code: public static String checkNonEmptyString(final String reference, final String errorMessage) {
          return precondition(reference, Predicates.notEmptyString(),
                  IllegalArgumentException.class, errorMessage);
      }
docs: Check that a String is not empty after removing whitespace.
  
  @param reference    the string
  @param errorMessage the message for the exception
  @throws java.lang.NullPointerException     if the String is null
  @throws java.lang.IllegalArgumentException if the string is non-null but zero length