ImageFetcherActivityHandler.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 static final int MSG_SHOW_PROGRESS = 1;
    private static final int MSG_SHOW_IMAGE = 2;

    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() {
            handler.obtainMessage(MSG_SHOW_PROGRESS).sendToTarget();
            // download image -- code removed
            final Bitmap bitmap = BitmapFactory.decodeStream(inputstream);
            handler.obtainMessage(MSG_SHOW_IMAGE, bitmap).sendToTarget();
        }
    }

    class UIHandler extends Handler {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case MSG_SHOW_PROGRESS: {
                    progressIndicator.setVisibility(View.VISIBLE);
                    break;
                }
                case MSG_SHOW_IMAGE: {
                    progressIndicator.setVisibility(View.GONE);
                    imageView.setImageBitmap((Bitmap) msg.obj);
                    break;
                }
            }
        }
    }

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