Scala AggregateByKey在抽象类中时无法编译
我对Scala和Spark都是新手,所以我希望有人能解释为什么aggregateByKey在抽象类中无法编译。这是我能想到的最简单的例子:Scala AggregateByKey在抽象类中时无法编译,scala,apache-spark,compiler-errors,abstract-class,Scala,Apache Spark,Compiler Errors,Abstract Class,我对Scala和Spark都是新手,所以我希望有人能解释为什么aggregateByKey在抽象类中无法编译。这是我能想到的最简单的例子: import org.apache.spark.{SparkConf, SparkContext} import org.apache.spark.rdd.RDD abstract class AbstractKeyCounter[K] { def keyValPairs(): RDD[(K, String)] def processData(
import org.apache.spark.{SparkConf, SparkContext}
import org.apache.spark.rdd.RDD
abstract class AbstractKeyCounter[K] {
def keyValPairs(): RDD[(K, String)]
def processData(): RDD[(K, Int)] = {
keyValPairs().aggregateByKey(0)(
(count, key) => count + 1,
(count1, count2) => count1 + count2
)
}
}
class StringKeyCounter extends AbstractKeyCounter[String] {
override def keyValPairs(): RDD[(String, String)] = {
val sc = new SparkContext(new SparkConf().setMaster("local").setAppName("counter"))
val data = sc.parallelize(Array("foo=A", "foo=A", "foo=A", "foo=B", "bar=C", "bar=D", "bar=D"))
data.map(_.split("=")).map(v => (v(0), v(1)))
}
}
其中:
Error:(11, 19) value aggregateByKey is not a member of org.apache.spark.rdd.RDD[(K, String)]
keyValPairs().aggregateByKey(0)(
^
如果我改为使用单个具体类,它将编译并成功运行:
import org.apache.spark.{SparkConf, SparkContext}
import org.apache.spark.rdd.RDD
class StringKeyCounter {
def processData(): RDD[(String, Int)] = {
val sc = new SparkContext(new SparkConf().setMaster("local").setAppName("counter"))
val data = sc.parallelize(Array("foo=A", "foo=A", "foo=A", "foo=B", "bar=C", "bar=D", "bar=D"))
val keyValPairs = data.map(_.split("=")).map(v => (v(0), v(1)))
keyValPairs.aggregateByKey(0)(
(count, key) => count + 1,
(count1, count2) => count1 + count2
)
}
}
我缺少什么?如果您更改:
abstract class AbstractKeyCounter[K] {
致:
这将编译
为什么aggregateByKey
是PairRDDFunctions
的一种方法(您的RDD
被隐式转换为该类),它具有以下签名:
class PairRDDFunctions[K, V](self: RDD[(K, V)])
(implicit kt: ClassTag[K], vt: ClassTag[V], ord: Ordering[K] = null)
这意味着它的构造函数需要类型为ClassTag[K]
和vt:ClassTag[V]
的隐式值。您的抽象类不知道K是什么,因此无法提供匹配的隐式值。这意味着到pairddfunctions
的隐式转换“失败”(编译器不执行转换),因此无法找到方法aggregateByKey
添加[K:ClassTag]
是将隐式参数隐式kt:ClassTag[K]
添加到抽象类构造函数的缩写,然后编译器使用它并将其传递给pairrdfunctions
的构造函数
有关类标记及其优点的更多信息,请参见
class PairRDDFunctions[K, V](self: RDD[(K, V)])
(implicit kt: ClassTag[K], vt: ClassTag[V], ord: Ordering[K] = null)