maintaining a Back stack for fragments as well as confirm on exit

I am actually working on fragments instead of activities .So on navigation drawer item click different fragments appear on the screen.Based on my code, if i am moving from Activity A to fragment b,then fragment c,again to b,then if i will click back button,then it will navigate me to the MainActivity A.But here is the problem.when i want to add confirm on exit dialog box on the MainActivity onBackPress(),it appears every time i click the back button regardless of which page is open,But i want it to appear only when there is no fragments left in the back stack.I need some help on that issue.

    Here is my MainActivity code:--

    package archerpenny.impdrawerfragment;


import android.content.DialogInterface;
import android.support.v4.app.Fragment;

import android.support.v4.app.FragmentManager;
import android.support.v4.widget.DrawerLayout;

import android.os.Bundle;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.support.v7.widget.Toolbar;

import android.view.MenuItem;

import android.view.View;


public class MainActivity extends AppCompatActivity  {
    ActionBarDrawerToggle mDrawerToggle;
    RecyclerView.Adapter mAdapter;
    RecyclerView recyclerView;
    DrawerLayout mDrawerLayout;
    FragmentManager fragmentManager;
    Fragment blankFragment=null;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mAdapter = new NavigationDrawerAdapter(this);
        mDrawerLayout=(DrawerLayout)findViewById(R.id.drawer_layout);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        NavigationDrawerAdapter adapter;

        recyclerView = (RecyclerView)findViewById(R.id.DrawerList);
        recyclerView.setHasFixedSize(true);
        LinearLayoutManager llm = new LinearLayoutManager(this);
        recyclerView.setLayoutManager(llm);
        recyclerView.setAdapter(mAdapter);

        getSupportActionBar().setDisplayShowTitleEnabled(false);

        mDrawerToggle = new ActionBarDrawerToggle(this,mDrawerLayout,toolbar,R.string.open,R.string.close) {
            @Override
            public void onDrawerOpened(View drawerView) {
                super.onDrawerOpened(drawerView);
            }

            @Override
            public void onDrawerClosed(View drawerView) {
                super.onDrawerClosed(drawerView);
            }
        };

        mDrawerLayout.setDrawerListener(mDrawerToggle);
        getSupportActionBar().setHomeButtonEnabled(true);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        recyclerView.addOnItemTouchListener(
                new RecyclerItemClickListener(MainActivity.this, new RecyclerItemClickListener.OnItemClickListener() {
                    @Override
                    public void onItemClick(View view, int position) {
                        // do whatever

                        if(position==0)
                        {
                            blankFragment=new BlankFragment();

                        }
                        if (position==1)
                        {
                            blankFragment=new BlankFragment2();
                        }
                        if (position==2)
                        {
                            blankFragment=new BlankFragment3();
                        }
                        android.support.v4.app.FragmentManager fragmentManager = getSupportFragmentManager();
                        getSupportFragmentManager().popBackStack();
                        fragmentManager.beginTransaction()
                                .add(R.id.container_body, blankFragment).addToBackStack("fragBack").commit();
                        mDrawerLayout.closeDrawers();
                    }
                })
        );
    }
    @Override
    public void onBackPressed() {
        if(getSupportFragmentManager().findFragmentByTag("fragBack") != null)
        {
            getSupportFragmentManager().popBackStack();
            getSupportFragmentManager().beginTransaction().add(R.id.container_body, blankFragment).addToBackStack("fragBack").commit();
        }
        else {
            new AlertDialog.Builder(this)
                    .setIcon(android.R.drawable.ic_dialog_alert)
                    .setTitle("Closing Activity")
                    .setMessage("Are you sure you want to close this activity?")
                    .setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                        @Override
                        public void onClick(DialogInterface dialog, int which) {
                            finish();
                        }

                    })
                    .setNegativeButton("No", null)
                    .show();



        }
    }

    @Override
    protected void onPostCreate(Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);
        mDrawerToggle.syncState();
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        if (mDrawerToggle.onOptionsItemSelected(item)) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

}

via Chebli Mohamed