尝试在空对象引用上调用虚拟方法'android.content.res.Resources $ Theme android.content.Context.getTheme（）'

Question

I want to run some OpenCv Filters over images inside of an Android App. I've hit an Error, of which I cant seem to find the origin.

Stacktrace:

java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources$Theme android.content.Context.getTheme()' on a null object reference
                                                                          at android.app.AlertDialog.resolveDialogTheme(AlertDialog.java:222)
                                                                          at android.app.AlertDialog$Builder.<init>(AlertDialog.java:452)
                                                                          at org.opencv.android.BaseLoaderCallback.onPackageInstall(BaseLoaderCallback.java:90)
                                                                          at org.opencv.android.AsyncServiceHelper.InstallService(AsyncServiceHelper.java:117)
                                                                          at org.opencv.android.AsyncServiceHelper.initOpenCV(AsyncServiceHelper.java:33)
                                                                          at org.opencv.android.OpenCVLoader.initAsync(OpenCVLoader.java:100)
                                                                          at project.fragment.OpenCvTestFragment.onCreateView(OpenCvTestFragment.java:94)
                                                                          at android.support.v4.app.Fragment.performCreateView(Fragment.java:1974)
                                                                          at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1067)
                                                                          at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1252)
                                                                          at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:742)
                                                                          at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1617)
                                                                          at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:517)
                                                                          at android.os.Handler.handleCallback(Handler.java:739)
                                                                          at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                          at android.os.Looper.loop(Looper.java:148)
                                                                          at android.app.ActivityThread.main(ActivityThread.java:5417)
                                                                          at java.lang.reflect.Method.invoke(Native Method)
                                                                          at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                                          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

My Code: ( OpenCvTestFragment.java )

import android.content.Context;
import android.graphics.Bitmap;
import android.net.Uri;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.ImageView;

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

import java.io.IOException;

import team_excellence.growup.R;

public class OpenCvTestFragment extends Fragment implements CameraBridgeViewBase.CvCameraViewListener2, View.OnTouchListener {

    private OnOpenCvTestInteractionListener mListener;
    private View view;
    private ImageView imgView1;
    private ImageView imgView2;

    public OpenCvTestFragment() {

    }

    public static OpenCvTestFragment newInstance(String param1, String param2) {
        OpenCvTestFragment fragment = new OpenCvTestFragment();
        Bundle args = new Bundle();
        fragment.setArguments(args);
        return fragment;
    }

    private static final String TAG = "OCVSample::Activity";

    private BaseLoaderCallback mOpenCVCallBack = new BaseLoaderCallback(getActivity()) {
        @Override
        public void onManagerConnected(int status) {
            switch (status) {
                case LoaderCallbackInterface.SUCCESS: {
                    Log.i(TAG, "OpenCV loaded successfully");
                }
                break;
                default: {
                    super.onManagerConnected(status);
                }
                break;
            }
        }
    };

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        if (container == null) {
            return null;
        }
        view = inflater.inflate(R.layout.fragment_open_cv_test, container, false);

        final ImageView imgView1 = (ImageView) view.findViewById(R.id.ocvImage1);
        imgView1.setImageResource(R.drawable.img1);

        ImageView imgView2 = (ImageView) view.findViewById(R.id.ocvImage2);
        imgView1.setImageResource(R.drawable.img2);

        final Button contoursButton = (Button) view.findViewById(R.id.contoursButton);
        contoursButton.setOnClickListener(new View.OnClickListener(){
            public void onClick(View v) {

                try {
                    applyContoursFilter("img1");
                } catch (IOException e) {
                    e.printStackTrace();
                }

            }
        });
        OpenCVLoader.initAsync(OpenCVLoader.OPENCV_VERSION_2_4_8, getContext(),
                mOpenCVCallBack);
        return view;
    }

    public void onButtonPressed(Uri uri) {
        if (mListener != null) {
            mListener.onOpenCvTestInteraction(uri);
        }
    }

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        if (context instanceof OnOpenCvTestInteractionListener) {
            mListener = (OnOpenCvTestInteractionListener) context;
        } else {
            throw new RuntimeException(context.toString()
                    + " must implement OnOpenCvTestInteractionListener");
        }
    }

    @Override
    public void onDetach() {
        super.onDetach();
        mListener = null;
    }

    @Override
    public void onCameraViewStarted(int width, int height) {

    }

    @Override
    public void onCameraViewStopped() {

    }

    @Override
    public Mat onCameraFrame(CameraBridgeViewBase.CvCameraViewFrame inputFrame) {
        return null;
    }

    @Override
    public boolean onTouch(View v, MotionEvent event) {
        return false;
    }

    public interface OnOpenCvTestInteractionListener {
        void onOpenCvTestInteraction(Uri uri);
    }

    public void applyContoursFilter(String resName) throws IOException {

        Mat matImg = convertImageToMat(resName);
        Mat matResult = findContours(matImg);
        Bitmap bm = Bitmap.createBitmap(matResult.cols(), matResult.rows(), Bitmap.Config.ARGB_8888);
        Utils.matToBitmap(matResult, bm);
        imgView1.setImageBitmap(bm);

    }

    private Mat convertImageToMat(String resName) throws IOException {
        int drawableResourceId = this.getResources().getIdentifier(resName, "drawable", getActivity().getPackageName());
        Mat img = Utils.loadResource(getContext() , drawableResourceId , Imgcodecs.CV_LOAD_IMAGE_COLOR);

        return img;
    }


    private Mat findContours (Mat image){

        Mat kernel = Imgproc.getStructuringElement(Imgproc.MORPH_ELLIPSE, new Size(19,19));
        Mat closed = new Mat(); // closed will have type CV_32F
        Imgproc.morphologyEx(image, closed, Imgproc.MORPH_CLOSE, kernel);
        Core.divide(image, closed, closed, 1, CvType.CV_32F);
        Core.normalize(closed, image, 0, 255, Core.NORM_MINMAX, CvType.CV_8U);
        Imgproc.threshold(image, image, -1, 255, Imgproc.THRESH_BINARY_INV
                + Imgproc.THRESH_OTSU);

        return closed;

    }



}

For clarification when the corresponding Button is pressed, the "applyContoursFilter" Method is called, which then converts the image from drawables ( shown in imgView1 ), into a Mat object. After that the "findContours" Method is called and the Mat-object then converted back into bmp format and set into the imgView1 again.

Well that's how it should work. But the nullpointer error posted above gets thrown and I cant figure out where its coming from or how to fix it.

I'm relatively new to Android and would be grateful for your Help.

Answer 1

I got it solved now, by initializing OpenCV in the MainActivity of my App and only calling it in the Fragments. I moved the BaseLoaderCallBack Code from the Fragment's OnCreateView , to the MainActivity's OnCreate. Seems to be working fine now.

Answer 2

Before I used GPSTracker.this in AlertDialog.So I got an exception 'android.content.res.Resources$Theme android.content.Context.getTheme()'

I solved this exception by adding context like this:

private final Context mContext;

public GPSTracker(Context context) {

    this.mContext = context;


}

public void showSettingsAlert(String provider) {
    AlertDialog.Builder alertDialog = new AlertDialog.Builder(mContext);

 ...........

 }