在Scala中循环流

Question

这是一个Scala类：

class CustomizedStream[K,V](private val queue: BlockingQueue[FetchedDataChunk],
                    FooTimeoutMs: Int,
                    private val keyDecoder: Decoder[K],
                    private val valueDecoder: Decoder[V],
                    val clientId: String)
  extends Iterable[MessageAndMetadata[K,V]] with java.lang.Iterable[MessageAndMetadata[K,V]] {
...
} 

和在Java代码中

 Map<String, List<FoStream<byte[], byte[]>>> foStreams = consumer
            .createMessageStreams(topicCount);
    List<CustomizedStream<byte[], byte[]>> streams = consumerStreams.get(topic);
for (final CustomizedStream stream : streams) {
        FoIterator<byte[], byte[]> it = stream.iterator();
        while (it.hasNext()) {
            System.out.println(it.next().message());
        }
    }

如何在Scala中重写上述Java代码？ 我尝试for（stream <-stream）并得到

value foreach is not a member of java.util.List[CustomizedStream(Array[Byte], Array[Byte]]]

Łukasz建议以下解决方案：

for{
  stream <- streams.asScala
  m <- stream
}{
   println(m)
}

我看不到任何输出，并且对于

for{
  stream <- streams.asScala
} {
    println(stream)  // <- this line shows up 
    for(m <- stream){
      println(m)
    }
}

和相同的

val foStreams = consumer.createMessageStreams(topicCount);
val streams : java.util.List[KafkaStream[Array[Byte], Array[Byte]]]= foStreams.get(topic);
val itr = streams.iterator()
while(itr.hasNext){
  val a = itr.next()
  println(a)  // <- can see this line
  val itr2 =   a.iterator()
  while(itr2.hasNext()) {
    println(itr2.next()) // but not this line
  }
}

Answer 1

您不想在这里循环流，而是在包含CustomizedStream的java.util.List上循环。 它包含的内容无关紧要。

显而易见的方法是将Java列表转换为scala seq。

import scala.collection.JavaConverters._

val streams: java.util.List[CustomizedStream[Array[Byte], Array[Byte]]] = ???

for (stream <- streams.asScala) {
  // .. code
}

由于CustomizedStream实现了scala Iterable您可以将Java代码简化为

for {
  stream <- streams.asScala
  m <- stream
} { 
  println(m.message)
}