Intent startMain = new Intent(Intent.ACTION_MAIN);
    startMain.addCategory(Intent.CATEGORY_HOME);
    startMain.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
    startActivity(startMain);

@Override
    public void onBackPressed()
    {
         // code here to show dialog
         super.onBackPressed();  // optional depending on your needs

finish();
    }

onBackPressed();