From RandomAccessFile.setLength(long)

If the present length of the file as returned by the length method is  
smaller than the newLength argument then the file will be extended. In  
this case, the contents of the extended portion of the file are not  
defined.

I suspect any solution that attemps to write to the end of a file will  
work much like setLength() - the contents of the file which you have not  
explicitly written to will be dependant on OS behavior. The only way to  
guarantee a zeroed file across all platforms is to zero it out yourself.

Once you have that algorithm in place, you can figure out what OSs you are  
most likely to be running on, and put guards in place to use functions /  
assumptions available on that OS that can do something more intelligent.

Eg, on Windows NTFS, all files are normal files unless explicity defined  
otherwise (IIRC). But, you can always use JNI to create a sparse file, or  
use Runtime.exec to call "fsutil sparse" to set up a sparse file. Note  
that JNI is the preferable approach: fsutil does many other things besides  
sparse file management, and thus has much tighter security restrictions.

HTH,

Zig