Yeah, and this is why I'm very careful about recording the versions I use for a calculation (version number and/or commit for the language, library version numbers, and commit for my code). The thing is, that's not your first step: the first step is to try and get the thing to run on whatever Java (/Julia/Matlab/Mathematica/Fortran) you have installed. If that doesn't work, there are a thousand things it could be (improper usage, some missing file you had no clue you might need, change in libraries, change in OS/distro/CPUarch, out-of-memory with a new set of parameters); installing an old version of your language and trying it on that is not necessarily easy, and certainly not the first bug-finding step.

Philip Guo (pgbovine) addressed many of these problems in his thesis work, on Burrito (http://www.pgbovine.net/burrito.html ) and CDE (http://www.pgbovine.net/cde.html ). Both of these look very cool, but I've never managed to get over the hump and actually start playing with them.

At this point I should note that I'm actually in favor of fixing a bad standard-library PRNG---I just think it's important to understand (a) what could break, and (b) why users might be unhappy about the breakage. This isn't quite a Chesterton's Fence sort of situation, but it's similar in spirit.

Sure, but it kind of sucks if your whole program has to remain on Java 1.3 forever because you need a particular RNG.

Until a required library (or Java itself) has a security flaw and the new version does not support that version of Java.