Combine Java with C

You didn't mention which platform or JDK version you are using so I'll note the most universal way of doing it.  You define a method as "native".  This tells Java that the underlying implementation of the function is not Java but implented in C (or some other language) on the hosting operating system.  Check the documentation for your Java development environment on how to implement and use a native method.

To use a C program you have to use a Java Native  Method. Basicly you have to defind the C program in Java, pass the varibles in vative types and recive the answers.

remember to generate the the C header not write it

p.s it is advisable to use C++ as it fits the OO programing
The only example i have is from Learn more java in 21 days so here goes

the pond example

pond.cpp-----------------------
#include "Pond.h"

// Global field IDs
jfieldID fLengthID;
jfieldID fWidthID;
jfieldID fDepthID;

void JNICALL Java_Pond_initPond(JNIEnv* env, jclass thisClass) {
  // Find the fields
  fLengthID = env->GetFieldID(thisClass, "length", "F");
  fWidthID = env->GetFieldID(thisClass, "width", "F");
  fDepthID = env->GetFieldID(thisClass, "depth", "F");
}

jfloat JNICALL Java_Pond_getVolume(JNIEnv* env, jobject thisObject) {
  // Get the field values
  jfloat length = env->GetFloatField(thisObject, fLengthID);
  jfloat width = env->GetFloatField(thisObject, fWidthID);
  jfloat depth = env->GetFloatField(thisObject, fDepthID);

  // Calculate and return the volume
  return length * width * depth * 7.5f;
}
_______________________________________

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

#ifndef _Included_Pond
#define _Included_Pond
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     Pond
 * Method:    initPond
 * Signature: ()V
 */
JNIEXPORT void JNICALL Java_Pond_initPond
  (JNIEnv *, jclass);

/*
 * Class:     Pond
 * Method:    getVolume
 * Signature: ()F
 */
JNIEXPORT jfloat JNICALL Java_Pond_getVolume
  (JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif

-------------------------------------------------------------

pond.java---------------------------------------------
import java.io.*;

public class Pond implements Serializable {
  private float length, width, depth;

  // Constructors
  public Pond() {
    this (0.0f, 0.0f, 0.0f);
  }

  public Pond(float l, float w, float d) {
    length = l;
    width = w;
    depth = d;
  }

  // Accessor methods
  public float getLength() {
    return length;
  }
 
  public void setLength(float l) {
    length = l;
  }

  public float getWidth() {
    return width;
  }

  public void setWidth(float w) {
    width = w;
  }

  public float getDepth() {
    return depth;
  }

  public void setDepth(float d) {
    depth = d;
  }

  // Natives
  private static native void initPond();
  public native float getVolume();

  // Static startup code
  static {
    System.loadLibrary("Pond");
    initPond();
  }
}
-----------------------------------------------------------------

pondTest.java--------------------------------------------
public class PondTest {
  public static void main(String[] args) {
    // Make sure we have the right number of args
    if (args.length != 3) {
      System.out.println("Usage: java PondTest Length Width Depth");
      System.exit(0);
    }

    // Convert the args to floats
    float length = Float.valueOf(args[0]).floatValue();
    float width = Float.valueOf(args[1]).floatValue();
    float depth = Float.valueOf(args[2]).floatValue();

    // Create the Pond object and display the volume in gallons
    Pond pond = new Pond(length, width, depth);
    System.out.println("Pond length = " + pond.getLength() + " feet");
    System.out.println("Pond width  = " + pond.getWidth() + " feet");
    System.out.println("Pond depth  = " + pond.getDepth() + " feet");
    System.out.println("Pond volume = " + pond.getVolume() + " gallons");
  }
}
-----------------------------------------------------------

the c++ program must be a DLL

This example will explain step-by-step the creation of an application use C native code. This example uses Sun's JNI 1.1. It will also demonstrate C if faster than Java by doing 500 000 calls to strlen and to
java.lang.String.length().

     1.Create the Main.java file and compile it using javac Main.java

                    class Main
                    {
                        public static void main(String [] args)
                        {
                            CJava j=new CJava();
                            System.out.println(j.perfTest("Just a Test!"));
                        }
                    }

     2.Create the CJava.java file and compile it using javac CJava.java

                    class CJava

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

                        public int compute(String s) {
                        return s.length();
                        }

                        public native String perfTest(String s);
                    }

          This class contains the definition for the perfTest method. It will be called by the main method to display a String. This class also loads the CJava.dll (Win32), or the CJava.so (Unix) dynamic library.
          Note that the mapping from a library name to a specific filename is done in a system-specific manner.
          The compute() method is used to return the string length to the native code. It would be faster to call directly the java.lang.String.length() method from native code, but the example would become less
          clear.

     3.Create the .h file using javah -jni CJava

          This tool generate a header file with a .h extension contains the header for the perfTest C native method. You don't need to modify this file.

     4.Create the CJava.cpp file and compile it.

                    #include <jni.h>
                    #include <stdio.h>
                    #include <string.h>
                    #include "CJava.h"

                    JNIEXPORT jstring JNICALL Java_CJava_perfTest(JNIEnv *env, jobject obj, jstring s)
                    {
                    char *buf=new char[100];
                    long a,i,j;
                    printf("Entering native method.\n\n");

                    const char *str=env->GetStringUTFChars(s,0);
                    printf("The input string is: %s\n",str);

                    jclass cls = env->GetObjectClass(obj);
                    jmethodID mid = env->GetMethodID(cls, "compute", "(Ljava/lang/String;)I");


                    printf("Using C function to get the string length:\n");
                    for (a=1;a<500000;a++) j=strlen(str);
                    printf("C size: %d\n",j);


                    printf("Calling java method from native code to get the string length:\n");
                    for (a=1;a<500000;a++) i=env->CallIntMethod(obj, mid,s);
                    printf("Java size: %d\n",i);


                    printf("Creating the string returned by the function.\n");
                    sprintf(buf,"\nString %s :\nJava length=%d\nC length=%d\n",str,i,j);


                    env->ReleaseStringUTFChars(s,0);
                    printf("Leaving native code.\n");
                    return env->NewStringUTF(buf);
                    }

          To compile it on a win32 machine using Visual C++ , simply enter the command:

                    cl -I"C:\program files\javasoft\jdk1.1.4\include" -I"C:\program files\javasoft\jdk1.1.4\include\win32" /LD CJava.cpp

     5.Launch the main class using java Main

          You should see a big performance difference between the first loop (using strlen() C function) and the other one (using java.lang.String.length()). A large overhead for calling java methods inside native
          code make this example unuseful.
           

Another site for interesting info on native methods

              http://www.inside-java.com/articles/native/index.htm