Java-8 Streams: Convert List<{String,List<String>}> to Map<String, List<String>>

Question

I have a Tag class which contains a list of Items

class Tag {

private String tagName
private List<String> items

}

I have a list of Tags in which each tag a list of items

List<Tag> tagList =   [
                       {"tagName": "popular", "items": ["Item1","Item2","Item3","Item4"]},
                       {"tagName": "expensive" , "items":  ["Item2","Item4","Item5"]},
                       {"tagName": "mostwanted", "items":  ["Item1","Item2","Item5"]}
                      ]

I wan to convert this to a map which have the items as key and tagName as values.

expected output :

{
    "Item1" : ["popular","mostwanted"],
    "Item2" : ["popular","expensive","mostwanted"],
    "Item3" : ["popular","mostwanted"],
    "Item4" : ["popular","expensive"],
    "Item5" : ["expensive","mostwanted"]
}

I tried various combination of stream/faltmap/groupingBy, but I didnt get the expected output. Can you please help. Thanks

Eklavya · Accepted Answer · 2020-07-24 13:53:07Z

2

You can flat the items using flatMap then create pair of item and tagName using SimpleEntry. Then grouping by item using groupingBy and map tagName to get list of tagName

Map<String, List<String>> tagMap = tagList.stream()
        .flatMap(t -> t.getItems()
                       .stream()
                       .map(item -> new AbstractMap.SimpleEntry<>(item, t.getTagName())))
        .collect(Collectors.groupingBy(m -> m.getKey(),
            Collectors.mapping(m -> m.getValue(), Collectors.toList())));

edited Jul 24, 2020 at 13:53

answered Jul 24, 2020 at 13:18

Eklavya

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2 Comments

Nikolas Over a year ago

It's a good one! Stream API here looks very clumsy :) Moreover, I struggled to compose it.

Eklavya Over a year ago

@Nikolas When you love Stream API more then it looks nice also :)

Nikolas · Accepted Answer · 2020-07-24 13:26:36Z

1

Here is a procedural way to go using new features from java-8 such as Map::computeIfPresent and Map::computeIfAbsent, but without java-stream:

Map<String, List<String>> map = new HashMap<>();
    tagList.forEach(tag -> {
        String tagName = tag.getTagName();
        tag.getItems().forEach(item -> {
            map.computeIfPresent(item, (k, v) -> { v.add(tagName); return v; });
            map.computeIfAbsent(item, k -> new ArrayList<>(Arrays.asList(tagName)));
   });
});

map.forEach((k, v) -> System.out.println(k + " " + v));

If you want to sort these items from Item1 to Item5, use the different implementation:

Map<String, List<String>> map = new TreeMap<>();

Moreover, your expected output doesn't match as long as there is only one occurence of Item3:

Item1 [popular, mostwanted]
Item2 [popular, expensive, mostwanted]
Item3 [popular]
Item4 [popular, expensive]
Item5 [expensive, mostwanted]

edited Jul 24, 2020 at 13:26

answered Jul 24, 2020 at 13:20

Nikolas

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2 Comments

Naman Over a year ago

You might have meant something simpler as

tagList.forEach(tag -> tag.getItems().forEach(item -> map.computeIfAbsent(item, k -> new ArrayList<>()).add(tag.getTagName())));

...though I would have preferred

for (Tag tag : tagList) {             for (String item : tag.getItems()) {                 map.computeIfAbsent(item, k -> new ArrayList<>())                         .add(tag.getTagName());             }         }

Nikolas Over a year ago

@Naman: You are right, I have forgotten about the return types of these methods (IMHO, the word "compute" is somehow associated with void in my mind). Feel free to post your answer.

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