CODEWITHSUNDEEP
javajava-8java-stream
fbielejec
fbielejec

Reputation: 3720

Java 8 streams - merge a collection of Maps

Lets say I have a method with a following signature:

Map<String, String> merge (String[][] ... maps)

It takes a (variadic) number of two-dim arrays which represent mappings, for example:

        String[][] m1 = new String[][] {
            {"k1", "a"},
            {"k2", "b"}
        };

        String[][] m2 = new String[][] {
            {"k1", "a"},
            {"k2", "b"},
            {"k3", "c"}   
        };

        String[][] m3 = new String[][] {
            {"k1", "x"},
            {"k2", "b"}
        };

Now I can merge any two of the maps left-to-right like this:

 Map<String, String> m1 = Stream.of(map1).collect(Collectors.toMap(k -> k[0], v -> v[1]));
 Map<String, String> m2 = Stream.of(map2).collect(Collectors.toMap(k -> k[0], v -> Optional.ofNullable(v[1]).orElse("null")));
 m1.forEach((key, value) -> m2.merge(key, value, (v1, v2) -> v1));

But how can I go about merging a variadic number of such array-maps, so that after passing m1, m2, m3 as defined above the result is: 

        String[][] m3 = new String[][] {
            {"k1", "x"},
            {"k2", "b"}
            {"k3", "c"}   
        };

Upvotes: 2

Views: 99

Answers (2)

shmosel
shmosel

Reputation: 50756

It's quite simple, if I'm understanding your question correctly. You just flatMap all the entries and collect to map, with a merge function that selects the later value:

Map<String, String> merge (String[][] ... maps) {
    return Arrays.stream(maps)
            .flatMap(Arrays::stream)
            .collect(Collectors.toMap(a -> a[0], a -> a[1], (a, b) -> b));
}

If you want to convert it back to an array, you can do

String[][] m3 = merge(maps)
        .entrySet()
        .stream()
        .map(e -> new String[] { e.getKey(), e.getValue() })
        .toArray(String[][]::new);

Upvotes: 6

Kartik
Kartik

Reputation: 7917

Map<String, String> merge(String[][]... maps) {
    return Stream.of(maps)
            .flatMap(Arrays::stream)          //flatten all maps into a single stream
            .map(s -> Map.entry(s[0], s[1]))  //convert array to Entry
            .collect(Collectors.toMap(Map.Entry::getKey,
                    Map.Entry::getValue,
                    (o1, o2) -> o2));
}

Edit: No need to convert to Entry. We can directly collect to map as @shmosel showed.

Upvotes: 3

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