what's the point of having both Iterator.forEachRemaining() and Iterable.forEach()?

Question

and both of them get a Consumer as parameter. so if Java 8, is meant to avoid confusions, like it has done in Time API, why has it added a new confusion? or am I missing some point?

Prolepsis

are you trying to compare Iterator and Iterable? #6863682 – Hoary

@JBNizet all right guys, don't throw tomatoes on me! I got it :D – Prolepsis

Answer 1

To understand why the two methods both exist, you need to first understand what are Iterator and Iterable.

An Iterator basically is something that has a "next element" and usually, an end.

An Iterable is something that contains elements in a finite or infinite sequence and hence, can be iterated over by keep getting the next element. In other words, Iterables can be iterated over by Iterators.

Now that you understand this, I can talk about what's the difference between the two methods in question.

Let's use an array list as an example. This is what is inside the array list:

[1, 3, 6, 8, 0]

Now if I call Iterable.forEach() and pass in System.out::print(), 13680 will be printed. This is because Iterable.forEach iterates through the whole sequence of elements.

On the other hand, if I get the Iterator of the array list and called next twice, before calling forEachRemaining with System.out::print(), 680 will be printed. The Iterator has already iterated through the first two elements, so the "remaining" ones are 6, 8 and 0.

Wier

all right. thanks for your clear and complete answer. – Prolepsis

Ok, but where is the difference between forEach() and forEachRemaining() ? pl.kotl.in/ehW6pBa5t – Rosenwald

@Rosenwald What part of my answer did you not understand? forEach iterates through the elements of an Iterable. forEachRemaining iterates through the remaining elements of an Iterator. – Wier

In my linked sample forEach iterates also through the remaining elements. Or does forEach() in Kotlin I used is not the same as Iterable.forEach() in Java? – Rosenwald

@Rosenwald Ah... I didn't understand what you meant by the link. Well, this answer is about Java's Iterable.forEach vs Iterator.forEachRemaining. I am not an expert in Kotlin. It appears that in Kotlin, Iterator also has a forEach method that works exactly like forEachRemaining. I suggest you ask a new question if you want more accurate info on that. – Wier

Answer 2

In Simple word, forEachRemaining() is an improved version of Iterator, where before JDK8 we were uses hasNext(), next() to traverse the any collection(ArrayList, HastSet etc).

List<String> fruits= new ArrayList<>();
fruits.add("Apple");
fruits.add("Banana");
fruits.add("Grapes");
fruits.add("Orange");
Iterator<String> iterator = fruits.iterator();

Before JDK8

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

Now In JDK8 if we want to we want to traverse any collection we can do like this:

After JDK8

iterator.forEachRemaining(System.out::println);

Now, JDK8 forEach() is improved version JDK7 with lamda expression.

Granivorous

Answer 3

Just summarizing for Scala and Kotlin users: in Java, Iterator's forEach is called forEachRemaining (while Iterable has a forEach method).

Cascarilla

Answer 4

Additional comment to @Sweeper's answer:

forEach is for Iterable, which corresponds to a "collection", thus "for each" is "for each element of the collection".

forEachRemaing is for Iterator, which corresponds to a pointer. There may or may not be a "collection" behind the pointer; "for each element of a pointer" is not valid statement.

Bemused