71

I have a List<Thing> and I would like to pass it to a method declared doIt(final Thing... things). Is there a way to do that?

The code looks something like this:

public doIt(final Thing... things)
{
    // things get done here
}

List<Thing> things = /* initialized with all my things */;

doIt(things);

That code obviously doesn't work because doIt() takes Thing not List<Thing>.

Is there a way to pass in a List as the varargs?

This is in an Android App, but I don't see why the solution will not apply to anything Java

xbakesx
  • 13,202
  • 6
  • 48
  • 76

2 Answers2

120

Just pass things.toArray(new Thing[things.size()]).

SLaks
  • 868,454
  • 176
  • 1,908
  • 1,964
  • 7
    It should be `things.toArray(new Thing[things.size()]);`. – Gray Aug 31 '12 at 18:03
  • 2
    That'll do it. (And yes @Gray is right, it's a little more efficient to pass in the array of the appropriate size.) – xbakesx Aug 31 '12 at 18:05
  • 4
    Both work @newacct, but my suggestion doesn't create an array object that will have to be immediately GC'd. – Gray Aug 31 '12 at 21:44
  • 5
    As of Java 6 and beyond, it is usually more performant to do `things.toArray(new Thing[0])`. – chairbender Jun 18 '18 at 23:02
  • 2
    @chairbender: Why? – SLaks Jun 18 '18 at 23:40
  • 3
    @SLaks from the Intellij refactoring suggestion "since late updates of OpenJDK 6 this call was intrinsified, making the performance of the empty array version the same and sometimes even better, compared to the pre-sized version. Also passing pre-sized array is dangerous for a concurrent or synchronized collection as a data race is possible between the size and toArray call which may result in extra nulls at the end of the array, if the collection was concurrently shrunk during the operation." Also, there is a benchmark showing it to be faster: https://stackoverflow.com/a/29444594/765419 – chairbender Jun 20 '18 at 00:24
  • Specifying the array length is overkill in almost all circumstances (and possible, literally all circumstances due to compiler-assistance with this). – Josh M. Nov 07 '19 at 02:08
  • What to do if `Thing` does not have a public constructor? – Anmol Singh Jaggi Aug 28 '21 at 12:15
1

The variadic argument is internally interpreted as an array. So you should convert it into an array beforehands. Also in your doIt method you should access things-s elements with array indexing.

zeller
  • 4,904
  • 2
  • 22
  • 40
  • Thanks for the quick response. That works great. Is there a reason you mentioned accessing the elements by array index, or is it more that you *can* do that? There's no reason not to for(Thing t : things) inside doIt(), right? – xbakesx Aug 31 '12 at 18:04
  • I can't mark both your answer and SLaks answer correct, so I think you should edit his answer, and add the reason that works. Then I'll make that the correct answer. Sorry, people like to see the code in the answer. – xbakesx Aug 31 '12 at 18:11
  • @xbakesx There's no reason that you can't use for-each. – Brian Aug 31 '12 at 18:29
  • Of course enhanced for loop will work on arrays. I just emphasized that method of the collection interface won't work in varargs since it is an array. – zeller Aug 31 '12 at 18:42