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you have to train the SVM on hog features, not images

so, processing the train data should look like:

HOGDescriptor hog = new HOGDescriptor(new Size(160, 96), new Size(8,8), new Size(4,4), new Size(4,4), 9);
for (each image) {
        Mat img = Imgcodecs.imread(file.getPath());
        Mat newImg = new Mat();
        Imgproc.cvtColor(img, newImg, Imgproc.COLOR_BGR2GRAY);
        Imgproc.resize(newImg, newImg, new Size(160 , 96));
        Mat descriptors = new Mat();
        hog.compute(newImg, descriptors);
        // now train your svm with the descriptors, not the images !
}