When String.hashCode() returns 0
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From: Jim Janney on 9 Aug 2010 11:42 Daniel Pitts <newsgroup.spamfilter(a)virtualinfinity.net> writes: > On 7/26/2010 12:12 AM, Andreas Leitgeb wrote: >> Jim Janney<jjanney(a)shell.xmission.com> wrote: >>> Andreas Leitgeb<avl(a)gamma.logic.tuwien.ac.at> writes: >>>> Has anyone found e.g. an english dictionary-word with hashCode 0, yet? >>>> Or perhaps the name of some commonly used class in Java standard library >>>> or some other String likely occurring in innocent code? >>> All strings of the form "\0", "\0\0", "\0\0\0", etc. >> Well, they're *almost* trivial ;-) >> >>> For ordinary words, I ran the following against the >>> dictionary file on a Linux system and got no hits. >> Thanks for the code (I guess I'd have been too lazy, myself ;) >> >> I ran it against all the pure class-names (like "java.lang.String", >> and "java/lang/String") from $JAVA_HOME/lib/rt.jar and found no >> match, either. Not even for each "Lpath/to/ClassName;". >> > The conditions under which String.hashCode() would return 0: > > 1. Empty String > 2. All characters in the string are \0. (Empty string is the > trivial case of this) To state it another way: 1) For any Java string s, s.hashCode() == ("\0" + s).hashCode() 2) "".hashCode() == 0 -- Jim Janney
From: Jim Janney on 9 Aug 2010 11:42 Andreas Leitgeb <avl(a)gamma.logic.tuwien.ac.at> writes: > Daniel Pitts <newsgroup.spamfilter(a)virtualinfinity.net> wrote: >> 3. The string is long enough that s[0]*31^(length-1) is close enough >> to, or over 2^32, and the rest of the characters add up just right. > > It's clear, that any 0-hashCode string with at least one non-\0 char > would have to have some minimum length around 5. ("clear" may be a > slight over-statement, as it's only a consequence of chars being treated > as unsigned when size-extended to int) > > My question was not intended to ask others to do the work of finding such > words, but rather, that if anyone had already accidentally found such a > String in the past, that he'd share it here > - leaving away empty or non-empty sequences of \0 (and empty sequences > of non-\0 as well ;-) ). > > Of course, I also don't want to stop those, you've made it their hobby > now, to find such a word for their own fun. > > Just so that one such String is named: "forty two 42 %4;2*" Applause. -- Jim Janney
From: Lew on 9 Aug 2010 13:03 Jim Janney wrote: > 1) For any Java string s, s.hashCode() == ("\0" + s).hashCode() > 2) "".hashCode() == 0 > Generally, 1) Given two Java Strings s and z where z.hashCode() == 0, s.hashCode() == (s + z).hashCode() and s.hashCode() == (z + s).hashCode() 3) "\0".hashCode() == 0 -- Lew
From: Lew on 9 Aug 2010 17:39
Lew wrote: > Generally, > > 1) Given two Java Strings s and z where z.hashCode() == 0, > s.hashCode() == (s + z).hashCode() and > s.hashCode() == (z + s).hashCode() > > 3) "\0".hashCode() == 0 > It occurs to me that s.hashCode() == (s + z).hashCode() is wrong. -- Lew |