准备以下User对象集合 ，构造⽅法User（Long Id, String username）
List userList = new ArrayList<>();
代码语言：javascript复制 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()
代码语言：javascript复制 .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
代码语言：javascript复制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
代码语言：javascript复制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));
