Here's one for the geeks, on how to use native functions in Java on Mac OS X. Start out by creating Java a class

public class NativeSum {}

In your class, declare a native method you will implement in c

public native static int nativeSum(int a, int b);

Load your c library, which we will create later on

static {
    System.loadLibrary("nativesum");
}

Make use of your native c function to calculate the sum of the numbers 10 and 20

public static void main(String[] args) {
    int c = nativeSum(10, 20);
    System.out.println(c);
}

By now you should have a Java class similar to the following

public class NativeSum {

    public native static int nativeSum(int a, int b);

    static {
        System.loadLibrary("nativesum");
    }

    public static void main(String[] args) {
        int c = nativeSum(10, 20);
        System.out.println(c);
    }
}

Compile your java class

$ javac NativeSum

Run the javah command to generate a JNI c header file

$ javah -jni NativeSum

This will generate something like the this as a NativeSum.h c header file

/* DO NOT EDIT THIS FILE - it is machine generated */
#include 
/* Header for class NativeSum */

#ifndef _Included_NativeSum
#define _Included_NativeSum
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     NativeSum
 * Method:    nativeSum
 * Signature: (II)I
 */
JNIEXPORT jint JNICALL Java_NativeSum_nativeSum
  (JNIEnv *, jclass, jint, jint);

#ifdef __cplusplus
}
#endif
#endif

Now create a NativeSum.c file to hold your c implementation of the NativeSum.h header file's method, like this

#include 

JNIEXPORT jint JNICALL 
Java_NativeSum_nativeSum (JNIEnv * e, jclass c, jint i, jint j) {
    int k = i + j;
    return k;
}

Compile your NativeSum.c file. Remember to specify the path of the jni.h file to include the library

$ cc -c -I"/System/Library/Frameworks/JavaVM.framework/Headers/" NativeSum.c

Create a shared library for java to access your c implementation

$ cc -dynamiclib -o libnativesum.jnilib NativeSum.o -framework JavaVM

Run your program

$ java NativeSum
30

If you end up with the number 30 in your terminal, you have successfully called your (first) c function from java