@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if(keyCode == KeyEvent.KEYCODE_BACK)
    {
        exitBy2Click();
    }
    return false;
}

private static Boolean isExit = false;

private void exitBy2Click() {
    Timer tExit = null;
    if (!isExit) {
        isExit = true;
        Toast.makeText(this, "click again to quit", Toast.LENGTH_SHORT).show();
        tExit = new Timer();
        tExit.schedule(new TimerTask() {
            @Override
            public void run() {
                isExit = false;
            }
        }, 2000);

    } else {
        finish();
        System.exit(0);
    }
}