Java8 Collectors.toConcurrentMap的key重复

项目场景：

Java8 Collectors.toConcurrentMap的key重复

问题描述

报错信息如下：java.lang.IllegalStateException: Duplicate key

Caused by: java.lang.IllegalStateException: Duplicate key SubnetDto(subnet_id=subId01, cidr=192.168.2.0/24, subnet_name=subName01, vpc_id=vpcId01)
	at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
	at java.util.concurrent.ConcurrentHashMap.merge(ConcurrentHashMap.java:1990)
	at java.util.stream.Collectors.lambda$toConcurrentMap$59(Collectors.java:1472)
	at java.util.stream.ReferencePipeline.lambda$collect$1(ReferencePipeline.java:496)
	at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184)
	at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
	at java.util.stream.ForEachOps$ForEachTask.compute(ForEachOps.java:291)
	at java.util.concurrent.CountedCompleter.exec(CountedCompleter.java:731)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)

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原因分析：

如这个地方，如果使用subnet_id为map的key, SubnetDto就会重复，这时候就会报Duplicate Key的问题，其实是map的key重复了

 // 默认不处理重复的key值
 ConcurrentMap<String, SubnetDto> map = subnetDtoList.parallelStream().collect(Collectors.toConcurrentMap(SubnetDto::getSubnet_id, Function.identity()));

报错截图如下：

查看源码：可以看到throwingMerger()是一个出现异常时默认执行的方法

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解决方案：

给重复的key值设置一个新值

// 当key值重复时，用value2值覆盖value1
ConcurrentMap<String, SubnetDto> map = subnetDtoList.parallelStream().collect(Collectors.toConcurrentMap(SubnetDto::getSubnet_id, Function.identity(),(SubnetDto value1, SubnetDto value2) -> value2));