forEach 循环期间的 java8 java.util.ConcurrentModificationException

Question

我使用 java8 流。这是我的数据结构:

Map< String, List< String >> mmessage = getSomeMessage();

然后我遍历 map 和列表:

 mmessage.entrySet().stream().forEach( entry -> {
            entry.getValue().stream().forEach( li -> {
                if ( lis.indexOf( li ) == - 1 )  {
                    lis.add( lineItem );
                }
            });
        });

但是得到并发修改异常:

java.util.ConcurrentModificationException
    at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1380)
    at java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:580)
    at com.web3.buyer.roomba.RoombaTurn.lambda$received$3(RoombaTurn.java:296)
    at java.util.Iterator.forEachRemaining(Iterator.java:116)
    at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801)
    at java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:580)
    at com.web3.buyer.roomba.RoombaTurn.received(RoombaTurn.java:295)
    at com.web3.buyer.SystemBus.lambda$publishToTheQueue$0(SystemBus.java:51)
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

根据我的理解，遍历 map\list 不应该导致这种行为。

Answer 1

我会使用完整的函数式风格编写此代码，您不应该在迭代列表时遇到修改列表的问题。

List<String> strs = mmessage.values().stream()
                            .flatMap(List::stream)
                            .distinct()
                            .collect(Collectors.toList());