系统Back键再按一次退出程序

private long exitTime = 0;

@Overridepublic 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);}