"Write once, run anywhere" is a cool motto that can't necessarily be true in practice. There are obvious O/S differences, such as drive letters on Windows, but there are less obvious differences which are hardware based, such as big-endian vs little-endian ordering, the way floating point numbers are represented (not all systems use IEEE arithmetic), basic code sets (EBCDIC vs ASCII; some older systems used 6-bit character sets), etc.

Quite a few languages try to achieve the nirvana of "write once, run anywhere;" it's not trivial for the people who build the underlying compilers|virtual machines|interpreters.