How to use Scala implicit class in Java

Question

I have a Scala Implicit class from RecordService API, which i wanted to use in Java file.

package object spark {

   implicit class RecordServiceContext(ctx: SparkContext) {
     def recordServiceTextFile(path: String) : RDD[String] = {
      new RecordServiceRDD(ctx).setPath(path)
          .map(v => v(0).asInstanceOf[Text].toString)
    }
  }

}

Now i am trying to import this in a Java file using below import.

import com.cloudera.recordservice.spark.*;

But i am not able to use recordServiceTextFile("path") from sparkContext.

In Scala the import is little different and its working.

Answer 1

Here is simple definition of implicit class in package object

package object spark {
  implicit class Ext(param: Int) {
    def a = param + 1
  }
}

and here is how you can use it from java

public class Test {
    public static void main(String[] args) {
        spark.package$.MODULE$.Ext(123).a();
    }
}

so you can basically use RecordServiceContext as a method that wraps your SparkContext and adds an extra method that you can call. That is optimization for implicit classes.

That would be something like this:

SparkContext c = ???
RDD<String> rdd = com.cloudera.recordservice.spark.package$.MODULE$.RecordServiceContext(c)
   .recordServiceTextFile("asdf");

Answer 2

A package object spark is compiled to a class package in the package spark. The implicit class RecordServiceContext will get compiled to a static method RecordServiceContext (that's scala's implicit def) in package and a class package$RecordServiceContext.

So the following code should do it:

import com.cloudera.recordservice.spark.*;

//some code

RDD<String> rdd = package.RecordServiceContext(myContext).recordServiceTextFile(pathToFile);

//some code

But package is probably a reserved keyword, and Java has no way of escaping them as far as I know. So you'll have to do some reflection to invoke the RecordServiceContext method.