合并具有重复键的数组映射 [英] Merge map of arrays with duplicate keys

问题描述

我有两个数组映射.

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

我想将它们合并到一张新地图中.
如果在两个映射中都存在键，那么在这种情况下，我应该合并数组.

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

例如:

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]}

我尝试使用流

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())
    ));

如果地图中没有相同的键，则此功能有效.否则，我会收到异常

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)

是否有一种方法可以通过流来完成此任务?
还是我必须遍历地图?

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

推荐答案

在键重复的情况下使用合并功能:

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;}
                ));

注意，我将e -> e.getValue().stream().collect(Collectors.toList())更改为new ArrayList<>(e.getValue())以确保我们始终有一个可变列表，可以将其添加到合并功能中.

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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