markvgti markvgti - 3 months ago 13
Java Question

Which implementation to use when creating a List from Iterable

I find myself frequently doing the following:

Iterator<A> itr = iterableOfA.getIterator();
List<B> list = new ArrayList<>(); // how about LinkedList?
while (itr.hasNext()) {
B obj =;
someMethod(list); // this method takes an Iterable

I have no idea just how many elements are likely to be in
— could be 5, could be 5000. In this case, would
be a better implementation to use here (since
would then be O(1))? As it stands, if
has 5000 elements, this will lead to many resizings of backing array of

Other option is to do:

Iterator<A> itr = iterableOfA.getIterator();
int size = Iterables.size(iterableOfA); // from Guava
List<B> list = new ArrayList<>(size);
// and the rest...

This means double iteration of
. Which option would be best when the size of the iterable is unknowns and can vary wildly:

  1. Just use

  2. Just use

  3. Count the elements in
    and allocate an

Edit 1

To clarify some details:

  1. I am optimizing primarily for performance and secondarily for memory usage.

  2. list
    is a short-lived allocation as at the end of the request no code should be holding a reference to it.

Edit 2

For my specific case, I realized that
doesn't handle an iterable with greater than 200 elements, so I decided to go with
new ArrayList<>(200)
which works well enough for me.

However, in the general case I would have preferred to implement the solution outlined in the accepted answer (wrap in a custom iterable, obviating the need for allocating a list).

All the other answers gave valuable insight into how suitable
is compared to
, so on behalf of the general SO community I thank you all!


I would completely skip copying the elements to a new collection.

We have utility code for easily wrapping Iterators into Iterables and Filter for converting between types, but the gist of it is:

final Iterable<A> iofA ... ;
Iterable<B> iofB = new Iterable<B>() {
  public Iterator<B> iterator() {
    return new Iterator<B>() {
      private final Iterator<A> _iter = iofA.iterator();
      public boolean hasNext() { return _iter.hasNext(); }
      public B next() { return; }

No additional storage, etc. necessary.