code: public void addValidator(Validator validator) {
  
          if (validator == null) {
              throw new NullPointerException();
          }
  
          if (validators == null) {
              validators = new AttachedObjectListHolder<Validator>();
          }
          validators.add(validator);
  
      }
docs: <p>Add a {@link Validator} instance to the set associated with
  this {@link UIInput}.</p>
  
  @param validator The {@link Validator} to add
  @throws NullPointerException if <code>validator</code>
  is null