Java count occurrence of each element in an intege

问题:

I've written the following snippet to count the number of occurrences of each element. Is it possible to achieve this in a much shorter way?

int[] arr = {1, 6, 2, 8, 5, 4, 7, 7, 5, 7};
Arrays.stream(arr)
        .collect(ArrayList::new, ArrayList::add, ArrayList::addAll)
        .stream()
        .collect(Collectors.groupingBy(s -> s))
        .forEach((k, v) -> System.out.println(k+" "+v.size()));

Also I would like to display only the elements which occur more than 1 time. So I tried modifying as below which resulted in an error.

.forEach((k, v) -> if(v.size() > 1) System.out.println(k+" "+v.size()));

What is the correct way to do this?

回答1:

For the latter question, you have to change

.forEach((k, v) -> if(v.size() > 1) System.out.println(k+" "+v.size()));

to

.forEach((k, v) -> {if(v.size() > 1) System.out.println(k+" "+v.size());});

For the first part, it's not clear why you need the first collect followed by a second Stream pipeline. If the purpose was to convert an IntStream to a Stream<Integer>, use boxed():

Arrays.stream(arr)
      .boxed()
      .collect(Collectors.groupingBy(s -> s))
      .forEach((k, v) -> System.out.println(k+" "+v.size()));

As Dici suggested, you can also chain Collectors to group each number with its number of occurrences :

Map<Integer,Integer> occurrences = 
    Arrays.stream(arr)
          .boxed()
          .collect(Collectors.groupingBy(s -> s, Collectors.counting()));

回答2:

If you are open to using a third-party library, Eclipse Collections has a Bag type which can be used as follows:

Bags.mutable.with(1, 6, 2, 8, 5, 4, 7, 7, 5, 7)
    .selectByOccurrences(count -> count > 1)
    .forEachWithOccurrences((k, count) -> System.out.println(k+" "+count));

If you have to keep int[] arr variable as an int array, then you can use an IntBag. Update: 9/9/16: I have added selectByOccurrences to primitive Bags and it is available in the EC 8.0 release. The following code should work now without having to box the ints using collect(i -> i):

IntBags.mutable.with(arr)
    .selectByOccurrences(count -> count > 1)
    .forEachWithOccurrences((k, count) -> System.out.println(k+" "+count));

Note: I am a committer for Eclipse Collections.

回答3:

I want to share my solution as well!!!

// Solution 1 [Improved from Eran's solution & suggestion]
int[] arr = {1, 6, 2, 8, 5, 4, 7, 7, 5, 7};
Map<Integer, Long> counts = Arrays.stream(arr)
    .boxed()
    .collect(collectingAndThen(groupingBy(n -> n, counting()),
        map -> map.entrySet().stream()
            .filter(n -> n.getValue() > 1)
            .collect(toMap(Entry::getKey, Entry::getValue))
));
System.out.println(counts.toString());

// Solution 2 [Improved from Dici's suggestion]
int[] arr = {1, 6, 2, 8, 5, 4, 7, 7, 5, 7};
Map<Object, Long> counts = Arrays.stream(arr)
    .collect(ArrayList::new, ArrayList::add, ArrayList::addAll)
    .stream()
    .collect(groupingBy(Function.identity(), counting()));
counts.values().removeIf(count -> count < 2);
System.out.println(counts.toString());  

回答4:

Also might be done using frequency :

 List<Integer> list = ImmutableList.of(1, 2, 3, 4, 5, 6, 3, 4, 5);
 Map<Integer, Integer> result = list.stream().distinct().collect(Collectors.toMap(Function.identity(), token -> Collections.frequency(list, token)));