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Re: Novice to Generics Trying to Implement a Generic Priority Queue

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On Fri, 8 Apr 2011, Lew wrote:

> You do, however, have to guard against a possible 
> 'NullPointerException'. The method is 'private', so it's up to its 
> callers not to screw that up.  You enforce that with an assertion:
>
> private static boolean inOrder( Da left, Da right)
> {
>   assert left != null;
>   return left.compareTo( right ) <= 0;
> }

compareTo will (or at least should) throw a NullPointerException if right 
is null (from Comparable's javadoc - "Note that null is not an instance of 
any class, and e.compareTo(null) should throw a NullPointerException even 
though e.equals(null) returns false"), so if you're going to check left, 
you ought to check right too.

But i don't see why you would check either. It is right and proper that if 
a method which requires non-null arguments is passed nulls, it should 
throw a NullPointerException (from NullPointerException's javadoc: 
"Applications should throw instances of this class to indicate other 
illegal uses of the null object"). This method will do that. The assertion 
is unnecessary.

A method which is going to handle a parameter in such a way that it will 
not naturally blow up if it is null, but which requires that it be 
non-null, should of course make the assertion you describe, or an 
equivalent explicit guard.

> If the method were 'public' you'd need to add an explicit guard against 
> the 'NullPointerException':
>
> public static boolean inOrder( Da left, Da right)
> {
>   if ( left == null )
>   {
>     return true;
>   }
>   assert left != null;
>   return left.compareTo( right ) <= 0;
> }

This means something different to the assertion. This says that nulls are 
smaller than any other element - and so implies that nulls are a 
permissible element, whereas the assertion rejected them. That would be a 
logically consistent thing to do, but in that case, the method also needs 
to deal with a null right, by inserting "if (right == null) return false;" 
after the first guard and before the compareTo call.

The other option would be to check both left and right for nullity, and 
throw a NullPointerException (or IllegalArgumentException if you prefer) 
if either is null. That would require all elements to be non-null. You can 
do this by omitting any guard clauses - the compareTo call will do it.

> Don't forget your Javadocs!

Sage advice.

tom

-- 
secular utopianism is based on a belief in an unstoppable human ability
to make a better world -- Rt Rev Tom Wright

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Novice to Generics Trying to Implement a Generic Priority Queue KevinSimonson <kvnsmnsn@hotmail.com> - 2011-04-07 16:03 -0700
  Re: Novice to Generics Trying to Implement a Generic Priority Queue markspace <-@.> - 2011-04-07 17:32 -0700
    Re: Novice to Generics Trying to Implement a Generic Priority Queue KevinSimonson <kvnsmnsn@hotmail.com> - 2011-04-08 14:55 -0700
      Re: Novice to Generics Trying to Implement a Generic Priority Queue markspace <-@.> - 2011-04-08 15:54 -0700
        Re: Novice to Generics Trying to Implement a Generic Priority Queue Lew <noone@lewscanon.com> - 2011-04-08 20:39 -0400
          Re: Novice to Generics Trying to Implement a Generic Priority Queue Tom Anderson <twic@urchin.earth.li> - 2011-04-09 10:36 +0100
  Re: Novice to Generics Trying to Implement a Generic Priority Queue Roedy Green <see_website@mindprod.com.invalid> - 2011-04-09 05:50 -0700
    Re: Novice to Generics Trying to Implement a Generic Priority Queue KevinSimonson <kvnsmnsn@hotmail.com> - 2011-04-11 08:08 -0700
      Re: Novice to Generics Trying to Implement a Generic Priority Queue markspace <-@.> - 2011-04-11 10:29 -0700
      Re: Novice to Generics Trying to Implement a Generic Priority Queue KevinSimonson <kvnsmnsn@hotmail.com> - 2011-04-11 12:10 -0700
        Re: Novice to Generics Trying to Implement a Generic Priority Queue Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2011-04-11 21:42 +0200
          Re: Novice to Generics Trying to Implement a Generic Priority Queue Tom Anderson <twic@urchin.earth.li> - 2011-04-11 22:41 +0100
            Re: Novice to Generics Trying to Implement a Generic Priority Queue Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2011-04-12 01:07 +0200

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