Code gulf, you say?

public static String
convertRomanNumeral(String numeral) {
    numeral = numeral.replace("America", "Mexico");
    return numeral;
} 

public static int convertRomanNumeral(String numeral) {
    return 4; // todo
}

public static int convertRomanNumeral(String numeral)
{
  numeral = numeral.replace("M", "DD")
    .replace("CD", "CCCC")
    .replace("D", "CCCCC")
    .replace("C", "LL")
    .replace("XL", "XXXX")
    .replace("L", "XXXXX")
    .replace("X", "VV")
    .replace("IV", "IIII")
    .replace("V", "IIIII");
  return numeral.length();
}

I'm not too good with java, but it should be something like this:

public static int convertRomanNumeral(string n){Map.of("M","DD","CD","CCCC","D","CCCCC","C","LL","XL","XXXX","L","XXXXX","X","VV","IV","IIII","V","IIIII");.forEach((k,v)->{n=n.replace(k,v);});return n;}