this.numberColumns() != that.numberRows())
            throw new MatrixException("incompatible matrix dimensions");
        Fraction[][] product = new Fraction[this.numberRows()][that.numberColumns()];
        int constant = this.numberRows();
        for (int row = 0; row < this.numberRows(); row++)
        {
            for (int col = 0; col < that.numberColumns(); col++)
            {
                for( int a = 0; a <= constant; a++ )
                {
                        sum = sum.add( this.get( row, a ).multiply( that.get( a, col ) ) );
                        System.out.println( sum );
                }
                product[row][col] = sum;
                sum = new Fraction( 0 , 1 );
            }
        }
        return new FractionMatrix( product );
    }