Hi Zoltan,
1. Well, obviously, there's an empty string at the end of all Strings which
you can index into! ;-)
(I guess it was a coin toss originally. Since it'd be a bit
inconsistent if say "foo".substring(0, 0) returned "" while, say,
"".substring(0, 0) threw a tantrum, I'd like to think the current impl was
favored for good reason)
2. https://bugs.openjdk.java.net/browse/JDK-8067471 was recently discussed
and a patch which actually optimizes away the array creation was pushed
into JDK9:
http://hg.openjdk.java.net/jdk9/dev/jdk/rev/c60cf8acabb2
Going further and removing the String allocation entirely (returning "")
might have unexpected side-effects for some code expecting a new, distinct
object. Any potential benefit might also be eaten up by increased byte
code,
more branches etc.
HTH!
/Claes
On 2015-02-19 01:16, Zoltan Sziladi wrote:
Hi,
There are 2 things I do not understand about how substring works, hopefully
someone can shed some light on it.
1. When you call substring and you provide the string's length as
beginIndex, why does it return an empty string instead of throwing an
exception? You are effectively indexing outside of the string: you index a
character that is after the last character. Obviously this won't be changed
since probably lots of code depend on this already, but still, I just don't
get the logic behind it.
2. In this specific case (when beginIndex is the string's length) there
seem to be a lot of unnecessary method calls for returning an empty string:
String constructor -> Arrays.copyOfRange -> System.arraycopy
All of these calls do some checks and in this case pass around 0's a lot,
even creating array of 0 length, etc.
My question is, would it make sense to include an if so that if subLen ==
0, then it would return an empty string? Or it has too much overhead if we
consider how often this case occurs and it would be better to lose on
performance on these rare occasions while keeping the general case 1 if
faster?
Thanks in advance!
Regards,
Zoltan
