java - 从 Java 实例化一个 Scala 类，并使用构造函数的默认参数

Question

我有一个Scala类:

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

我想从 Java 调用它，但使用默认参数

传递 null 不起作用(它分配 null，正如预期的那样)

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

这个技巧对我有用，但它很丑，我想知道是否有更好的方法:

斯卡拉

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

但是我想摆脱构造函数重载技巧，并使用 0 参数构造函数

这可能吗？

Answer 1

好像没有这个办法: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

然后 Lukas 提出了与您发现的相同的解决方案:

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) }
}