This solution sohuld not pass:

class MultiplyAsString {
  public static String multiply (String a, String b) {
    return new java.math.\u0042igInteger(a).multiply(new java.math.\u0042igInteger(b)).toString();
  }
}

In general, Java does not seem to be the best fit for this challenge, because it's difficult to forbid use of built-in BigInteger.

Random tests are fixed, thanks.

Hey!
Already solved the issues, hope the kata is good to go.

Thanks.

Tests are susceptible to mutation of inputs: when my solution changes tracks, it affects presentation of inputs in messages displayed by tests

This solution passes all random tests:

return new HashMap<Character, List<Character>>();

Java translation!!

p.s. thanks for checking :)