how to convert double in C++ to Java

Question

I have a question about data-types, I want to have it clear before I start coding! Let's say I have a method in C++ that returns me an array, or a buffer of doubles. Then I need to convert this double[] into jdouble, using JNI header, so Java can process the data.

My first question is should I configure my C++ method to return an array of doubles, or a buffer of doubles. I know they are almost the same, but don't know which one is better. Also, how do I convert a buffer in a way Java can understand?

This is what I have so far:

JNIEXPORT jdouble[] JNICALL Java_Test_getDouble[](JNIEnv *env, jobject obj) 
{
      double[] temp= new double[someSize];
      temp = // call my C++ code

      jdouble result = new jdouble[someSize];
      for(i = 0; i < someSize; i++)
           converting double in temp to jdouble in result;

       return result;     
}

and then Java can do whatever it wants with the data.

Does is my code make sense? I read some post about ByteGetElement() but I do not know if it can be applied here. If you can provide me a simple example that would be really helpful. And ultimately, I still need to know whether I should go with an array or buffer of doubles.

Thank you very much.

This may help, http://stackoverflow.com/questions/1610045/how-to-return-an-array-from-jni-to-java It's for an integer array, but I guess the method for doubles is similar. — john, Oct 23 '12 at 21:54

score 2 · Answer 1 · answered Jan 09 '14 at 16:31

A jdouble is actually the same as a double as far as C is concerned.

// just for the sake of the example  this buffer exists and holds the double values in C
double* sourceBuffer;

// allocate the jdouble Array
jdoubleArray output = (*env)->NewdoubleArray(env, someSize);

// check if it was allocated correctly
if (NULL == output) return NULL;

// commit the contents of the double* array (sourceBuffer in this example) into the Java array 
(*env)->SetDoubleArrayRegion(env, result, 0, someSize, sourceBuffer);
return result;

how to convert double in C++ to Java

1 Answers1