Without desiring to start a debate (as my posts sometimes do) I think
that "write once run anywhere" is not 100% accurate.  A better wording
should be "write once *correctly*, run anywhere".  For example, in the
past couple of weeks, there was a rather interesting discussion thread
in c.l.j.h which talked about the difference in the way newline
characters were coded in Java.  As it turns out, different operating
systems handle this differently.  Andrew Thompson pointed out

System.getProperty( "line.separator" );

*Here is the actual link:  http://shorterlink.com/?WUMPTW*

which is something that I didn't know about.

So, a java program coded on Windows with the windows system of dealing
with newlines will crap out on Linux or other platforms which deal with
it differently unless it is coded correctly by consulting the
System.getProperty method.  For a newbie like me, this was a very
important thing to realize.  If one is to write code that is truly
portable, one must never assume that something is handled the same way
on all platforms, and code to handle different operating systems, if the
code is going to be used on multiple platforms.

JMTC