使用可选参数从Java访问Scala的case类

Question

I have a case class having an optional parameter. When the parameter is not defined it should be set to some default. Eg:

case class Foo(bar: Option[Int] = Some(100))

The problem arises when I try to access this case class from Java:

Foo foo = new Foo(Option.empty())

It doesn't assign the default value 100 to variable bar

Answer 1

You can maybe use apply() method to get an instace of the class with default parameters in the following way:

object Foo{
  def getDefault: Foo = new Foo
}

Which can be used like this in Java:

Foo.getDefault()

or if you have more than one default parameters then you will have to use the builder pattern like this:

case class Foo(a: Int = 1, b: Option[Int] = None, c: String = "Hello")

object Foo{
  class Builder{
    var a = 1
    var b = None
    var c = "Hello"
    def withA(x: Int) = {a = x; this}
    def withB(x: Some) = {b = x; this}
    def withC(x: String) = {c = x; this}
    def build = Foo(a,b,c)
  }
}

Answer 2

You can do this:

case class Foo(bar: Option[Int]) {
  def this() = this(Some(100))
} 

If you run it in a scala worksheet with:

object Foo{
  val a = new Foo()                         //> a  : week3.Foo = Foo(Some(100))
}

Hope this helps