Java8 stream操做toMap的key重複問題

準備如下User對象集合 ,構造方法User(Long Id, String username)java

List<User> 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"));

當進行普通toMap操做時lua

Map<Long, String> map = userList.stream()
            .collect(Collectors.toMap(User::getId, User::getUsername);

 就會報錯,說明處理到已存在的key,其對應value爲bbbcode

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)

解決方式就在Java8提供的Collectors.toMap() 方法中,其第三個參數就是當出現 duplicate key的時候的處理方案對象

方案一: 出現重複時,取前面value的值,或者取後面放入的value值,則覆蓋先前的value值ip

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

方案二: Map的value能夠儲存一個list,把重複key的值放入list,再存到value中ci

userList.stream().collect(Collectors.toMap(User::getId,
                e -> Arrays.asList(e.getUsername()),
                (List<String> oldList, List<String> newList) -> {
                    oldList.addAll(newList);
                    return oldList;
                }));

....get

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