Create a class, say Test.java, that has fucntions to be imported embedded inside it.

Compile the class, and then run the autmatic C header generator

javac Test.java
javah Test

You will get Test.class and Test.h

Create your implementation for external functions Test.c and compile it a a library using a C/C++ compiler. See additional details below.

In case of Microsoft Visual C++ you need to do:
Start a new project with target set to DLL
Set the additinal include file location. That can be changed from the compiler menu be selecting Project/Settings.

Now you can run the example program. In case of running Win32 OS you can download: Test.class Test.dll