Duplicate key

说明toMap的时候有重复的key，程序也不知道该取哪个了

Exception in thread "main" java.lang.IllegalStateException: Duplicate key User(id=1, name=null, role=null)
 at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
 at java.util.HashMap.merge(HashMap.java:1255)
 at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
 at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
 at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1384)
 at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
 at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
 at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
 at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
 at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:566)
 at com.seed.server.utils.SftpUtilTest.main(SftpUtilTest.java:44)

解决办法，根据业务需要，如果不允许出现这样的数据，那就直接抛出异常。如果对于key重复的，根据实际业务人为决定取拿一个，一般要么去第一个要么去最后一个。

NullPointerException

说明toMap的时候有的value是null

Exception in thread "main" java.lang.NullPointerException
 at java.util.HashMap.merge(HashMap.java:1226)
 at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
 at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
 at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1384)
 at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
 at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
 at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
 at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
 at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:566)
 at com.seed.server.utils.SftpUtilTest.main(SftpUtilTest.java:44)

解决办法，根据业务需要，如果不允许出现这样的数据，那就直接抛出异常。如果对于value为null的不进行处理，那就先filter再toMap

连阿里规约都说了