Question

I’m learning Scala. I want to implement a subclass of Exception that gets a name as parameter and builds a message with that name embedded on it. Something similar to this:

class InvalidItem(itemName: String) extends Exception(msg: name) {
  def this(itemName)= {
    super("Invalid item: " + itemName)
  }
}

In this case, I simply want to prepend itemName with "Invalid item:" before passing it to the superconstructor. But I can’t find the way.

I’ve tried several similar syntaxes (i.e. replacing super by this) but kept getting cryptic errors.

What is the correct way of doing this in Scala?

Answer 1

You’re actually calling the parent constructor in the extends clause, so the following works:

class InvalidItem(itemName: String) extends Exception("Invalid item name" + itemName)

For a discussion of this syntax and its motivation, see for example this blog post by Daniel Spiewak:

That little bit of extra syntax in the extends clause is how you
call to a superclass constructor… This may seem just a bit odd at
first glance, but actually provides a nice syntactical way to ensure
that the call to the super constructor is always the first statement
in the constructor. In Java, this is of course compile-checked, but
there’s nothing intuitively obvious in the syntax preventing you
from calling to the super constructor farther down in the
implementation. In Scala, calling the super constructor and calling a
superclass method implementation are totally different operations,
syntactically. This leads to a more intuitive flow in understanding
why one can be invoked arbitrarily and the other must be called prior
to anything else.