Java Native Interface - setting and retrieving object fields

My reference: Java Native Interface

This table was specifically handy:

Java Type	JNI Type	machine dependent C/C++ typedef	Signature	Call...Method Get...Field
boolean	jboolean	unsigned char	Z	Boolean
byte	jbyte	signed char	B	Byte
char	jchar	unsigned short	C	Char
short	jshort	short	S	Short
int	jint	int	I	Int
long	jlong	long	J	Long
float	jfloat	float	F	Float
double	jdouble	double	D	Double
void	void		V	Void
nonprimitive	jobject	*...	L...;	Object

Examples

method definition	signature
int m1 ()	()I
double m2 (long l, char c)	(JC)D
void m3 (String s, int[] a)	(Ljava/lang/String;[I)V
String m4 (boolean b)	(Z)Ljava/lang/String;
Object m4 (BigDecimal b)	(Ljava/math/BigDecimal;)Ljava/lang/Object;

My code testing implementing this field access:

HelloWorldNative.c (builds into .so linked from Java)

JNIEXPORT void JNICALL Java_helloworld_Main_nativePrintNumber
(JNIEnv *env, jobject obj, jint ji)
{
  jfieldID fid;
  jint version;
  jclass c;
  c = (*env)->GetObjectClass(env, obj);
  fid = (*env)->GetFieldID(env, c, "version_number", "I");
  version = (*env)->GetIntField(env, obj, fid);
  (*env)->SetIntField(env, obj, fid, ji*ji);
  printf("::: %d, %d\n", fid, version);
  hello_number(ji);
}

My Main.java, which the version_number integer field:

package helloworld;

/**
*
* @author thomas
*/
public class Main {
   public int version_number;
   private native void nativePrint();
   private native void nativePrintNumber(int i);
   /** Creates a new instance of Main */
   public Main() {
       version_number = 5;
   }
  
   static {
       System.load("/home/thomas/testlib/HelloWorldNative/dist/HelloWorldNative.so");
   }
   /**
    * @param args the command line arguments
    */
   public static void main(String[] args) {
       Main me = new Main();
       me.nativePrint();
       me.nativePrintNumber(2);
       me.nativePrintNumber(5);
       me.nativePrintNumber(3);
       // TODO code application logic here
   }
}