jni - convert int[][] to jobjectArray and return it to java

Question

I want to create a new array in c holding original array doubled. this is my c code, It compiles but the app crashes:

 #include <jni.h>
    JNIEXPORT jobjectArray JNICALL Java_com_example_jninew_MainActivity_getNum(JNIEnv *env, jobject obj, jintArray arr)
    {
      int i,j, sum = 0;
      jsize width = (*env)->GetArrayLength(env, arr);
      int array[2][2];
       for (i=0; i<width; i++){

            jintArray *line =   (*env)->GetObjectArrayElement(env, arr, i);
            int height =       (*env)->GetArrayLength(env, line);
            jint *pos = (*env)->GetIntArrayElements(env, line, 0);
            for (j=0; j<height; j++){
                   array[i][j] = 2*pos[j];
              }
            (*env)->ReleaseIntArrayElements(env, arr, pos, 0);
       }


         return array;
    }

Main java code:

package com.example.jninew;

import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.TextView;

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        TextView textView = (TextView) findViewById(R.id.tv);
        int[][] a = {{1,2},{3,55}};
        a = getNum(a);
        textView.setText("G"+a[0][1]);


    }
    static{
        System.loadLibrary("getNum");
    }
    native int[][] getNum(int[][] a);
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }


}

Can you help me to return the array to java side? Please I need your help!

Where does `a` come from? You are returning an automatic variable (`array`), which is no longer valid after the function returns. — Ctx, Jan 30 '16 at 16:28
Have a look here: http://stackoverflow.com/questions/1610045/how-to-return-an-array-from-jni-to-java This should help you on how to return a proper java int array from jni — Ctx, Jan 30 '16 at 16:38
@AndrewHenle I want to return a 2D array I changed it to jobjectArray. — yanisB, Jan 30 '16 at 16:53

milevyo · Accepted Answer · 2016-01-31T21:06:00.183

According to manuals that i read, here is where i land so far.

#include <jni.h>
#include<stddef.h>

JNIEXPORT jobjectArray JNICALL Java_com_example_jninew_MainActivity_getNum(JNIEnv *env, jobject obj, jobjectArray arr)
{
    int i,j, sum = 0;

    jclass intClass =         (*env)->FindClass(env,"[I");//
    jsize width =             (*env)->GetArrayLength(env, arr);
    jobjectArray jObjarray =  (*env)->NewObjectArray(env,width,intClass, NULL);    //**//


    for (i=0; i<width; i++){

        jintArray *line =   (*env)->GetObjectArrayElement(env, arr, i);
        int height =        (*env)->GetArrayLength(env, line);
        jintArray jline =   (*env)->NewIntArray(env,height);             //**//
        jint *pos =         (*env)->GetIntArrayElements(env, line, 0);
        jint jpos[ height ];

        for (j=0; j<height; j++){
               jpos[j] = 2*pos[j];
        }

        (*env)->SetIntArrayRegion(env, jline, 0, height, jpos);    //**//
        (*env)->ReleaseIntArrayElements(env, line, pos, 0);
        (*env)->ReleaseIntArrayElements(env, jline, jpos, 0);     //**//

        (*env)->SetObjectArrayElement(env,jObjarray,i,jline);
        (*env)->DeleteLocalRef (env,jline); //**//
   }


     return jObjarray;
}

tried it but I get this error in logcat:` Caused by: java.lang.ArrayStoreException: int[] cannot be stored in an array of type java.lang.Integer[]` Any help? — yanisB, Jan 31 '16 at 16:56
thanks a lot @milevyo, it's working. PS: I edited your answer to my working version(I added #include to recognize NULL....). thanks — yanisB, Jan 31 '16 at 17:07

jni - convert int[][] to jobjectArray and return it to java

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