完美退出应用程序

//返回鍵監聽
@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 {
Intent intent = new Intent(Intent.ACTION_MAIN);
intent.addCategory(Intent.CATEGORY_HOME);
intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(intent);

finish();

System.exit(0);

}
return true;
}
return super.onKeyDown(keyCode, event);
}