Merge map of arrays with duplicate keys

Question

I have two maps of arrays.

Map<String, List<String>> map1 = new HashMap<>();
Map<String, List<String>> map2 = new HashMap<>();

I want to merge them in one new map.
If a key exists in both maps, in that case, I should merge arrays.

For example:

map1.put("k1", Arrays.asList("a0", "a1"));
map1.put("k2", Arrays.asList("b0", "b1"));

map2.put("k2", Arrays.asList("z1", "z2"));

// Expected output is 
Map 3: {k1=[a0, a1], k2=[b0, b1, z1, z2]}

I tried to do that with streams

Map<String, List<String>> map3 = Stream.of(map1, map2)
    .flatMap(map -> map.entrySet().stream())
    .collect(Collectors.toMap(
        Map.Entry::getKey,
        e -> e.getValue().stream().collect(Collectors.toList())
    ));

This work if there are no the same keys in maps. Otherwise, I get the exception

Exception in thread "main" java.lang.IllegalStateException: Duplicate key k2 (attempted merging values [b0, b1] and [z1, z2])
    at java.base/java.util.stream.Collectors.duplicateKeyException(Collectors.java:133)
    at java.base/java.util.stream.Collectors.lambda$uniqKeysMapAccumulator$1(Collectors.java:180)
    at java.base/java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
    at java.base/java.util.HashMap$EntrySpliterator.forEachRemaining(HashMap.java:1751)
    at java.base/java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:658)
    at java.base/java.util.stream.ReferencePipeline$7$1.accept(ReferencePipeline.java:274)
    at java.base/java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:948)
    at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484)
    at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474)
    at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913)
    at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:578)
    at im.djm.Test.main(Test.java:25)

Is there a way to accomplish this task with streams?
Or I have to iterate throug maps?

Answer 1

Use a merge function in the case of duplicate keys:

Map<String, List<String>> map3 = Stream.of(map1, map2)
                .flatMap(map -> map.entrySet().stream())
                .collect(Collectors.toMap(
                        Map.Entry::getKey,
                        e -> new ArrayList<>(e.getValue()),
                        (left, right) -> {left.addAll(right); return left;}
                ));

Note, I've changed e -> e.getValue().stream().collect(Collectors.toList()) to new ArrayList<>(e.getValue()) to guarantee that we always have a mutable list which we can add into in the merge function.

Answer 2

Maybe. But you are more likely to get everything right by combining the entries manually, using iteration. I don't know if anyone else will have to work on this code, but they will likely be grateful for an easy to read approach.