Converting C++ double* to a Java double

Question

I am struggling with a basic problem. I am trying to call a .dll-function from Java with the following signature:

int function(double* a, double b);

My problem is that I don't know how to use the double* a parameter in Java and how to print the manipulated value after the function call is done.

Answer 1

I think for the simplest case of just getting an output back use a double [] with length of 1.

This was my java file (usefunc.java) :

public class usefunc {

    static {
        System.loadLibrary("usefunc");
    }
    public native int function(double a[], double b);

    public static void main(String args[]) {
        double a[] = new double[1];
        new usefunc().function(a,1.0);
        System.out.println("a[0] = " + a[0]);
    }
}

This was my C++ file (usefunc.cpp) :

#include <jni.h>

extern "C" {
    JNIEXPORT jint JNICALL Java_usefunc_function
    (JNIEnv *env, jobject obj, jdoubleArray a, jdouble b);
    int function(double *a, double b);
}

int function(double* a, double b) {
    *a = 77.7;
    return 0;
}

/*
 * Class:     usefunc
 * Method:    function
 * Signature: ([DD)I
 */
JNIEXPORT jint JNICALL Java_usefunc_function
(JNIEnv *env, jobject obj, jdoubleArray a, jdouble b) {
    jboolean iscopy;
    double *nativeA = env->GetDoubleArrayElements(a, &iscopy);
    jint ret = function(nativeA, b);
    env->ReleaseDoubleArrayElements(a, nativeA, 0);
    return ret;
}

Compiled with:

 g++ -I /usr/local/jdk1.8.0_60/include/ -I /usr/local/jdk1.8.0_60/include/linux/ -shared usefunc.cpp  -o libusefunc.so
 javac usefunc.java

Ran with:

 java usefunc

produces:

 a[0] = 77.7