EDIT

    /**

     * EnumSets being part of the model should be immutable to make hibernate know if they have been changed.

     * Therefore any change to the enum set should result in a new enum set.

     */

    protected EnumSet<CdmClass> newEnumSet(@NotNull EnumSet<CdmClass> enumSet, CdmClass additionalClass, CdmClass classToRemove) {

        EnumSet<CdmClass> result = EnumSet.copyOf(enumSet);

        if (additionalClass != null){

            result.add(additionalClass);

        }

        if (classToRemove != null){

            result.remove(classToRemove);

        }

        return result;

    }