Hi Claes, Thanks a lot for the clarification! I'm glad this has been optimized in JDK9! Also, your example with "foo" and "" makes sense.
Thanks! Zoltan On Wed, Feb 18, 2015 at 4:51 PM, Claes Redestad <claes.redes...@oracle.com> wrote: > 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 >> > >
