How To Use Java Primitive Datatypes In C?

Previous Article:- Steps To Call A C Program From Java?

Datatypes in Java and in C are not directly related. When you call C programs from Java, you need to pass arguments and accept the returned results or create Java objects in the C program. This requires you to handle primitive datatypes, arrays, and strings coded in C.

Primitive Datatypes
To establish communication between the Java and C datatypes, you must define native equivalents to the Java primitive datatypes, which are then used in the C code as representatives of the Java primitive datatypes.

Java Datatype	Native Language Equivalent
Void	void
Byte	jbyte
Boolean	jboolean
Char	jchar
Short	jshort
Int	jint
Long	jlong
Float	jfloat
Double	jdouble

Following code shows how to access an integer variable declared in a Java program in C and modify it:

Code:

class JMapC 
{
	static int send;
	//Declaring the native method
	private native void intMapjint();
	public static void main(String args[]) 
	{
    	JMapC map = new JMapC();
	    JMapC.send = 100;
	    System.out.println("In Java:");
	    System.out.println(" JMapC.send = " + JMapC.send);
		//Invoking the native method
		map.intMapjint();
	    System.out.println("Back in Java:");
	    System.out.println(" JMapC.send = " + JMapC.send);
 	}
	static 
	{
		//loading library
		System.loadLibrary("Demo_lib");
  	}
}

Above code invokes a native method that accesses an integer variable defined in the code. The implementation of the native method is provided by the Demo_lib library.
The following code shows how to generate the header file for the JMapC file

Code:

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class JMapC */
#ifndef _Included_JMapC
#define _Included_JMapC
#ifdef __cplusplus
extern "C" 
{
	#endif
	/* Inaccessible static: send */
	/*
	 * Class:     JMapC
	 * Method:    intMapjint
	 * Signature: ()V
	 */
	JNIEXPORT void JNICALL Java_JMapC_intMapjint (JNIEnv *, jobject);
	#ifdef __cplusplus
}
#endif
#endif

The code shows how the header file is generated for the JMapC class file with the help of the javah tool.

The following code shows how to generate the header file for the JMapC file:

Code:

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class JMapC */
#ifndef _Included_JMapC
#define _Included_JMapC
#ifdef __cplusplus
extern "C" 
{
	#endif
	/* Inaccessible static: send */
	/*
	 * Class:     JMapC
	 * Method:    intMapjint
	 * Signature: ()V
	 */
	JNIEXPORT void JNICALL Java_JMapC_intMapjint (JNIEnv *, jobject);
	#ifdef __cplusplus
}
#endif
#endif

This code shows how the header file is generated for the JMapC class file with the help of the javah tool.

The following code shows how to implement the intMapJint method:

Code:

#include <stdio.h>
#include <jni.h>
#include "JMapC.h"
JNIEXPORT void JNICALL 
Java_JMapC_intMapjint(JNIEnv *exeenv, jobject javaobj)
{
	jclass cls = (*exeenv)->GetObjectClass(exeenv, jaavobj);
	jfieldID fid;
	jint num;
	printf("\nAccessing send in C:\n");
	fid = (*exeenv)->GetStaticFieldID(exeenv, cls, "send", "I");
	if (fid == 0) 
	{
    	return;
  	}
	num = (*exeenv)->GetStaticIntField(exeenv, cls, fid);
	printf("JMapC.send = %d\n\n", num);
	printf("Modifying the value send\n\n");
	(*exeenv)->SetStaticIntField(exeenv, cls, fid, num*9);
}

This code accesses an integer variable from the Java code, modifies it, and displays the modified value. The value of the variable is modified using the SetStaticIntField method.

Next Article:- How To Use Java Arrays In C?