Stream distinct Array elements in List

Question

I have a List of String[6]s. I'm trying to build a List<List> that contains the unique String elements at each index.

For some reason, I frequently run into this problem, search to see if answers exist, experiment a lot, and end up building a working method that does not use stream, but takes up like 20 lines.

To clarify, with an example:

//Sample items: String[] items = "AD", "AR", "BC", "DA", "RA", "DD";
//              String[] items2 = "AE", "AZ", "BU", "DI", "RE", "DP";
//              String[] items3 = "AD", "AO", "BU", "DZ", "RW", "DP";
List<String[]> itemsList;
List<String>[] distinctItems;

If itemsList contains the sample Items, how can .stream() put them in distinctItems? distinctItems should look like:

//distinctItems[0].get(0) == "AD"
//distinctItems[0].get(1) == "AE"
//distinctItems[0].size() == 2         ; Has 2 unique elements in index 0

//distinctItems[1].get(0) == "AR"
//distinctItems[1].get(1) == "AZ"
//distinctItems[1].get(2) == "AO"
//distinctItems[1].size() == 3         ; Has 3 unique elements in index 1

I'm getting pretty good at .stream() but on certain compound data types (List of Array[]), I can't seem to find the right methods and solutions.

Any help greatly appreciated.

Unclear! A List<String[]> is basically a two-dimensional data structure. A List<List>[] is a three-dimensional data structure. I am not getting how you want to transform your source list. Besides that, where are your code attempts? — Seelenvirtuose
– Seelenvirtuose, Commented Apr 3, 2019 at 18:41
Ah, I think I copied that wrong: List<String>[]. I have working solutions without stream but I think stream() would be more concise. I really have been working on a project using this throughout the night so I'm exhausted. If you think this question is invalid, I'll just take it down. Attempts using .stream() include .foreach(), .distinct(), but I am not sure how to apply .distinct() to the String[] within the original List. — user176692
– user176692, Commented Apr 3, 2019 at 18:49

JB Nizet · Accepted Answer · 2019-04-03 18:54:22Z

1

If I understand correctly, you actually want a List<List<String>>, which has the same size as your 3 List<String[]>, and which contain, for each index; the distinct elements for this index.

So

    List<String[]> itemLists = new ArrayList<>();
    itemLists.add(new String[] { "AD", "AR", "BC", "DA", "RA", "DD" });
    itemLists.add(new String[] { "AE", "AZ", "BU", "DI", "RE", "DP" });
    itemLists.add(new String[] { "AD", "AO", "BU", "DZ", "RW", "DP" });

    List<List<String>> distinctItems =
        IntStream.range(0, itemLists.get(0).length)
                 .mapToObj(i -> itemLists.stream().map(itemArray -> itemArray[i]).distinct().collect(toList()))
                 .collect(toList());

    System.out.println("distinctItems = " + distinctItems);

edited Apr 3, 2019 at 18:54

answered Apr 3, 2019 at 18:47

JB Nizet

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user176692 Over a year ago

I think so, yes, a List<List<String>> as you describe would be just fine. A lot in here, going to study each of these .mapToObj(), .range() methods and stream()ing within a stream in more detail soon. This is opening up some very exciting doors! I am getting this though, "Cannot infer type argument(s) for <U> mapToObj(intFunction<? extends U>). Maybe I need an extra parentheses? Quick answer here, you seem very expert with stream()! Will accept once I make sure I understand this.

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