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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