Scala: is it possible to override default case class constructor?

Question

Just wondering if this is possible. What I would actually like to do is check and possibly modify one of the arguments before it is stored as a val.

Alternatively, I could use an overload and make the default constructor private. In which case I would also like to make private the default factory constructor in the companion object, how would I do that?

Many thanks.

Adam

edit: well i figured out that making the default constructor private also makes the default factory constructor private, so i have a solution, i'm still interested to know if the default constructor is overridable though

Answer 1

You can overload constructors. It's the same as in C++ or Java. Simply make another constructor.

class Foo( _input:Int ){
    def this() = this( 0 )
}

Or you can see this SO post.

Answer 2

The presence of secondary case class constructors don't cause the compiler to produce additional factory methods in the class's companion, so you won't get the convenience of CaseClaseName(«secondary constructor parameter list»>) for creating them. You'll have to use the new keyword.

It's better to put the sort of logic you're describing in alternate factory methods in the companion object and stick to using the primary constructor.

Answer 3

You can turn a regular class into a pseudo-case-class by writing your own apply (for the factory) and unapply (for the pattern match) methods in the companion object. Or, you could simply write a named factory method in the case class's companion object.

Answer 4

You do not have the option of changing the way the default constructor stores its parameters (e.g. by modifying the parameters before they are stored as vals) but you do have the option of throwing an exception if the parameters are wrong (this will occur after the parameters are stored)

case class Foo(x:Int){
    if (x<0) throw SomeException;
}

You also have the option of implementing additional constructors that call the first constructor

case class Foo(x:Int){
     def this(x:Int,y:Int) = this(x+y)
}

but those don't get factory methods.

You could easily create the factory method yourself by adding it to the companion object

object Foo{
     def apply(x:Int,y:Int) = new Foo(x,y)
}

Anything else more complicated than that, and you have to forgo the case class and implement it's parts on your own: apply, unapply, equals, and hashCode. Programming in Scala talks about how to do all of these, giving good formulas for equals and hashCode.