java.lang.UnsatisfiedLinkError: No implementation found for long org.opencv.core.Mat.n_Mat(int, int, int)

Question

I have been following some tutorials on how to uses opencv and ndk in android studio. I have executed some youtube channel tutorial.Badly, I have been stuck on this problem for a while now.
this is the detailed error :

    Process: medianet.com.helloopencv, PID: 17079
                                               java.lang.UnsatisfiedLinkError: No implementation found for long org.opencv.core.Mat.n_Mat(int, int, int) (tried Java_org_opencv_core_Mat_n_1Mat and Java_org_opencv_core_Mat_n_1Mat__III)
          at org.opencv.core.Mat.n_Mat(Native Method)
       at org.opencv.core.Mat.<init>(Mat.java:37)
        at medianet.com.helloopencv.MainActivity.onCameraViewStarted(MainActivity.java:95)

and this is my cpp file , which is supposed to detect Human face. It s a simple code from opencv.org:

   #include "medianet_com_helloopencv_OpencvClass.h"
JNIEXPORT void JNICALL             Java_medianet_com_helloopencv_OpencvClass_faceDetection
  (JNIEnv *, jclass, jlong addRgba){
  Mat& frame= *(Mat*) addRgba;

  detect(frame);
  }

  void detect(Mat& frame){

  String face_cascade_name =                 "/storage/emulated/0/data/haarcascade_frontalface_alt.xml";
  String eyes_cascade_name = "/storage/emulated/0/data/haarcascade_eye_tree_eyeglasses.xml";
  CascadeClassifier face_cascade;
   CascadeClassifier eyes_cascade;
   if( !face_cascade.load( face_cascade_name ) ){ printf("--(!)Error         loading\n"); return ; };
   if( !eyes_cascade.load( eyes_cascade_name ) ){ printf("--(!)Error         loading\n"); return ; };

 std::vector<Rect> faces;
  Mat frame_gray;

  cvtColor( frame, frame_gray, CV_BGR2GRAY );
  equalizeHist( frame_gray, frame_gray );

  //-- Detect faces
  face_cascade.detectMultiScale( frame_gray, faces, 1.1, 2, 0|CV_HAAR_SCALE_IMAGE, Size(30, 30) );

  for( size_t i = 0; i < faces.size(); i++ )
  {
    Point center( faces[i].x + faces[i].width*0.5, faces[i].y +     faces[i].height*0.5 );
    ellipse( frame, center, Size( faces[i].width*0.5, faces[i].height*0.5), 0, 0, 360, Scalar( 255, 0, 255 ), 4, 8, 0 );

        Mat faceROI = frame_gray( faces[i] );
    std::vector<Rect> eyes;

    //-- In each face, detect eyes
    eyes_cascade.detectMultiScale( faceROI, eyes, 1.1, 2, 0         |CV_HAAR_SCALE_IMAGE, Size(30, 30) );

for( size_t j = 0; j < eyes.size(); j++ )
 {
   Point center( faces[i].x + eyes[j].x + eyes[j].width*0.5, faces[i].y + eyes[j].y + eyes[j].height*0.5 );
   int radius = cvRound( (eyes[j].width + eyes[j].height)*0.25 );
   circle( frame, center, radius, Scalar( 255, 0, 0 ), 4, 8, 0 );
     }
  }

}

And her is my java Class. This class is my main activity, and it will be launching the camera and call the methode implemented in the native script which will detect the face

    public class MainActivity extends AppCompatActivity implements
    CameraBridgeViewBase.CvCameraViewListener2 {
private static final String TAG = "MainActivity";
JavaCameraView javaCameraView;
Mat mRgba/*, imggray, imgCanny*/;
BaseLoaderCallback mloderCallback = new BaseLoaderCallback(this) {
    @Override
    public void onManagerConnected(int status) {
        switch (status) {
            case BaseLoaderCallback.SUCCESS: {
                javaCameraView.enableView();
                break;
            }
            default: {
                super.onManagerConnected(status);
                break;
            }
        }
    }
};


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    javaCameraView =  findViewById(R.id.cameraview);

    initOpencv();

    /* set for front camera
    * if index=0 then it s back camera
    * else it s front camera
     */
    javaCameraView.setCameraIndex(1);


    javaCameraView.setVisibility(SurfaceView.VISIBLE);
    javaCameraView.setCvCameraViewListener(this);


}

@Override
protected void onPause() {
    super.onPause();
    if (javaCameraView != null) {
        javaCameraView.disableView();
    }
}


@Override
protected void onDestroy() {
    super.onDestroy();
    if (javaCameraView != null) {
        javaCameraView.disableView();
    }
}

@Override
protected void onResume() {
    super.onResume();
        initOpencv();
}

@Override
public void onCameraViewStarted(int width, int height) {
    mRgba = new Mat(height, width, CvType.CV_8UC4);
  /*  imggray=new Mat(height, width, CvType.CV_8UC1);
    imgCanny=new Mat(height, width, CvType.CV_8UC1);
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN);
    getWindow().addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);*/

}

@Override
public void onCameraViewStopped() {
    mRgba.release();

}
Display display;

@Override
public Mat onCameraFrame(CameraBridgeViewBase.CvCameraViewFrame inputFrame) {
    mRgba = inputFrame.rgba();
    OpencvClass.faceDetection(mRgba.getNativeObjAddr());


    // return dummy value for compiler's sake
    return mRgba;
}



// initialisation opencv called in on activity start et on resume
private void initOpencv(){


    if (!OpenCVLoader.initDebug()) {
        Log.d(TAG, "fail firas");
        mloderCallback.onManagerConnected(LoaderCallbackInterface.SUCCESS);


    } else {
        Log.d(TAG, "sucess firas");
        OpenCVLoader.initAsync(OpenCVLoader.OPENCV_VERSION_3_0_0, this, mloderCallback);


    }

}
}