Android中連續按2下返回鍵退出，而且提示

一：連按兩次返回鍵退出程序

private long exitTime = 0;

    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN) {
            if ((System.currentTimeMillis () - exitTime ) > 2000 ) {
                Toast.makeText(getApplicationContext(), "再按一次退出程序", Toast.LENGTH_SHORT).show();
                exitTime = System.currentTimeMillis();
            } else {
                finish();
                System.exit(0 );
            }
            return true;
        }
        return super .onKeyDown(keyCode, event);
    }

}