Android连续两次后退键退出程序

@Overridepublic void onBackPressed() {// TODO Auto-generated method stubif (firstTime + 2000 > System.currentTimeMillis()) {finish();System.exit(0);super.onBackPressed();} else {Toast.makeText(MainActivity.this, "再按一次退出程序", 2).show();}firstTime = System.currentTimeMillis();}