ImageFetcherActivityRunnable.java

public class MainActivity extends AppCompatActivity {
    private static final String IMAGE_URL = "https://www.android.com/static/2016/img/logo-android-green_2x.png";

    private ProgressBar progressIndicator;
    private ImageView imageView;
    private Handler handler;

    class ImageFetcher implements Runnable {
        final String url;

        ImageFetcher(String url) {
            this.url = url;
        }

        @Override
        public void run() {
            // download image -- code removed
            final Bitmap bitmap = BitmapFactory.decodeStream(inputstream);
            handler.post(new Runnable() {
                @Override
                public void run() {
                    imageView.setImageBitmap(bitmap);
                }
            });
        }
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        final Thread workerThread = new Thread(new ImageFetcher(IMAGE_URL));
        workerThread.start();
    }
}

Missing handler initialization on main Thread. i.e handler = new Handler()