How can I convert a generic list to an array?

Question

I'm beginning to discover the wonders of Java generics...

Apparently, you can't create a generic array:

Stuff<Thing>[] array = new Stuff<Thing>[5]; // doesn't compile

So obviously, you can't do this either (which works perfectly when generics aren't involved):

// thingsList is an ArrayList<Stuff<Thing>>
Stuff<Thing>[] array = thingsList.toArray(new Stuff<Thing>[0]);

So my question is, how can I get around this? How can I easily convert a generic list to an array?

Try going through this post - http://stackoverflow.com/q/18581002/1679863, you might get your answer. — Rohit Jain, Aug 21 '14 at 18:40
Why would you expect an `ArrayList` to convert to `Stuff[]`, and not `Thing[]`? That would certainly not work. — Rohit Jain, Aug 21 '14 at 18:43
See my answer to [this question](http://stackoverflow.com/questions/18413571/java-2d-array-of-arraylists/18413894#18413894); one possibility is to define a non-generic type that behaves the same as `Stuff`. — ajb, Aug 21 '14 at 18:59

score 4 · Accepted Answer · answered Aug 21 '14 at 18:43

4

ArrayList<Stuff<Thing>> thingsList = new ArrayList<Stuff<Thing>>();
Stuff<Thing>[] array = (Stuff<Thing>[] )thingsList.toArray(new Stuff[0]);

answered Aug 21 '14 at 18:43

satellite779

Actually, Object solution would throw an exception. Updated. – satellite779 Aug 21 '14 at 18:46
I once had someone suggest using `>`, i.e. `new Stuff>[0]` in your example. I don't know whether that makes any difference. – ajb Aug 21 '14 at 18:59

EpicPandaForce · Answer 2 · 2014-08-22T08:03:23.230

It would be the following:

Stuff<Thing>[] array = thingsList.toArray(new Stuff<Thing>[thingsList.size()]);

However, it is a restriction on Generics that you can't make parametrized arrays, see http://docs.oracle.com/javase/tutorial/java/generics/restrictions.html#createArrays

So only the following would work:

Stuff[] array = thingsList.toArray(new Stuff[thingsList.size()]);

But that's a raw type.

Therefore, it's recommended to just use a List<Stuff<Thing>> instead rather than an array.

2 Answers2