How to Exit App When Press Back Button - Android

OnBackPressed.java

///////////////////////////////BACK-BUTTON-PRESSED/////////////////////////////
//////////////////////////////////creatorb////////////////////////////////////

//You can choose one method ;)

//PopUp
@Override
public void onBackPressed() {
    new AlertDialog.Builder(this)
        .setTitle("Really Exit?")
        .setMessage("Are you sure you want to exit?")
        .setNegativeButton(android.R.string.no, null)
        .setPositiveButton(android.R.string.yes, new OnClickListener() {

            public void onClick(DialogInterface arg0, int arg1) {
                WelcomeActivity.super.onBackPressed();
            }
        }).create().show();
}

//Set Class to Top of App and no history
Intent launchNextActivity;
launchNextActivity = new Intent(B.class, A.class);
launchNextActivity.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
launchNextActivity.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);                  
launchNextActivity.addFlags(Intent.FLAG_ACTIVITY_NO_ANIMATION);
startActivity(launchNextActivity);

//Exit When Back and Set no History
Intent intent = new Intent(Intent.ACTION_MAIN);
          intent.addCategory(Intent.CATEGORY_HOME);
          intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);//***Change Here***
          startActivity(intent);
          finish();
          System.exit(0);

//Styling for double press back button
private static long back_pressed;
@Override
public void onBackPressed(){
    if (back_pressed + 2000 > System.currentTimeMillis()){
        super.onBackPressed();
    }
    else{
        Toast.makeText(getBaseContext(), "Press once again to exit", Toast.LENGTH_SHORT).show();
        back_pressed = System.currentTimeMillis();
    }
}

//For clear your history when back pressed, You can add android:noHistory="true" on your manifest.xml
<-- <activity
            android:name="id.creatorb.math.Quiz"
            android:configChanges="orientation|keyboardHidden|screenSize|smallestScreenSize"
            android:label="@string/app_name"
            android:noHistory="true"
            android:screenOrientation="portrait" >
        </activity> -->

//