Get the Strings that occur exactly three times from Arraylist<String>

Question

I have an ArrayList which contains some values with duplicates and elements that occur thrice, I want to collect those values that occur thrice specifically into another ArrayList like

Arraylist<String> strings;   //contains all strings that are duplicates and that occur thrice

Here, I want to get only the Strings that occur thrice in another array list.

Arraylist<String> thrice;    //contains only elements that occur three times.

Currently, I have a solution for dealing with duplicates but I cannot extend this for only getting strings that occur thrice, this please help me to find out.

Answer 1

You can do it via a stream as follows:

 List<String> result = strings.stream()
                .collect(Collectors.groupingBy(Function.identity(), counting()))
                .entrySet().stream()
                .filter(e -> e.getValue() == 3) // keep only elements that occur 3 times
                .map(Map.Entry::getKey)
                .collect(Collectors.toList());

You could also do it as follows, but I'd recommend the above as it's more preferable.

List<String> result = new HashSet<>(strings).stream()
                            .filter(item -> strings.stream()
                                  .filter(e -> e.equals(item)).limit(3).count() == 3)
                          .collect(Collectors.toList());