RSA problem

Hi friends,

my trying to create implementation of RSA algorithm, when I follow the algorithm I create:
random big prime integers - p and q
calculate n = p*q
phi = (p-1)*(q-1)
also, I calculate, e and d

I knoe that n and e represent the public key, but I have to represent the public key as single string, is there way how to this conversions:

n, e ---> public_key_as_string_variable

public_key_as_string -> n and e BigInteger variables

n, d ---> private_key_as_string_variable

private_key_as_string_variable ---> n, d

Thanks you very much in advanced for every help