CODEWITHSUNDEEP
javaiterator
graviton
graviton

Reputation: 463

When would it be useful to implement Iterator but not Iterable?

When would it be useful to implement iterator without implementing iterable?

Or is implementing iterator simply a by product of implementing iterable?

Upvotes: 3

Views: 220

Answers (3)

Boris the Spider
Boris the Spider

Reputation: 61128

These two are related but not the same.

A List is Iterable - you can get its Iterator. It is not an Iterator.

An Iterator is a single use class that can iterate along a Collection of objects using the hasNext and next methods.

An Iterable is a Collection class that returns an Iterator instance when then the iterator() method is called.

I would go as far as to say that I see no case where an Iterable should implements Iterator. And as a Iterator is single use I cannot see a case where Iterator should implements Iterable.

Upvotes: 4

Chris Gerken
Chris Gerken

Reputation: 16392

Generally speaking, you'd implement either an Iterable or an Iterator to hide the implementation of a collection from the code that's iterating over the collection. Either one would work well. The difference lies in how many times the collection can be traversed. An iterator can only traverse the collection once while the Iterable can traverse the collection many times. You can traverse the Iterable by asking it for an Iterator and you can do that multiple times.

Upvotes: 1

Andrey Chaschev
Andrey Chaschev

Reputation: 16476

They are very similar in semantics and use cases, but differ in implementation. Iterable is a simple factory for Iterators which can be used inside for loops. For this reason it's might be more convenient to implement an Iterable.

for(String s : getIterable()){
    ...
}

Versus:

Iterator<String> it = getIterator();

while(it.hasNext()){
    String s = it.next();
    ...
}

However, in some case Iterator might not be re-instantiated, i.e. when you're running through a db request results, so in this case you can't make it Iterable without re-sending your request.

Upvotes: 0

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