public interface DownloadCallback<T> {
    interface Progress {
        int ERROR = -1;
        int CONNECT_SUCCESS = 0;
        int GET_INPUT_STREAM_SUCCESS = 1;
        int PROCESS_INPUT_STREAM_IN_PROGRESS = 2;
        int PROCESS_INPUT_STREAM_SUCCESS = 3;
    }

    /**
     * Call from main thread to update the UI
     * @param result
     */
    void updateFromDownload(T result);

    /**
     * Get the device's active network status as a NetworkInfo object
     * @return
     */
    NetworkInfo getActiveNetworkInfo();

    /**
     * Show progress update
     * @param progressCode one of the constants defined above
     * @param percentComplete 0-100
     */
    void onProgressUpdate(int progressCode, int percentComplete);

    /**
     * Called when process has finished regardless of success of download.
     */
    void finishDownloading();
}

public class DummyActivity extends AppCompatActivity implements DownloadCallback {
    private NetworkFragment networkFragment;
    private boolean downloading = false;
    private TextView mTextView2;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_dummy);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
        mTextView2 = findViewById(R.id.textView2);
        networkFragment = NetworkFragment.getInstance(getSupportFragmentManager(), getString(R.string.base_url));
    }

    @Override
    private void updateFromDownload(String result) {
        mTextView2.setText(result);
    }

    private void startDownload() {
        if (!downloading && networkFragment != null) {
            networkFragment.startDownload();
            downloading = true;
        }
    }
}