I don't know which version of java  documentation you have but mine says:

<quote_jdk_1.3>
freeMemory

public long freeMemory()

Returns the amount of free  memory in the system. Calling the gc method  may
result in increasing the value returned by freeMemory.

Returns:
an approximation to the total amount of memory currently available for
future allocated objects, measured in bytes.
</quote_jdk_1.3>

But you can use the Runtime class  and use exec to call for example a dos
command and then grab the output
and parse it. Nasty, but AFAIK there is no other way.