Java 8 int array to map

Question

I want to convert int array to

Map<Integer,Integer>

using Java 8 stream api

int[] nums={2, 7, 11, 15, 2, 11, 2};
Map<Integer,Integer> map=Arrays
                .stream(nums)
                .collect(Collectors.toMap(e->e,1));

I want to get a map like below, key will be integer value, value will be total count of each key

map={2->3, 7->1, 11->2, 15->1}

compiler complains "no instance(s) of type variable(s) T, U exist so that Integer confirms to Function"

appreciate any pointers to resolve this

Arrays.stream(nums).boxed().collect(Collectors.toMap(Function.identity(), i -> 1, Integer::sum)); — Hadi
– Hadi, Commented Jun 1, 2019 at 12:21

Naman · Accepted Answer · 2019-06-01 12:15:25Z

8

You need to box the IntStream and then use groupingBy value to get the count:

Map<Integer, Long> map = Arrays
        .stream(nums)
        .boxed() // this
        .collect(Collectors.groupingBy(e -> e, Collectors.counting()));

or use reduce as:

Map<Integer, Integer> map = Arrays
        .stream(nums)
        .boxed()
        .collect(Collectors.groupingBy(e -> e,
                Collectors.reducing(0, e -> 1, Integer::sum)));

answered Jun 1, 2019 at 12:15

Naman

32.7k32 gold badges240 silver badges385 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Buddhi Over a year ago

Thanks for the answer, if the map value is just 1 instead of key count, is it possible to use Collectors.toMap(e->e,1), by modifying it

Hadi Over a year ago

@Buddhi, No, should be ...Collectors.toMap(e->e,v->1)...

Holger Over a year ago

@HadiJ but you’d need Collectors.toMap(e->e, v->1, Integer::sum) in this specific case, to handle the duplicates.

Hadi Over a year ago

@Holger, yeah, I meant for this ...toMap(e->e,1). thanks for clearing

Samuel Philipp · Accepted Answer · 2019-06-01 12:30:58Z

4

You have to call .boxed() on your Stream to convert the IntStream to a Stream<Integer>. Then you can use Collectors.groupingby() and Collectors.summingInt() to count the values:

Map<Integer, Integer> map = Arrays.stream(nums).boxed()
        .collect(Collectors.groupingBy(Function.identity(), Collectors.summingInt(i -> 1)));

answered Jun 1, 2019 at 12:30

Samuel Philipp

11.1k12 gold badges41 silver badges58 bronze badges

Comments

Donald Raab · Accepted Answer · 2019-06-04 04:51:52Z

2

You can also accomplish counting the ints without boxing the int values into a Map<Integer, Integer> or Map<Integer, Long>. If you use Eclipse Collections, you can convert an IntStream to an IntBag as follows.

int[] nums = {2, 7, 11, 15, 2, 11, 2};
IntBag bag = IntBags.mutable.withAll(IntStream.of(nums));
System.out.println(bag.toStringOfItemToCount());

Outputs:

{2=3, 7=1, 11=2, 15=1}

You can also construct the IntBag directly from the int array.

IntBag bag = IntBags.mutable.with(nums);

Note: I am a committer for Eclipse Collections.

answered Jun 4, 2019 at 4:51

Donald Raab

6,8362 gold badges40 silver badges47 bronze badges

Collectives™ on Stack Overflow

Java 8 int array to map

3 Answers 3

4 Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

4 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related