问题描述:
最近使用Java8中Steam()流进行tomap转换编程时,遇到以下错误
java.lang.IllegalStateException: Duplicate key bbbat 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)
原因分析:
正如错误提示中所说的那样,tomap时遇到了重复键的问题,这里举个例子并记录一下解决方法。
准备以下User对象集合 ,构造方法User(Long Id, String username)
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"));
当进行普通toMap操作时,就会出现我们上面的报错提示
Map<Long, String> map = userList.stream().collect(Collectors.toMap(User::getId, User::getUsername);
解决方案:
解决方式就在Java8提供的Collectors.toMap() 方法中,其第三个参数就是当出现 duplicate key的时候的处理方案
方案一: 出现重复时,取前面value的值,或者取后面放入的value值,则覆盖先前的value值
// 取后面的值,舍弃前面的值Map<Long, String> map = userList.stream().collect(Collectors.toMap(User::getId, User::getUsername, (v1, v2) -> v2));// 取前面的值,舍弃后面的值Map<Long, String> map = userList.stream().collect(Collectors.toMap(User::getId, User::getUsername, (v1, v2) -> v1));
方案二: Map的value可以储存一个list,把重复key的值放入list,再存到value中
userList.stream().collect(Collectors.toMap(User::getId,e -> Arrays.asList(e.getUsername()),(List<String> oldList, List<String> newList) -> {oldList.addAll(newList);return oldList;}));