java Lambda表达式List快速转Map

柔情只为你懂 2021-09-28 14:54 375阅读 0赞

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**  
![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2RoajE5OTE4MQ_size_16_color_FFFFFF_t_70][]

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**

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2RoajE5OTE4MQ_size_16_color_FFFFFF_t_70]: /images/20210923/238e99eb2d9f4dc6a3a1a112f2382ff0.png