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!

return null from the catch block

Kakarot
– Kakarot

2014-02-25 23:54:34 +00:00
Commented Feb 25, 2014 at 23:54 — Kakarot
– Kakarot, Commented Feb 25, 2014 at 23:54
I tried that but it gave me a type mismatch

user3335040
– user3335040

2014-02-25 23:55:30 +00:00
Commented Feb 25, 2014 at 23:55 — user3335040
– user3335040, Commented Feb 25, 2014 at 23:55
is Data in the deque of primitive type or an Object ?

Kakarot
– Kakarot

2014-02-25 23:56:11 +00:00
Commented Feb 25, 2014 at 23:56 — Kakarot
– Kakarot, Commented Feb 25, 2014 at 23:56
class LinkedDeQue[A] extends Deque[A]

user3335040
– user3335040

2014-02-25 23:57:02 +00:00
Commented Feb 25, 2014 at 23:57 — user3335040
– user3335040, Commented Feb 25, 2014 at 23:57

gourlaysama · Accepted Answer · 2014-02-26 00:49:36Z

8

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)

edited Feb 26, 2014 at 0:49

answered Feb 26, 2014 at 0:42

gourlaysama

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Comments

Asqan · Accepted Answer · 2014-02-25 23:56:44Z

0

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

answered Feb 25, 2014 at 23:56

Asqan

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1 Comment

user3335040 Over a year ago

My list is empty when last = null.

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