Fragment fragment=null;

public void onBackPressed() {
        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        if (drawer.isDrawerOpen(GravityCompat.START)) {
            drawer.closeDrawer(GravityCompat.START);
        } else { if (fragment.equals(fragment_I)) {
                AlertDialog.Builder builder = new AlertDialog.Builder(this).setTitle("Salir de la aplicación").setMessage("¿Estas seguro de salir de la aplicación?")
                        .setPositiveButton("Si", new DialogInterface.OnClickListener() {
                            @Override
                            public void onClick(DialogInterface dialogInterface, int i) {

                                try {
                                    this.finalize();
                                    MainActivity.super.onBackPressed();
                                } catch (Throwable throwable) {
                                    throwable.printStackTrace();
                                }


                            }
                        }).setNegativeButton("No", new DialogInterface.OnClickListener() {
                            @Override
                            public void onClick(DialogInterface dialogInterface, int i) {
                                dialog.dismiss();
                            }
                        });
                dialog = builder.create();
                dialog.show();
            } else {
                fragment = new Fragment_Inicio();
                FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
                fragmentTransaction.replace(R.id.content_main, fragment);
                fragmentTransaction.commit(); }
        }
    }