Re: looping through a list, starting at 1

From:
Lew <lewbloch@gmail.com>
Newsgroups:
comp.lang.java.programmer
Date:
Thu, 11 Aug 2011 09:20:10 -0700 (PDT)
Message-ID:
<4b71d212-c055-4af0-b9a9-13e3afbc5785@glegroupsg2000goo.googlegroups.com>
Andreas Leitgeb wrote:

Lew wrote:

Andreas Leitgeb wrote:

Lew wrote:

Volker Borchert wrote:

if (l instanceof RandomAccess) {

Tests on type like this are an antipattern.

Are marker-interfaces (which RandomAccess is, iirc) already an
antipattern, or is there a different way to check for them, [...]

With generics you can use type intersections so that something has to be
both 'List' and 'RandomAccess' (again, say) even to reach the call or th=

e

class or whatever.

 
Interesting stuff! (but still falls a bit short):
 
--- snip SSCCE Test.java ---
import java.util.*;
public class Test {
    public static void main(String[] args) {
        method( new ArrayList(42) );
        method( new Vector(42) );
        //method( new LinkedList(42) ); // would error: good!
    }
 
    public static <T extends List,RandomAccess> T method(T raAcList) {
       return raAcList;
    }
 
    //Provide a fallback overload for all Lists: doesn't compile
    // public static <T extends List> T method(T anyList) {
    // return anyList;
    // }


You cannot have two erasure-equivalent methods in the same class, so that's=
 why both won't co-exist.

The generics trick is suitable when you want to insist that the argument be=
 of the particular type, not when you want to bifurcate as you indicated. =
It seems as though perhaps you are in a situation where you want to fork on=
 the type at run-time, but I am not convinced yet.
 

The object is asked at runtime if it is a RandomAccess, and
either way correct (hope so) and appropriate code is executed
for the object. It's not principially different from asking
a (list-)object if it is empty(), and do something appropriate
in each case.


It is different, inherently so, in that type assertions normally should be =
done at compile time, and are less expensive to aver at that time, but list=
 emptiness is inherently a run-time issue, being based on data and not the =
program's own types.

I only wished to point out a general principle here.

 
Thanks for pointing out the trick with generics, even though
it doesn't yet fully work. I hadn't thought of that, and might
run into a situation where it could prove useful.


It does "fully work", if the problem is fully thought out.

--
Lew

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