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

package com.example.mhk.dropbox;

import android.content.Context;
import android.content.SharedPreferences;
import android.os.AsyncTask;
import android.os.Environment;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.EditText;

import com.dropbox.client2.DropboxAPI;
import com.dropbox.client2.android.AndroidAuthSession;
import com.dropbox.client2.exception.DropboxException;
import com.dropbox.client2.session.AppKeyPair;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileWriter;
import java.io.IOException;

public class MainActivity extends AppCompatActivity
{
    private DropboxAPI<AndroidAuthSession> mDBApi;//global variable
    final static private String APP_KEY = "jkj";
    final static private String APP_SECRET = "xyx";

    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;
            try
            {
                inputStream = new FileInputStream(file);
                Log.d("mhk","file created..");
                Log.d("mhk","inputstream is :"+inputStream);
            }
            catch (FileNotFoundException e)
            {
                Log.e("mhk", "inputstreamException is : " + e.getMessage());
            }
            try
            {
                DropboxAPI.Entry response = mDBApi.putFile("/hello.txt", inputStream,file.length(), null, null);
                Log.d("mhk","The uploaded file's rev is: " + response.rev);
             }
            catch (DropboxException e)
            {
                e.printStackTrace();
                Log.e("mhk", "dropboxException is :" + e.getMessage());
            }
            return false;
        }
    }

    void savingToDropbox()
    {

       try
            {
                AppKeyPair appKeys = new AppKeyPair(APP_KEY, APP_SECRET);
                AndroidAuthSession session = new AndroidAuthSession(appKeys);
                mDBApi = new DropboxAPI<AndroidAuthSession>(session);
                mDBApi.getSession().startOAuth2Authentication(MainActivity.this);
            }
            catch (Exception e1)
            {
                Log.e("mhk", "Exception" + e1.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();
                SharedPreferences sp=getSharedPreferences("DropBox App", MODE_PRIVATE);
                SharedPreferences.Editor ed=sp.edit();
                ed.putString("accessToken",accessToken);
            }
            catch (IllegalStateException e)
            {
                Log.e("mhk", "Error authenticating", e);
            }
        }
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        savingToDropbox();
        new forPutFile().execute("Hii");
    }

    @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);
    }

This is the full code snippet i am using. Gregory had done by giving simple path also.Getting the same error giving under:

11-16 14:49:57.977 22637-22675/? D/mhk﹕ indoInBackground
11-16 14:49:57.987 22637-22675/? D/mhk﹕ file created..
11-16 14:49:57.987 22637-22675/? D/mhk﹕ inputstream is :java.io.FileInputStream@41aba9f0
11-16 14:49:57.997 22637-22675/? E/mhk﹕ dropboxException is :null (This is the exception...)

Please provide a solution .I am studying Core API for the first time.And it had really irritated me.

}