Why is my convexity defect OpenCV in Android getting error?

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Java

Mobile

Android

OpenCV

I'm beginner in android development and newbie in OpenCV too. I have project about hand gesture recognition in Android. I want to do convexity defect on it. Before I do convexity defect, I should have done finding contour and convex hull. I have done them already. Now, I'm doing convexity defect to get defect of the hand and fingers. But there is something wrong. When I build my code with convexity defect in my Android, It's unfortunately stopped. I don't know whether my convexity defect function is wrong, or the way I get value of defect (MatOfInt4) to Point is wrong. Somebody, please help me. Thank you. This is my code :

List<MatOfInt> hull = new ArrayList<MatOfInt>();
List<MatOfInt4> defect = new ArrayList<MatOfInt4>();
for(int i = 0; i < newContours.size(); i++)
     {
         hull.add(new MatOfInt());
         Imgproc.convexHull(newContours.get(i), hull.get(i));
         Imgproc.convexityDefects(newContours.get(i), hull.get(i), defect.get(i));
     }

     Point defectSP= new Point();
     Point defectEP= new Point();
     Point defectFP= new Point();
     for(int i = 0; i < defect.size(); i++)
     {
         Point[] startP = new Point[newContours.get(i).rows()];
         Point[] endP = new Point[newContours.get(i).rows()];
         Point[] farP = new Point[newContours.get(i).rows()];
         Point[] depthP = new Point[newContours.get(i).rows()];

         for(int j = 0; j < defect.get(i).rows(); j++)
         {
             int distP = (int) defect.get(i).get(j, 3)[3];
             if (distP > 20*256)
             {
                 startId = (int) defect.get(i).get(j, 0)[0];
                 endId = (int) defect.get(i).get(j, 1)[1];
                 farId = (int) defect.get(i).get(j, 2)[2];

                 defectSP.x = startId;
                 defectSP.y = startId;
                 defectEP.x = endId;
                 defectEP.y = endId;
                 defectFP.x = farId;
                 defectFP.y = farId;
             }
          }
      }