public class HelloWorld
{
  public HelloWorld() //constructor
  {
  
  }
public native double getPopulation(double w, double x, double y, double z);

 static
 {
   System.loadLibrary("populationLib");
 }
        public static void main(String[] args)
        {
                HelloWorld hello = new HelloWorld();
                double value = hello.getPopulation(-89.3, 30.1, -89.6, 30.8);
                System.out.println("value returned is: " + value);
        }

}


now i use javac to compile and javah to create my C Header Stub. 

//C header stub created
//**note i had to add the shapefil.h header file myself because i want to use this to call the C Library Methods available through this. I also had to add a C++ function that i want to use to make these calls. (is this right??? )


/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
#include <shapefil.h>
/* Header for class HelloWorld */

#ifndef _Included_HelloWorld
#define _Included_HelloWorld
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     HelloWorld
 * Method:    getPopulation
 * Signature: (DDDD)D
 */
JNIEXPORT jdouble JNICALL Java_HelloWorld_getPopulation
  (JNIEnv *, jobject, jdouble, jdouble, jdouble, jdouble);

double setPopulation(double, double, double, double);
#ifdef __cplusplus
}
#endif
#endif

#include <iostream>
#include "HelloWorld.h"

using namespace std;

JNIEXPORT double JNICALL Java_HelloWorld_getPopulation(JNIEnv *env, jobject obj, jdouble w, jdouble x,
                                                                                  jdouble y, jdouble z)

{
  jdouble pop;
  pop = setPopulation(w, x, y, z);
  return (jfloat)pop;
}

double setPopulation(double x1, double y1, double x2, double y2)
{
  SHPHandle mySHPhandle = NULL;
  DBFHandle myDBFhandle = NULL;
  SHPObject *mySHPobject = NULL;
  DBFFieldType stuff;
  int field_count;
  int record_count;

  mySHPhandle = SHPOpen("file1.shp", "rb");
  //myDBFhandle = DBFOpen("file1.dbf", "rb");
  //field_count = DBFGetFieldCount(myDBFhandle);
  //record_count = DBFGetRecordCount(myDBFhandle);

  return 10.0; //arbitrary number/cant return anything of use yet. 
}