Java 8 streams - merge a collection of Maps

fbielejec :

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"}   
        };

shmosel :

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);