How to access a directory programatically with java android code in dropbox

private DropboxAPI<AndroidAuthSession> mDBApi;//global variable
final static private String APP_KEY = "abc";
final static private String APP_SECRET = "xyz";

class forPutFile extends AsyncTask<String,String,Boolean>
{
    @Override
    protected Boolean doInBackground(String... params)
    {
        Log.d("mhk","indoInBackground");
        File file = new File(Environment.getExternalStorageDirectory()+"/hello.txt");
        FileInputStream inputStream = null;
        Log.d("mhk","file created..");
        try
        {
            Log.d("mhk","updating inputstream.");
            inputStream = new FileInputStream(file);
            Log.d("mhk","inputstream is :"+inputStream);
        }
        catch (FileNotFoundException e)
        {
            Log.e("mhk", "inputstreamException is : " + e.getMessage());
        }

        try
        {
            DropboxAPI.Entry response = new DropboxAPI.Entry();
            Log.d("mhk","Uploading file..."+response.root);
            response = mDBApi.putFile(response.root+"/hello.txt", inputStream,file.length(), null, null);
            Log.d("mhk","File Uploaded..."+response);
         }
        catch (DropboxException e)
        {
            Log.e("mhk", "dropboxException is :" + e.getMessage());
        }
        Log.d("mhk", "The uploaded file's rev is: ");//+ response.rev);
        return false; }}
void savingToDropbox(){   try   {

try
{
    AppKeyPair appKeys = new AppKeyPair(APP_KEY, APP_SECRET);
    AndroidAuthSession session = new AndroidAuthSession(appKeys);
    mDBApi = new DropboxAPI<AndroidAuthSession>(session);
    mDBApi.getSession().startOAuth2Authentication(MainActivity.this);

new forPutFile().execute("Hii");
            }
            catch (Exception e1)
            {
                Log.e("mhk", "DropboxException" + e1.getMessage());
            }
        }
        catch (Exception e)
        {
            Log.e("mhk", "FileNotFoundException" + e.getMessage());
        }
    }

    protected void onResume()
    {
        super.onResume();
        if (mDBApi.getSession().authenticationSuccessful())
        {
            try
            {
                // Required to complete auth, sets the access token on the session
                mDBApi.getSession().finishAuthentication();
                String accessToken = mDBApi.getSession().getOAuth2AccessToken();


            }
            catch (IllegalStateException e)
            {
                Log.e("mhk", "Error authenticating", e);
            }
        }
    }

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

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

11-13 08:27:49.385 18716-18736/? D/mhk﹕ indoInBackground

11-13 08:27:49.385 18716-18736/? D/mhk﹕ file created..

11-13 08:27:49.385 18716-18736/? D/mhk﹕ updating inputstream.
11-13 08:27:49.505 18716-18736/? D/mhk﹕ inputstream is :java.io.FileInputStream@419d4e98
11-13 08:27:49.555 18716-18736/? D/mhk﹕ Uploading file...null
11-13 08:27:49.555 18716-18736/? E/mhk﹕ dropboxException is :null
11-13 08:27:49.555 18716-18736/? D/mhk﹕ The uploaded file's rev is:

This is the full snippet. But it is giving exception ... Having all those permissions and everything in manifest file.