code: public String createFieldTypeSignature(VariableElement variable) {
      if (!hasGenericSignature(variable.asType())) {
        return null;
      }
      StringBuilder sb = new StringBuilder();
      genTypeSignature(variable.asType(), sb);
      return sb.toString();
    }
docs: Create a field signature string for a specified variable.
  
  @return the signature if field type is a type variable, else null.