Instantiate a Scala class from Java, and use the default parameters of the constructor

Question

I have a Scala class:

class Foo(val x:String = "default X", val y:String = "default Y" ) 

I want to call it from Java, but using the default parameters

Passing null doesn't work (it assigns null, as expected)

new Foo(null,null); //both are instantiated as null

This trick did work for me, but it's ugly, and I wonder if there is a better way:

Scala

class Foo(val x:String = "default X", val y:String = "default Y" ) {
  def this(x:Object) = this()
}

Java

new Foo(null); //no matter what I pass it should work

However I would like to get rid of the constructor overload trick, and use a 0 param constructor

Is that possible?

Answer 1

It seems, there is no such way: https://issues.scala-lang.org/browse/SI-4278

Issue: default no-args constructor should be generated for classes with all-optional arguments
...

Lukas Rytz: in respect of language uniformity we decided not to fix this one - since it's a problem of interoperability with frameworks, we think it should not be fixed at the language level.

workarounds: repeat a default, or abstract over one, or put one default int the zero-argument constructor

Then Lukas proposes the same solution as you found:

class C(a: A = aDefault, b: B = C.bDefault) {
  def this() { this(b = C.bDefault) }
}
object C { def bDefault = ... }

// OR

class C(a: A = aDefault, b: B) {
  def this() { this(b = bDefault) }
}

Answer 2

There is a solution, please check out section "Default Arguments" from article: https://lampwww.epfl.ch/~michelou/scala/using-scala-from-java.html

It's possible to invoke both constructors and methods by passing the appropriate positional argument from java using .$default$[number] format.

Here scope is as follows:

• Class constructor: << ClassName >>.init$default$1 for the value of the first arguments default value set in the constructor definition default$2 for second arguments default value and so on.
• Method call: object.methodName$default$1 to resolve the methods first parameters default value assigned in the method signature etc.

Example:

import za.co.absa.spline.core.SparkLineageInitializer;
SparkLineageInitializer.SparkSessionWrapper lineage = SparkLineageInitializer.SparkSessionWrapper(spark);
lineage.enableLineageTracking(lineage.enableLineageTracking$default$1());

For this example the maven dependency is: groupId: za.co.absa.spline artifactId: spline-core version: 0.3.1