Hi Tumzadoc!

You can write any method of your Java classes either in Java-language
(which is the usual way) or in a native language like C or C++.
In your Java code you have to declare such a method with the "native"
key-word, for example:
  public native int myMethod(int arg);
Note that there is no { } here.

The implementation of such methods will be in a *.DLL file (in case of
Windows platform) or in a lib*.so file (in case of UNIX platform).

The technique to be used is called JNI (Java Native Interface).
There is a good on-line tutorial about this topic:
http://java.sun.com/docs/books/tutorial/native1.1/index.html

You probably do not want to create a DLL. If you want to put some code
into a reusable format, consider packaging it into a jar. This will allow
other java programs to easily use your classes.

Sadly, other languages are not very well suited to use java libraries. You
could try using some java->native compiler, but i doubt you'll end up with
a very usable library.