Object recognition knnmatch

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Hello... I extracted features from a train image and query image using

FeatureDetector detector = FeatureDetector.create(FeatureDetector.ORB);
detector.detect(pic, keypoints1);
DescriptorExtractor extractor = DescriptorExtractor.create(DescriptorExtractor.ORB);
extractor.compute(pic, keypoints1, featuresofqimg);

I did this for both images...i wanna match these features and i used

       DescriptorMatcher matches =DescriptorMatcher.create(DescriptorMatcher.FLANNBASED);
       matches.knnMatch(featuresofqimg, featuresoftrainimg,  matc, 2);

but the app stops unexpectedly... pls help me over here...

I initialized matc as follows

private List<MatofDMatch>             matc1;

and i don't know how to initialise it..

Also a 2-d variable is required for match...Mat is already 2d ,I wonder why List<matofdmatch> is needed..Its the syntax of the arguments in knnmatch();

pls help me over here.