Apache Flink Streaming type mismatch in flatMap function - scala

Question

Trying to use streaming api of 0.10.0 flink version in scala 2.10.4. While trying to compile this first version:
import org.apache.flink.streaming.api.environment.StreamExecutionEnvironment
import org.apache.flink.streaming.api.scala.DataStream
import org.apache.flink.streaming.api.windowing.time._
object Main {
def main(args: Array[String]) {
val env = StreamExecutionEnvironment.getExecutionEnvironment
val text = env.socketTextStream("localhost", 9999)
val words : DataStream[String] = text.flatMap[String](
new Function[String,TraversableOnce[String]] {
def apply(line:String):TraversableOnce[String] = line.split(" ")
})
env.execute("Window Stream wordcount")
}
}
I am getting compile time error:
[error] found : String => TraversableOnce[String]
[error] required: org.apache.flink.api.common.functions.FlatMapFunction[String,String]
[error] new Function[String,TraversableOnce[String]] { def apply(line:String):TraversableOnce[String] = line.split(" ")})
[error] ^
And in decompiled version of DataStream.class that I have included to project there are functions that accept such type (the last one):
public <R> DataStream<R> flatMap(FlatMapFunction<T, R> flatMapper, TypeInformation<R> evidence$12, ClassTag<R> evidence$13) {
if (flatMapper == null) {
throw new NullPointerException("FlatMap function must not be null.");
}
TypeInformation outType = (TypeInformation)Predef..MODULE$.implicitly(evidence$12);
return package..MODULE$.javaToScalaStream((org.apache.flink.streaming.api.datastream.DataStream)this.javaStream.flatMap(flatMapper).returns(outType));
}
public <R> DataStream<R> flatMap(Function2<T, Collector<R>, BoxedUnit> fun, TypeInformation<R> evidence$14, ClassTag<R> evidence$15) {
if (fun == null) {
throw new NullPointerException("FlatMap function must not be null.");
}
Function2<T, Collector<R>, BoxedUnit> cleanFun = this.clean((F)fun);
.anon flatMapper = new /* Unavailable Anonymous Inner Class!! */;
return this.flatMap((FlatMapFunction<T, R>)flatMapper, evidence$14, evidence$15);
}
public <R> DataStream<R> flatMap(Function1<T, TraversableOnce<R>> fun, TypeInformation<R> evidence$16, ClassTag<R> evidence$17) {
if (fun == null) {
throw new NullPointerException("FlatMap function must not be null.");
}
Function1<T, TraversableOnce<R>> cleanFun = this.clean((F)fun);
.anon flatMapper = new /* Unavailable Anonymous Inner Class!! */;
return this.flatMap((FlatMapFunction<T, R>)flatMapper, evidence$16, evidence$17);
}
What could be wrong here? I would be grateful if you could give some insight.
Thank you in advance.

Answer 1

The problem is that you are importing the Java StreamExecutionEnvironment of Flink: org.apache.flink.streaming.api.environment.StreamExecutionEnvironment.
You have to use the Scala variant of the StreamExecutionEnvironment like this: import org.apache.flink.streaming.api.scala.StreamExecutionEnvironment.
With that change, everything is successfully building!
Original answer:
The problem is that you are passing a Function to the flatMap() method. However flatMap() expects a FlatMapFunction.
val words : DataStream[String] = text.flatMap[String](
new FlatMapFunction[String,String] {
override def flatMap(t: String, collector: Collector[String]): Unit = t.split(" ")
})