CODEWITHSUNDEEP
javalistdictionaryjava-8java-stream
user6123723
user6123723

Reputation: 11136

Efficient way to create a list of specific values from a bunch of maps

Say I have a List of maps

List<Map<String, Double>> maps = new ArrayList<>();

Map<String, Double> map1 = new HashMap();
map1.put("height",60D);
map1.put("weight",144D);

maps.add(map1);


Map<String, Double> map2 = new HashMap();
map2.put("height",63D);
map2.put("weight",192D);

maps.add(map2);

and so on...

What's the quickest way to create a list of heights or weights? Something like List heights etc.

The classic way is to look through the map, use an if condition and search for a height key and then if found, add it to a return list.

What would the equivalent way to do it using streams?

Upvotes: 0

Views: 85

Answers (3)

Pankaj Singhal
Pankaj Singhal

Reputation: 16053

List<Double> listHeights = maps.stream()
                                .map(map -> map.get("height"))
                                .filter(Objects::nonNull)
                                .collect(Collectors.toList());

Similarly, you can do it for weight also.

Upvotes: 7

Ryuzaki L
Ryuzaki L

Reputation: 40078

If you wanna get both height and weight list in one shot, then use Collectors.groupingBy no need of contains or null check

Map<String, List<Double>> result = maps.stream()
                                           .flatMap(m->m.entrySet().stream())
                                           .collect(Collectors.groupingBy(Map.Entry::getKey,
                                                   Collectors.mapping(Map.Entry::getValue, Collectors.toList())));

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

Output

weight...[144.0, 192.0]
height...[60.0, 63.0]

And also you can use getOrDefault to get the List of height or weight

List<Double> height = result.getOrDefault("height", new ArrayList<Double>());

Upvotes: 1

Mureinik
Mureinik

Reputation: 311808

You could stream over the list and map the Maps to the relevant values:

List<Double> heights =
    maps.stream()
        .filter(m -> m.containsKey("height"))
        .map(m -> m.get("height"))
        .collect(Collectors.toList());

Upvotes: 2

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