How to get indexes of all duplicate elements by using the Java stream API

Question

How can I get the indexes of all duplicate elements by using the Java stream API?

For example, we have:

List<Integer> input =
        new ArrayList<>(Arrays.asList(312, 312, 195, 195, 195, 851, 335, 851, 99));

I could group same elements like this:

result = input.stream().collect(Collectors.groupingBy(input::indexOf))
//result = {0=[312, 312], 2=[195, 195, 195], 5=[851, 851], 6=[335], 8=[99]}

But I want get a result like this:

{312=[0, 1], 195=[2, 3, 4], 851=[5, 7]}

or just like this:

(0,1,2,3,4,5,7)

How can I do this?

Sayan Bhattacharya · Accepted Answer · 2022-07-07 07:04:24Z

4

You can Use IntStream

Map<Integer, List<Integer>> map = IntStream.range(0, input.size())
                .boxed()
                .collect(Collectors.groupingBy(i -> input.get(i), HashMap::new,
                        Collectors.toCollection(ArrayList::new)));

// output : {99=[8], 851=[5, 7], 195=[2, 3, 4], 312=[0, 1], 335=[6]}

Once you have all the values and their indexes you can simply filter out the entries which have more than 1 value.
Full example:

Map<Integer, List<Integer>> map = IntStream.range(0, input.size())
                .boxed()
                .collect(Collectors.groupingBy(i -> input.get(i), HashMap::new,
                        Collectors.toCollection(ArrayList::new)))
                .entrySet()
                .stream()
                .filter(e -> e.getValue().size()>1)
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

edited Jul 7, 2022 at 7:04

answered Jul 7, 2022 at 6:55

Sayan Bhattacharya

1,3681 gold badge6 silver badges14 bronze badges

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

2 Comments

Ivan Ivanov Over a year ago

great, thx! it's give me fresh lookup on streams, and will help to lean it better

M. Justin Over a year ago

.collect(Collectors.groupingBy(input::get, Collectors.toList())) would usually suffice; there's rarely a need to specify that it's should specifically be a HashMap or ArrayList.

Eritrean · Accepted Answer · 2022-07-07 07:41:25Z

2

Another approach using Collections.frequency

int[] dupIndexes = IntStream.range(0, input.size())
                            .filter(i -> Collections.frequency(input, input.get(i)) > 1)
                            .toArray();

answered Jul 7, 2022 at 7:41

Eritrean

16.6k3 gold badges25 silver badges28 bronze badges

1 Comment

M. Justin Over a year ago

Note that while this is a straightforward approach, it's not very efficient. It needs to iterate over the entire list for each element of the list to calculate the frequency of that item. This gives it a time complexity of O(n^2). Much more efficient algorithms exist to solve this problem.

Ruslan · Accepted Answer · 2022-07-07 07:20:22Z

0

Looks like it's quite easy to solve by just for loop. Just keep a Map with input value as a key and it's index as a value of map:

Set<Integer> result = new HashSet<>();
Map<Integer, Integer> map = new HashMap<>();
for (int i = 0; i < input.size(); i++) {
    if (map.containsKey(input.get(i))) {
        result.add(map.get(input.get(i)));
        result.add(i);
    } else {
        map.put(input.get(i), i);
    }
}
System.out.println(result);

Output

[0, 1, 2, 3, 4, 5, 7]

answered Jul 7, 2022 at 7:20

Ruslan

6,3201 gold badge23 silver badges41 bronze badges

Comments

user19443549 · Accepted Answer · 2022-07-07 07:21:30Z

0

You can try below code -

     List<Integer> input = new ArrayList<>(Arrays.asList(312, 312, 195, 195, 195, 851, 335, 851, 99));
    result = IntStream.range(0, input.size()).boxed()
            .collect(Collectors.groupingBy(input::get, HashMap::new,
                    Collectors.toList()));

answered Jul 7, 2022 at 7:21

user19443549

Comments

Ashish Patil · Accepted Answer · 2022-07-07 07:44:55Z

0

    HashMap<Integer, List<Integer>> map = new HashMap<Integer, List<Integer>>();
    List<Integer> ls = null;
    for (int i = 0; i < input.size(); i++) {
        if (map.get(input.get(i)) != null) {
            ls = map.get(input.get(i));
        } else {
            ls = new ArrayList<Integer>();
        }
        ls.add(i);
        map.put(input.get(i), ls);
    }
    System.out.println(map);

edited Jul 7, 2022 at 7:44

answered Jul 7, 2022 at 7:23

Ashish Patil

4,6421 gold badge17 silver badges38 bronze badges

2 Comments

Ivan Ivanov Over a year ago

sorry dude, but i searching stream solution (on studying practice) and i find it in answers

Ashish Patil Over a year ago

no worries, glad you got your solutions & I too learned few new things,

Collectives™ on Stack Overflow

How to get indexes of all duplicate elements by using the Java stream API

5 Answers 5

2 Comments

1 Comment

Comments

Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

2 Comments

1 Comment

Comments

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related