某个函数返回一个特定的代码，用来表示某种错误情况

public int withdraw(int amount) {if (amount > balance) {return -1;} else {balance -= amount;return 0;}
}

重构：改用异常

public int withdraw(int amount) {if (amount > balance) {throw new BalanceException("余额不足。");} balance -= amount;
}