public interface Iterator<E> {

	boolean hasNext();

	default void remove() {
		throw new UnsupportedOperationException("remove");
	}

	default void forEachRemaining(Consumer<? super E> action) {
		Objects.requireNonNull(action);
		while(hasNext())
			action.accept(next());	
	}
}

List<Integer> list = new ArrayList<>();
Iterator it = list.iterator();

while(it.hasNext()) {
	System.out.println(it.next());
}

Iterator it = map.keySet().iterator();
Iterator it = map.entrySet().iterator();
Iterator it = map.values().iterator();