CODEWITHSUNDEEP
java
Carla
Carla

Reputation: 3380

Using Stream API to fill up a Collection

I'm just learning the java.util.stream API and I'm looking for a way to quickly fill up a Collection with some data.

I've come up with this code to add 5 random numbers:

List<Integer> lottery = Stream.of(random.nextInt(90), random.nextInt(90), random.nextInt(90),random.nextInt(90),
random.nextInt(90)).collect(Collectors.toList());

That would be however a problem in case I have to add hundreds of items. Is there a more concise way to do it using the java.util.stream API?

(I could obviously loop in the ordinary way...)

Thanks

Upvotes: 3

Views: 1562

Answers (3)

AstroMan
AstroMan

Reputation: 71

This should do:

List<Integer> randomList = new ArrayList<>();
SecureRandom random = new SecureRandom();
IntStream.rangeClosed(1,100).forEach(value->randomList.add(random.nextInt(90)));

Upvotes: 0

Olivier Gr&#233;goire
Olivier Gr&#233;goire

Reputation: 35427

Use Random.ints(long, int, int)

Everyone else on this page is generating the stream because they can, but Java already has such an out of the box method, Random.ints(long, int, int), available since Java 8 (and therefore the beginning of streams).

Just use it like this:

List<Integer> lottery = rnd.ints(5, 0, 90)
                           .boxed()
                           .collect(Collectors.toList());

The instruction rnd.ints(5L, 0, 90) means: "create a stream of 5 integers between 0 (included) and 90 (excluded)". So if you want 100 numbers instead of 5, just change 5 to 100.

Upvotes: 4

Kayaman
Kayaman

Reputation: 73548

In this case the stream API only makes things more complicated vs. a regular loop, but you could create an IntStream (or Stream<Integer> depending on where you convert the int to Integer) that generates infinite random numbers with

IntStream rndStream = IntStream.generate(() -> rnd.nextInt(90));

Now you can use limit() to create non-infinite substreams that can be collected, such as

rndStream.limit(5).boxed().collect(Collectors.toList());

However this is just stream trickery and has no advantages over say having a method called List<Integer> getRandoms(int number) with a regular loop inside.

There's no reason to use the code displayed in this answer as it's less readable and more complex than alternatives. This just demonstrates how to get finite amounts of elements from an infinitely generated stream.

Upvotes: 4

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