java8 for each循环期间的java.util.ConcurrentModificationException

Question

我使用java8流。下面是数据结构，我有：

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

然后我通过地图和列表迭代：

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

，但得到的并发修改例外：

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) 

从我的理解通过迭代map \ list不应该导致这种行为。

Answer 1

我会使用全功能风格编写此代码，并且您不应该遇到在迭代代码时修改列表的问题。

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