如何在android中的导航抽屉中实现后退按钮

Question

如何在Android中的导航抽屉中实现后退按钮？我实现了一个包含多个片段（1-7）的导航抽屉。但是我不知道如何启用应该切换到上一个片段的后退按钮。

这个包含导航抽屉代码的主要活动....在定义4个片段....我知道如何实现后退按钮

Mainactivity.java：

package com.example.admin.realstate;

import android.os.Bundle;
import android.support.design.widget.CollapsingToolbarLayout;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.support.v7.widget.RecyclerView;
import android.view.View;
import android.support.design.widget.NavigationView;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.view.ViewGroup;

import butterknife.Bind;
import butterknife.ButterKnife;

public class MainActivity extends AppCompatActivity
        implements NavigationView.OnNavigationItemSelectedListener
{
    CollapsingToolbarLayout collapsingToolbarLayout;
    ActionBarDrawerToggle toggle;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
       //ButterKnife.bind(this);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        if (toolbar != null) {
            setSupportActionBar(toolbar);
        }
        setSupportActionBar(toolbar);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);

if(savedInstanceState==null)
{
    Home_Fragment fragment=new Home_Fragment();
    FragmentManager frgmentmanager=getSupportFragmentManager();
    FragmentTransaction fragmentTransaction=frgmentmanager.beginTransaction();
    // fragmentTransaction.add(R.id.fragment_container, fragment);
    fragmentTransaction.replace(R.id.fragment_container,fragment);
    fragmentTransaction.addToBackStack(null);
    fragmentTransaction.commit();
}


        final DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
       toggle = new ActionBarDrawerToggle(
                this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.setDrawerListener(toggle);
        toggle.syncState();
        NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(this);


 }

    @Override
    public void onBackPressed()
    {
       DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
     if (drawer.isDrawerOpen(GravityCompat.START))
        {
            drawer.closeDrawer(GravityCompat.START);
        }
        else
        {            super.onBackPressed();
 }

    }

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

    @SuppressWarnings("StatementWithEmptyBody")
    @Override
    public boolean onNavigationItemSelected(MenuItem item) {
        // Handle navigation view item clicks here.
        int id = item.getItemId();
        item.setChecked(true);
        // Set action bar title
        setTitle(item.getTitle());
        if (id == R.id.nav_Home)
        {
            Home_Fragment fragment=new Home_Fragment();
            FragmentManager frgmentmanager=getSupportFragmentManager();
            FragmentTransaction fragmentTransaction=frgmentmanager.beginTransaction();
            // fragmentTransaction.add(R.id.fragment_container, fragment);
            fragmentTransaction.replace(R.id.fragment_container,fragment);
            fragmentTransaction.addToBackStack("home_fragment");
            fragmentTransaction.commit();
        }
        else if (id == R.id.nav_project)
        {
            Projects_Fragments fragment=new Projects_Fragments();
            FragmentManager frgmentmanager=getSupportFragmentManager();
            FragmentTransaction fragmentTransaction=frgmentmanager.beginTransaction();
            // fragmentTransaction.add(R.id.fragment_container, fragment);
            fragmentTransaction.replace(R.id.fragment_container,fragment);
            fragmentTransaction.addToBackStack("project_fragment");
            fragmentTransaction.commit();
        }
        else if (id == R.id.nav_amenities)
        {
            Amenities fragment=new Amenities();
            FragmentManager frgmentmanager=getSupportFragmentManager();
            FragmentTransaction fragmentTransaction=frgmentmanager.beginTransaction();
            // fragmentTransaction.add(R.id.fragment_container, fragment);
            fragmentTransaction.replace(R.id.fragment_container,fragment);
            fragmentTransaction.addToBackStack("amenities_fragment");
            fragmentTransaction.commit();
        }
        else if (id == R.id.nav_contactsus)
        {

            Contactus_Fragment fragment=new Contactus_Fragment();
            FragmentManager frgmentmanager=getSupportFragmentManager();
            FragmentTransaction fragmentTransaction=frgmentmanager.beginTransaction();
            // fragmentTransaction.add(R.id.fragment_container, fragment);
            fragmentTransaction.replace(R.id.fragment_container,fragment);
            fragmentTransaction.addToBackStack("contact_fragment");
            fragmentTransaction.commit();
        }

        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        drawer.closeDrawer(GravityCompat.START);
        return true;
    }



    }
}