How can I count the number of verticies on a shape in an image using the Imgproc.approxPolyDP() method in Java?

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I have put an image through the Imgproc.approxPolyDP() method in OpenCV Java as to make it binary. Now, I want to only find the heptagons in that image by finding all of the contours, and only counting those that have seven verticies. How can I use the Imgproc.approxPolyDP() method to do this?

Your help is greatly appreciated.