Detecting a square object from an image using OpenCv in android

Question

Below is the code i used

import android.app.AlertDialog;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.media.Image;
import android.os.Bundle;
import android.app.Activity;
import android.util.Log;
import android.widget.ImageView;
import android.widget.Toast;

import org.opencv.android.BaseLoaderCallback;
import org.opencv.android.LoaderCallbackInterface;
import org.opencv.android.OpenCVLoader;
import org.opencv.android.Utils;
import org.opencv.core.Mat;
import org.opencv.imgcodecs.Imgcodecs;
import org.opencv.imgproc.Imgproc;

import java.io.InputStream;

public class MainActivity extends Activity {
    ImageView iv;
    protected BaseLoaderCallback OpenCVCallBack = new BaseLoaderCallback(this) {
        @Override
        public void onManagerConnected(int status) {
            switch (status) {
                case LoaderCallbackInterface.SUCCESS:
                {
                    System.out.println("tester");
                    onOpenCVReady();
                } break;
                default:
                {
                    super.onManagerConnected(status);
                } break;
            }
        }
    };

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);




}
private void OutputGrayMatToFile(Mat mGaryMat, String Filename)
{
    ImageView img = (ImageView) findViewById(R.id.imgBelow);
    Mat mRgba = new Mat();
    Imgproc.cvtColor(mGaryMat, mRgba, Imgproc.COLOR_GRAY2BGRA, 4);
    Bitmap bmp = Bitmap.createBitmap(mRgba.cols(), mRgba.rows(), Bitmap.Config.ARGB_8888);
    Utils.matToBitmap(mRgba, bmp);
    img.setImageBitmap(bmp);

}

@Override
protected void onResume() {
    super.onResume();
    OpenCVLoader.initAsync(OpenCVLoader.OPENCV_VERSION_3_0_0,this,OpenCVCallBack);
}

protected void onOpenCVReady(){
    //this should crash if opencv is not loaded
    Mat img = new Mat();
    Toast.makeText(getApplicationContext(), "opencv ready", Toast.LENGTH_LONG).show();
    Bitmap footbm = BitmapFactory.decodeResource(getResources(), R.drawable.img);
  //  Bitmap footbm = BitmapFactory.decodeStream(is);

    Mat footMat = new Mat();
    //convert bitmap to opencv Mat
    Utils.bitmapToMat(footbm, footMat);

    //Convert to Gray image
    Mat footGrayMat = new Mat();
    Imgproc.cvtColor(footMat, footGrayMat, Imgproc.COLOR_BGR2GRAY, 1);

    //Do canny
    Mat outCannyMat = new Mat();
    Imgproc.Canny(footGrayMat, outCannyMat, 80, 100, 3, false);

    //output to file
    OutputGrayMatToFile(outCannyMat, "Canny");
}

}

Question How can i get the square image from it I am not getting any idea as This is my first time using OpenCV and Image Processing.. So far i reached this point and quite frustrated with it or Anyone please tell me about any other better way with Object detection in an Image