convert java to scala code - change of method signatures

问题

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.

回答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.

回答2:

The accepted answer is correct in saying that raw types should be avoided. However Scala can interoperate with Java code that has raw types. Scala interprets the raw type java.util.List as the existential type java.util.List[_].

Take for example this Java code:

// Test.java
import java.util.Map;

public class Test {
  public boolean foo(Map map, String s) {
    return true;
  }
}

Then try to call it from Scala:

Welcome to Scala 2.12.1 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_131).
Type in expressions for evaluation. Or try :help.

scala> import java.util.{Map,HashMap}
import java.util.{Map,HashMap}

scala> new Test().foo(new HashMap[String,Integer], "a")
res0: Boolean = true

scala> val h: Map[_,_] = new HashMap[String,Integer]
h: java.util.Map[_, _] = {}

scala> new Test().foo(h, "a")
res1: Boolean = true

So it looks like there must be some other problem.

来源：https://stackoverflow.com/questions/43626048/convert-java-to-scala-code-change-of-method-signatures

标签

java

scala

overloading

raw-types