[英]How do I return concrete type when overriding a generically typed method in Scala?
请查看以下代码段:
import org.apache.spark.streaming.dstream.DStream
import org.apache.spark.streaming.StreamingContext
import org.apache.spark.rdd.RDD
import org.apache.spark.streaming.Time
import org.apache.spark.streaming.Seconds
abstract class MQTTDStream[T <: Any](ssc: StreamingContext) extends DStream(ssc) {
override def compute(validTime: Time): Option[RDD[T]] =
Some(ssc.sparkContext.parallelize(Seq(1, 2, 3), 1)) //This line doesn't compile
override def dependencies = Nil
override def slideDuration = Seconds(1) // just an example
}
我收到以下错误:
类型不匹配; 找到:Int(1)必需:T
我已经声明T扩展了Any,那么为什么编译器会抱怨呢? Int是Any的子类型,不是吗?
非常感谢!
更新:2.9.16:
更改为从DStream [Int]扩展,但仍然是相同的错误:
abstract class MQTTDStream[T](ssc: StreamingContext) extends DStream[Int](ssc) {
override def compute(validTime: Time): Option[RDD[T]] =
Some(ssc.sparkContext.parallelize(Seq(1, 2, 3), 1)) //This line doesn't compile
override def dependencies = Nil
override def slideDuration = Seconds(1) // just an example
}
编辑:2.9.16:
感谢Alexey,这是有效的解决方案:
import org.apache.spark.streaming.dstream.DStream
import org.apache.spark.streaming.StreamingContext
import org.apache.spark.rdd.RDD
import org.apache.spark.streaming.Time
import org.apache.spark.streaming.Seconds
abstract class MQTTDStream[T](ssc: StreamingContext) extends DStream[Int](ssc) {
override def compute(validTime: Time): Option[RDD[Int]] =
Some(ssc.sparkContext.parallelize(Seq(1, 2, 3), 1))
override def dependencies = Nil
override def slideDuration = Seconds(1) // just an example
}
呼叫者可以选择T
,而不是您。 因此,您的类定义必须适用于所有T
(满足类型界限,但所有T
都是Any
子类型)。
也就是说,如果有人创建了例如MQTTDStream[String]
,则其compute
方法必须返回Option[RDD[String]]
。 但是它没有:它返回Some[RDD[Int]]
。
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