Java8 stream操作toMap的key重複問題
準備以下User物件集合 ，構造方法User（Long Id, String username）

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"));
當進行普通toMap操作時

Map<Long, String> map = userList.stream()
.collect(Collectors.toMap(User::getId, User::getUsername);
就會報錯，說明處理到已存在的key，其對應value為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)
解決方式就在Java8提供的Collectors.toMap() 方法中，其第三個引數就是當出現 duplicate key的時候的處理方案

方案一： 出現重複時，取前面value的值，或者取後面放入的value值，則覆蓋先前的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可以儲存一個list，把重複key的值放入list，再存到value中

userList.stream().collect(Collectors.toMap(User::getId,
e -> Arrays.asList(e.getUsername()),
(List<String> oldList, List<String> newList) -> {
oldList.addAll(newList);
return oldList;
}));