Confused about passing arrays

Question

Hi,
I'm stuck, I can't find any answers in the JNI tutorial, or in Sheng Liangs book.
I basically want to pass from Java to a C function a null 2D array of doubles, the C function to create a 2D array of doubles, which it then assigns to the passed array. Sheng Liangs book has an example where a 2D array is returned as a return value, but this doesn't help me since I have functions with more that one return value, so I have to use pass by reference.
Is this even possible with JNI?
Here my Java method signature:
public native void createCont(int[] noges,
                                                     double[][] probEx,
                                                     double[][] probEx2,
                                                     double root);I can't assigne probEx or probEx2 to anything other than null before I make this call and I have no idea what size these arrays will be.
Any suggestions here would be greatly appreciated.

Answer 1

I have done something similar, only with a 1D array. You should be able to treat the 2D array as if it were a 1D array on the C end, as long as the product of the two dimension lengths is equal to the length of the 1D array.
I basically passed in a byte[] with a known size, and filled it in C.
private native int method(byte[] b);
JNIEXPORT jint JNICALL Java_package_class_method
(JNIEnv* env, jobject canvas, jbyteArray array)
   //do stuff
   jbyte *data = env->GetByteArrayElements(array,0);
   // treat data as an array and populate it
   // you could also do a memcpy if the values you need are already stored.
   // data[x][y] should work here.
   env->ReleaseByteArrayElements(array,data,0);
   return ret; //some number
}You should be able to pass a byte[ i ] or a byte[ x ][ y ] in this case, both should be handled the same as long as i == x*y. Correct me if I'm wrong, I haven't actually tried it, but I believe it will work.
You cannot pass in null and construct it in C.