How to pass Encoder as parameter to dataframe's as method

问题

I want to convert dataFrame to dataSet by using different case class. Now, my code is like below.

case Class Views(views: Double)
case Class Clicks(clicks: Double)

def convertViewsDFtoDS(df: DataFrame){
    df.as[Views]
}

def convertClicksDFtoDS(df: DataFrame){
    df.as[Clicks]
}

So, my question is "Is there anyway I can use one general function to this by pass case class as extra parameter to this function?"

回答1:

It seems a bit obsolete (as method does exactly what you want) but you can

import org.apache.spark.sql.{Encoder, Dataset, DataFrame}

def convertTo[T : Encoder](df: DataFrame): Dataset[T] = df.as[T]

or

def convertTo[T](df: DataFrame)(implicit enc: Encoder[T]): Dataset[T] = df.as[T]

Both methods are equivalent and express exactly the same thing (existence of an implicit Encoder for a type T).

If you want to avoid implicit parameter you can use explicit Encoder all the way down:

def convertTo[T](df: DataFrame, enc: Encoder[T]): Dataset[T] = df.as[T](enc)

convertTo(df, encoderFor[Clicks])

来源：https://stackoverflow.com/questions/40692691/how-to-pass-encoder-as-parameter-to-dataframes-as-method