Converting a TreeSet to ArrayList?

Question

I have a TreeSet which contains > 100k objects. I have another method which requires ArrayList as an param.

Is there any way I can accomplish this without iterating whole TreeSet and then adding each object manually to ArrayList ?

Is it possible to relax the method signature to receive a mere Collection? That way no copying is needed. — Brent Worden, Feb 17 '12 at 05:11

score 69 · Accepted Answer · edited Jun 19 '20 at 12:24

69

How about this:

new ArrayList<T>(set);

For Java 7 and later, this can be simplified, as type arguments <T> can be replaced with diamond type <>:

new ArrayList<>(set);

edited Jun 19 '20 at 12:24

Sadeq Dousti

answered Feb 17 '12 at 03:58

yegor256

4

This is the best you're going to get. There is no solution that doesn't require linear time. – Louis Wasserman Feb 17 '12 at 07:01

score 5 · Answer 2 · answered Feb 17 '12 at 03:21

5

ArrayList has a convenience method addAll that fits the bill nicely:

final Set<Object> set = ...
List<Object> list = new ArrayList<Object>(someBigNum);
list.addAll(set);

answered Feb 17 '12 at 03:21

Perception

6

someBigNum could be set.size() – Kuldeep Jain Feb 17 '12 at 04:21
3

someBigNum is very misleading here... You don't need a big num. You just need set.size() as kuldeep pointed out – Joshua Kissoon May 18 '14 at 03:29

score 1 · Answer 3 · answered Jul 02 '20 at 06:28

In case one uses Eclipse Collections, which has super-powers and is developed by Goldman Sachs, there is toList():

MutableSortedSet<Object> set = SortedSets.mutable.empty();
...
return set.toList();

Providing a comparator is also possible:

MutableSortedSet<Object> set = SortedSets.mutable.of(comparator);
...
return set.toList();

3 Answers3