From lange.fabian at gmail.com  Wed Aug 27 13:48:19 2014
From: lange.fabian at gmail.com (Fabian Lange)
Date: Wed, 27 Aug 2014 16:48:19 +0300
Subject: Impact of code difference in Collection#contains() worth improving?
Message-ID: <CAFNq5NPy6oEowxsZ9DH=Xh9uYRS5ypXbjEaSQv3fCYM6Vez6cg@mail.gmail.com>

Hi all,
I have been involved recently in some theoretical or nonsensical
discussions about microbenchmarking, jit compiling assemblies and so fort.
One example was LinkedList vs ArrayList.

What I noticed is that those two have a different implementation for
contains():

ArrayList:

    *public* *boolean* contains(Object o) {
        *return* indexOf(o) >= 0;
    }

LinkedList:

    *public* *boolean* contains(Object o) {
        *return* indexOf(o) != -1;
    }

Logically this is of course identical due to the contract of contains which
returns either -1 or the >=0 index of the element.

This code has been like this almost forever, and I was wondering if this
actually makes a difference in CPU cycles.

And in fact this code compiles into different assembler instructions. The
array list does a test against 0 and conditional move, while the linked
list does a jump equals on -1.

Again that is not surprising, because the actual java source is different.
But I wonder if both options are equally good in cold performance and when
jitted based on parameter values.

Wouldn't one implementation be better than the other? And why is not the
"better" implementation taken in both classes (and maybe other Collections
which use indexOf) ?

Is the answer that this has always been like this and the benefit is not
worth the risk of touching ancient code?

And if not for performance, would code clarify and similarity be an
argument?

Or can the answer be found on a different mailing list? Then let me know :)


Fabian