Merge map of arrays with duplicate keys

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Category:Languages

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?


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.

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