using java Stream to get a sum, average, and sort

Question

I am studying java Stream and having hard time solving questions below. The reason why I got stuck is because I have no idea about dealing with Stream<Integer>.

I stumbled upon a solution of "count" by doing list.stream().count(), but other than this, I can't proceed further. Please help me how to deal with these problems and tell me why list.stream().count() works in this situation. So far, I've tried everything I've learned.

public class Question {
    public static void main(String[] args) {
        List<Integer>list = Arrays.asList(5,3,4,1,2);
        System.out.println("sum by using Stream : " + sum);
        System.out.println("count by using Stream: " + count);
        System.out.println("average by using Stream : " + avg);
        System.out.println("sort by using Stream");
    }
}

Is it me or is your code here incomplete ? How is `sum`, `count` and `avg` being initialized? — KarelG, Feb 21 '18 at 12:59
Did you have a look at the Streams API? It tells you what methods exist. And there are lots, lots of informations at the Oracle site. — glglgl, Feb 21 '18 at 13:00
Also [`.count()`](https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#count--) from a streamable gives you the _number_ of elements in that stream. — KarelG, Feb 21 '18 at 13:00
it is incomplete, so i need lines of code to work out those system.out.println() and i don`t know how to get the sum, avg, and sort. — KingJinho, Feb 21 '18 at 13:01
Start here: https://docs.oracle.com/javase/8/docs/api/java/util/stream/package-summary.html the example even shows sample code of a sum. — AlexC, Feb 21 '18 at 13:04
If one of the answers solved the problem, can you [accept one of them](https://meta.stackexchange.com/questions/5234/how-does-accepting-an-answer-work) as the correct solution? (and upvote on the useful ones) — Juan Carlos Mendoza, Mar 14 '18 at 20:23

score 35 · Answer 1 · edited Feb 21 '18 at 14:13

35

IntSummaryStatistics stats = Arrays.asList(1,2,3,4)
    .stream()
    .mapToInt(Integer::intValue)
    .summaryStatistics();

stats.getSum();
stats.getCount();
stats.getAverage();

For the sorted, you will have to stream again.

edited Feb 21 '18 at 14:13

banan3'14

3,810
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answered Feb 21 '18 at 13:09

Eugene

117,005
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5

or `Arrays.asList(1,2,3,4) .stream() .collect(Collectors.summarizingInt(Integer::intValue));` – Holger Feb 21 '18 at 13:29
@Holger Is there a way to know when to choose one implementation or the other? I mean, many times, there are operations that can be done either with stream intermediate operations or with collectors, i.e. `Stream.map` vs `Collectors.mapping`, reducing, fitlering, flatmapping and summarizing, etc – fps Feb 21 '18 at 16:00
3

@FedericoPeraltaSchaffner for this specific case, it doesn’t matter and the choice is yours. It’s worth knowing both, as it may make a difference when you want to extend the code, see [this answer](https://stackoverflow.com/a/37025002/2711488) for details. And [this one](https://stackoverflow.com/a/44259061/2711488). – Holger Feb 21 '18 at 17:28
@Eugene The IntSummaryStatistics idea was a recent memory finally :) – davidxxx Feb 25 '18 at 21:42

Juan Carlos Mendoza · Answer 2 · 2018-02-21T13:27:26.157

The reason of why list.stream().count() works but list.stream().sum() doesn't is because list.stream() returns a Stream<Integer> and there is a Stream::count method, but there isn't a Stream::sum or Stream::average.

To get the sum and avg first you have to map each integer value in the Stream<Integer> that you get when you do list.stream() to an IntStream. This is the int primitive specialization of Stream. This can be done using the Stream::mapToInt method:

list.stream().mapToInt(Integer::intValue)

Doing this you can use the methods IntStream::sum and IntStream::average:

System.out.println("sum by using Stream : " + list.stream().mapToInt(Integer::intValue).sum());
System.out.println("average by using Stream : " + list.stream().mapToInt(Integer::intValue).average());

Or even better, you can use the IntStream::summaryStatistics to get the sum, count and avg together (also the min and the max value):

System.out.println("sum, count, avg, min and max using Stream : " + list.stream().mapToInt(Integer::intValue).summaryStatistics());

To sort the values you can use the Stream::sorted method:

System.out.println("sort by using Stream: " + list.stream().sorted().collect(Collectors.toList()));

Here is a good post that can help you to understand how to use the Java 8 Stream Api.

Yaroslav · Answer 3 · 2018-02-21T13:18:24.767

You can do just like this:

List<Integer> list = Arrays.asList(5,3,4,1,2);
        System.out.println("sum by using Stream : " + list.stream().mapToInt(Integer::intValue).sum());
        System.out .println("count by using Stream: " + list.stream().mapToInt(Integer::intValue).count());
        System.out.println("average by using Stream : " + list.stream().mapToInt(Integer::intValue).average());
        System.out.println("sort by using Stream: " + list.stream().sorted().collect(Collectors.toList()));

Azzabi Haythem · Answer 4 · 2018-02-21T13:45:18.110

You should use mapToInt or mapToDouble

and you can print list by forEach(System.out::println)

List<Integer> list = Arrays.asList(5,3,4,1,2);

System.out.println("sum by using Stream : " + list.stream().mapToInt(Integer::intValue).sum());

System.out .println("count by using Stream: " + list.stream().count());

System.out.println("average by using Stream : " + list.stream().mapToInt(Integer::intValue).average());

System.out.println("sort by using Stream: " );

list.stream().sorted().forEach(System.out::println);

using java Stream to get a sum, average, and sort

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