java Lambda表达式List快速转Map_格斗机器人-CSDN博客 ---- 试了不可行还是报错 ---- 问题大概就是博主说的 value 不能有 null

安宇雨 - 随手采集
2020-09-24 17:29:40
随手采集
0000-未整理-等待研究

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

List<UserBean> userList = new ArrayList<>();        UserBean userBean = new UserBean("张三","热爱学习", 21);        userList.add(userBean);                 userBean = new UserBean("张三2","热爱学习2", 25);        userList.add(userBean);                 userBean = new UserBean("张三33","热爱学习3", 27);        userList.add(userBean);                 userBean = new UserBean("张三4","热爱学习2", 26);        userList.add(userBean);                System.out.println(userList);        Map<String,Integer>  map =                 userList.stream().collect(Collectors.toMap(UserBean::getUserName, UserBean::getAge,         (key1,key2) ->key2));                System.out.println(map);

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.copyInto(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)

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

Original url: Access
Created at: 2020-09-24 17:29:40
Category: default
Tags: none

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