Shulou(Shulou.com)06/02 Report--

This article focuses on "how to solve the key repetition problem of Java8 stream operating toMap". Interested friends may wish to have a look. The method introduced in this paper is simple, fast and practical. Let's let the editor take you to learn "how to solve the key repetition problem of Java8 stream operating toMap".

Prepare the following collection of User objects, constructor User (Long Id, String username)

List userList = new ArrayList (); 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"))

When performing a normal toMap operation

Map map = userList.stream () .cake (Collectors.toMap (User::getId, User::getUsername)

An error will be reported, indicating that the existing key is processed, and the corresponding value is bbb.

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)

The solution is in the Collectors.toMap () method provided by Java8, whose third parameter is the processing scheme when duplicate key occurs.

Option 1: when there is a repetition, take the value of the previous value, or take the value put after it, then overwrite the previous value

Map map = userList.stream () .duration (Collectors.toMap (User::getId, User::getUsername, (v1, v2)-> v1); Map map = userList.stream () .duration (Collectors.toMap (User::getId, User::getUsername, (v1, v2)-> v2))

Option 2: the value of Map can store a list, put the value of the duplicate key into list, and then save it in value.

UserList.stream () .collect (Collectors.toMap (User::getId, e-> Lists.newArrayList (e.getUsername ()), (List oldList, List newList)-> {oldList.addAll (newList); return oldList;})) At this point, I believe you have a deeper understanding of "how to solve the key repetition problem of Java8 stream operating toMap". You might as well do it in practice. Here is the website, more related content can enter the relevant channels to inquire, follow us, continue to learn!

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.