Scala/Java Error handling a NullPointerException

Question

I am working on a method which gets elements out of a double linked deque. When the linkedlist is empty, I get a nullpointerexception and I am trying to figure out how to handle it. I am using to following code but it still requires me to return A. Any ideas how I can get to this compile?

def peekBack():A = {
  try {
    last.data // return if the list is full if not, catch the nullpointerexception
  } catch {
    case ex: NullPointerException => {
      println("There are no elements in this list.")
         //Error is here, it is requiring me to return something!!!
    }
  }
}

Thanks!

Answer 1

If last is some var that ends up being null at some point, what about a simple if:

def peekBack(): A = {
  if (last == null) 
    throw new NoSuchElementException("empty list")
  else last.data
}

Edit : if you do want to return null , you need a proof that A is nullable:

def peekBack()(implicit ev: Null <:< A): A = {
  if (last == null) ev(null)
  else last.data
}

Of course the proper way to do this would be to return an Option[A] :

def peekBack(): Option[A] = Option(last).map(_.data)

Answer 2

You dont take a nullpointerexception when your list is empty. you take it when your object is not initialized. ensure that you call a "initialized" empty object or best, foresee that also in your method