convert java to scala code - change of method signatures

Question

Trying to convert some java to scala code I face the problem of a different method signature which compiled fine in the java world:

The following code in java (from https://github.com/DataSystemsLab/GeoSpark/blob/master/babylon/src/main/java/org/datasyslab/babylon/showcase/Example.java#L122-L126)

visualizationOperator = new ScatterPlot(1000,600,USMainLandBoundary,false,-1,-1,true,true);
visualizationOperator.CustomizeColor(255, 255, 255, 255, Color.GREEN, true);
visualizationOperator.Visualize(sparkContext, spatialRDD);
imageGenerator = new SparkImageGenerator();
imageGenerator.SaveAsFile(visualizationOperator.distributedVectorImage, "file://"+outputPath,ImageType.SVG);

Is translated to https://github.com/geoHeil/geoSparkScalaSample/blob/master/src/main/scala/myOrg/visualization/Vis.scala#L45-L57

val vDistributedVector = new ScatterPlot(1000, 600, USMainLandBoundary, false, -1, -1, true, true)
vDistributedVector.CustomizeColor(255, 255, 255, 255, Color.GREEN, true)
vDistributedVector.Visualize(s, spatialRDD)
sparkImageGenerator.SaveAsFile(vDistributedVector.distributedVectorImage, outputPath + "distributedVector", ImageType.SVG)

Which will throw the following error:

overloaded method value SaveAsFile with alternatives:
[error]   (x$1: java.util.List[String],x$2: String,x$3: org.datasyslab.babylon.utils.ImageType)Boolean <and>
[error]   (x$1: java.awt.image.BufferedImage,x$2: String,x$3: org.datasyslab.babylon.utils.ImageType)Boolean <and>
[error]   (x$1: org.apache.spark.api.java.JavaPairRDD,x$2: String,x$3: org.datasyslab.babylon.utils.ImageType)Boolean
[error]  cannot be applied to (org.apache.spark.api.java.JavaPairRDD[Integer,String], String, org.datasyslab.babylon.utils.ImageType)
[error]     sparkImageGenerator.SaveAsFile(vDistributedVector.distributedVectorImage, outputPath + "distributedVector", ImageType.SVG)

Unfortunately, I am not really sure how to fix this / how to properly call the method in scala.

Answer 1

This is a problem in ImageGenerator, inherited by SparkImageGenerator. As you can see here, it has a method

public boolean SaveAsFile(JavaPairRDD distributedImage, String outputPath, ImageType imageType)

which uses a raw type (JavaPairRDD without <...>). They exist primarily for compatibility with pre-Java 5 code and shouldn't normally be used otherwise. For this code, there is certainly no good reason, as it actually expects specific type parameters. Using raw types merely loses type-safety. Maybe some subclasses (current or potential) might override it and expect different type parameters, but this would be a misuse of inheritance and there must be a better solution.

Scala doesn't support raw types in any way and so you can't call this method from it (AFAIK). As a workaround, you could write a wrapper in Java which used correct types and call this wrapper from Scala. I misremembered, it's extending Java classes extending raw types which was impossible, and even then there are workarounds.

You might be able to call it by explicit type ascription (preferable to casting):

sparkImageGenerator.SaveAsFile(
  (vDistributedVector.distributedVectorImage: JavaPairRDD[_, _]), 
  outputPath + "distributedVector", ImageType.SVG)

But given the error message shows just JavaPairRDD, I don't particularly expect it to work. If this fails, I'd still go with a Java wrapper.