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java Lambda表达式List快速转Map

蚁族的乐土 2022-04-19 阅读 30
java后端

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 ( U n k n o w n S o u r c e ) a t j a v a . u t i l . H a s h M a p . m e r g e ( U n k n o w n S o u r c e ) a t j a v a . u t i l . s t r e a m . C o l l e c t o r s . l a m b d a 0(Unknown Source) at java.util.HashMap.merge(Unknown Source) at java.util.stream.Collectors.lambda 0(UnknownSource)atjava.util.HashMap.merge(UnknownSource)atjava.util.stream.Collectors.lambdatoMap$58(Unknown Source)
at java.util.stream.ReduceOps 3 R e d u c i n g S i n k . a c c e p t ( U n k n o w n S o u r c e ) a t j a v a . u t i l . A r r a y L i s t 3ReducingSink.accept(Unknown Source) at java.util.ArrayList 3ReducingSink.accept(UnknownSource)atjava.util.ArrayListArrayListSpliterator.forEachRemaining(Unknown Source)
at java.util.stream.AbstractPipeline.copyInto(Unknown Source)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(Unknown Source)
at java.util.stream.ReduceOps$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

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