String parameter in nsIOutputStream.write(...)

Hello,

I have a few interrelated questions about the write() method of the
interface org.mozilla.interfaces.nsIOutputStream that takes two
arguments; aBuf of type String and aCount of type long.

1) The C++ counterpart of the method takes a char array. Exactly what
is being done to convert
    a Java String to a C++ char*?
2) What should the value of aCount be? Should it be aBuf.length() or
aBuf.getBytes().length or some other value?
3) What if I want to write binary data? Is this interface suitable for
that task at all?

Thank you.

-Seong