List<User> userList = new ArrayList<>();
userList.add(new User(1L, "aaa"));
userList.add(new User(2L, "bbb"));
userList.add(new User(3L, "ccc"));
userList.add(new User(2L, "ddd"));
userList.add(new User(3L, "eee"));
当进行普通toMap操作时
Map<Long, String> map = userList.stream()
.collect(Collectors.toMap(User::getId, User::getUsername);
就会报错，说明处理到已存在的key，其对应value为bbb
java.lang.IllegalStateException: Duplicate key bbb
at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
at java.util.HashMap.merge(HashMap.java:1253)
at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1374)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
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:499)
解决方式就在Java8提供的Collectors.toMap() 方法中，其第三个参数就是当出现 duplicate key的时候的处理方案
方案一： 出现重复时，取前面value的值，或者取后面放入的value值，则覆盖先前的value值
Map<Long, String> map = userList.stream()
.collect(Collectors.toMap(User::getId, User::getUsername, (v1, v2) -> v1));
Map<Long, String> map = userList.stream()
.collect(Collectors.toMap(User::getId, User::getUsername, (v1, v2) -> v2));
方案二： Map的value可以储存一个list，把重复key的值放入list，再存到value中
userList.stream().collect(Collectors.toMap(User::getId,
e -> Lists.newArrayList(e.getUsername()),
(List<String> oldList, List<String> newList) -> {
oldList.addAll(newList);
return oldList;
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