Ufdjrw

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" 
 ;

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);

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.