Converting from Integer, to BigInteger

The method you want is BigInteger#valueOf(long val).

E.g.,

BigInteger bi = BigInteger.valueOf(myInteger.intValue());

Making a String first is unnecessary and undesired.

converting Integer to BigInteger

        BigInteger big_integer = BigInteger.valueOf(1234);

converting BigInteger back to Integer

int integer_value = big_integer.intValue();

converting BigInteger back to long

long long_value = big_integer.longValue();

converting string to BigInteger

        BigInteger bigInteger = new BigInteger("1234");

converting BigInteger back to string

        String string_expression = bigInteger.toString();