工作经常会遇到list转为 map 的情况，可能我们只需要某个字段的值转为map集合，在jdk8以前，我们更多地可能会用 for循环 的方式，然后put到map中去，有了jdk8之后，我们可以使用简单的一句话，就可以搞定

关键语句:Map<String,Integer> map = userList.stream().collect(Collectors.toMap(UserBean::getUserName, UserBean::getAge,(key1,key2) ->key2));解释

key值重复报错信息：

Exception in thread “main” java.lang.IllegalStateException: Duplicate key 21

at java.util.stream.Collectors.lambda$throwingMerger$0(Unknown Source)

at java.util.HashMap.merge(Unknown Source)

at java.util.stream.Collectors.lambda$toMap$58(Unknown Source)

at java.util.stream.ReduceOps$3ReducingSink.accept(Unknown Source)

at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(Unknown Source)

at java.util.stream.AbstractPipeline. copy Into(Unknown Source)

at java.util.stream.AbstractPipeline.wrapAndCopyInto(Unknown Source)

at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(Unknown Source)

at java.util.stream.AbstractPipeline.evaluate(Unknown Source)

at java.util.stream.ReferencePipeline.collect(Unknown Source)

at test.LamdaTest.abc(LamdaTest.java:32)

at test.LamdaTest.main(LamdaTest.java:12)

我们也可以指定返回的Map为哪种类型：

Map<String,Integer> map = userList.stream().collect(Collectors.toMap(UserBean::getUserName, UserBean::getAge,(key1,key2) ->key2, LinkedHashMap ::new ));

要注意的坑:value的值不能为null，否则会报错