How to get a random element from a list with stream api?

Question

What is the most effective way to get a random element from a list with Java8 stream api?

Arrays.asList(new Obj1(), new Obj2(), new Obj3()); 

Thanks.

Answer 1

Why with streams? You just have to get a random number from 0 to the size of the list and then call get on this index:

Random r = new Random(); ElementType e = list.get(r.nextInt(list.size())); 

Stream will give you nothing interesting here, but you can try with:

Random r = new Random(); ElementType e = list.stream().skip(r.nextInt(list.size())).findFirst().get(); 

Idea is to skip an arbitrary number of elements (but not the last one!), then get the first element if it exists. As a result you will have an Optional<ElementType> which will be non empty and then extract its value with get. You have a lot of options here after having skip.

Using streams here is highly inefficient...

Note: that none of these solutions take in account empty lists, but the problem is defined on non-empty lists.

Answer 2

There are much more efficient ways to do it, but if this has to be Stream the easiest way is to create your own Comparator, which returns random result (-1, 0, 1) and sort your stream:

 List<String> strings = Arrays.asList("a", "b", "c", "d", "e", "f");     String randomString = strings             .stream()             .sorted((o1, o2) -> ThreadLocalRandom.current().nextInt(-1, 2))             .findAny()             .get(); 

ThreadLocalRandom has ready "out of the box" method to get random number in your required range for comparator.