Puzzle: implement this

19 06 2009

This is something I stumbled on recently when trying to implement javax.jcr.NodeIterator in Scala.

Assume you are using a library which exports an Iterator2 interface:

public interface Iterator2 extends java.util.Iterator {}

Note that Iterator is a raw type and Iterator2 does not take any type parameters. So how would you implement Iterator2 in Scala?

Here is a start:

class MyIterator extends Iterator2 {
  def hasNext = false
  def remove = throw new Error
  def next: Nothing = throw new Error

But if the next method should return an actual value, what would be it’s return type? It turn’s out that any other type than Nothing results in a compiler error:

error overriding method next in trait Iterator of type ()E;
method next has incompatible type ()Any

So how would you implement Iterator2?



7 responses

19 06 2009
Jorge Ortiz

Scala does not understand raw types. You must implement MyIterator in Java. See: http://lampsvn.epfl.ch/trac/scala/ticket/1737

20 06 2009

Oh, I wasn’t aware of that. Thanks for pointing me to the ticket.

So I probably found a workaround? Hmm or probably my workaround has limitations I cannot see yet 😉 Anyway, I’ll post it in a couple of days.

19 06 2009
James Iry

There’s no perfect answer. One answer is to write an abstract Java class that implements all the raw methods in terms of Object based methods

public abstract class UnRawIterator implements Iterator2 {
public abstract Object unRawNext();
final public Object next() { return unRawNext();}

Now we can write Scala

class MyIterator extends UnRawIterator {
def unRawNext = “hello”
def hasNext = true
def remove = error(“no can doo”)

It’s seriously ugly. I think there’s a long standing trac issue around this.

20 06 2009
Daniel Spiewak

What’s “seriously ugly” about this is the fact that Java *allows* the use of raw types as in the example. Scala, having a safer and more consistent type system, doesn’t have any way of reasonably processing an override like the example.

If you can think of a sound solution to this problem, I’m willing to be that the compiler folks are open to suggestions.

24 06 2009
Puzzle: implement this (solution) « Michid’s Weblog

[…] I wasn’t aware of Ticket #1737 when I was trying to find a solution to the problem from my previous post. Thanks to Jorge Ortiz for pointing this out. However, I reviewed my approach to solving this and […]

30 08 2009
Implementing Java Interfaces and Generics « Michid’s Weblog

[…] Java Interfaces and Generics 30 08 2009 In an earlier post I asked for an implementation of the following Java interface in […]

30 03 2012

plz suggest me how implement puzzle in java

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