踩坑记录之spark：value toDF is not a member of org.apache.spark.rdd.RDD

用 sbt 打包 scala 程序时，遇到如下错误：

[error] /home/hadoop/sparkapp/src/main/scala/RecommendApp.scala:25:144: value toDF is not a member of org.apache.spark.rdd.RDD[Movie]
[error] val moviesDF = moviesRDD.map(x => Movie (x.split("::")(0).toInt,x.split("::")(1).replaceAll("[0-9()]","").trim,x.split("::")(2).trim)).toDF
[error]                                                ^
[error] /home/hadoop/sparkapp/src/main/scala/SimpleApp.scala:33:61: value toDF is not a member of org.apache.spark.rdd.RDD[Rating]
[error] val ratingsDF = ratingsRDD.map(parseRating).toDF
[error]                                                                                   ^

经授课老师指点，解决上述错误的过程中，有两个地方需要我们注意：

         1、启用隐式转换时，需要在 main 函数中自行创建 SparkSession 对象，然后使用该对象来启用隐式转换，而非在 object 对象之前启用。

         2、case class 类的声明需要放在 main 函数之前。

import org.apache.spark.sql.SparkSession
# import spark.implicits._ 错误示范
object RecommendApp {
    case class Rating(user_id:Int, movie_id:Int, rating:Float, timestamp:Long)        
    case class Movie(movie_id:Int, movie_name:String, movie_type:String)   
    def main(args: Array[String]) {
        val conf = new SparkConf().setAppName("Simple Application")
        val sc = new SparkContext(conf)
        val spark=  SparkSession.builder().getOrCreate()
        import spark.implicits._
        # 错误示范
        # case class Rating(user_id:Int, movie_id:Int, rating:Float, timestamp:Long)    
        # case class Movie(movie_id:Int, movie_name:String, movie_type:String) 

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