من تو اکتیوتی متد onBackPressed اومدم پیادش کردم :

 private static final long BACK_PRESS_DELAY = 2000;
    private boolean mBackPressCancelled = false;
    private long mBackPressTimestamp;
    private Toast mBackPressToast;
 @Override
    public void onBackPressed() {
        // Do nothing if the back button is disabled.
        if (!mBackPressCancelled) {
            // Pop fragment if the back stack is not empty.
            if (getSupportFragmentManager().getBackStackEntryCount() > 0) {
                super.onBackPressed();
            } else {
                if (mBackPressToast != null) {
                    mBackPressToast.cancel();
                }
 if(getSupportFragmentManager().findFragmentById(R.id.frame_main_fragmentContainers) instanceof MainFragment) {
                    long currentTimestamp = System.currentTimeMillis();
                    if (currentTimestamp < mBackPressTimestamp + BACK_PRESS_DELAY) {
                        super.onBackPressed();
                    } else {
                        mBackPressTimestamp = currentTimestamp;
                        mBackPressToast = Toast.makeText(this, "برای خروج کلیک مجدد را برنید", Toast.LENGTH_SHORT);
                        mBackPressToast.show();
                    }
                }
            }
        }
    }