@Override
	@SuppressWarnings("unchecked")
	public boolean addAll(Collection<? extends E> c) {
		if(c.isEmpty()) return false;
		if(c instanceof Set) return addAll((Set<? extends E>)c);
		else throw new UnsupportedOperationException("May only add sets");
	}
@ Override @ SuppressWarnings ( "unchecked" ) public boolean addAll ( Collection < ? extends E > c ) { if ( c . isEmpty ( ) ) return false ; if ( c instanceof Set ) return addAll ( ( Set < ? extends E > ) c ) ; else throw new UnsupportedOperationException ( "May only add sets" ) ; }
Must be a set.

@see  #addAll(java.util.Set)