Question

目标是创建withdraw方法，只有当withdrawAmount不超过该余额时才会将withdrawAmount从当前余额减去。如果是，那么我们打印一条消息并保持平衡不变。使用 if ... else ，例如

，这很容易

public void withdraw(double withdrawAmount)
{
    if (withdrawAmount > 0.0)
        if (withdrawAmount > balance)
            System.out.println("Withdrawal amount exceeded account balance");
        else
            balance = balance - withdrawAmount;
}

有没有办法在不使用 if ... else 结构或＆amp;＆amp; 及其兄弟的情况下实现相同的效果？

Answer 1

是的，使用例外。

在异常情况下会抛出异常。

public void withdraw(double amount) { if (amount > 0.0d) { throw new IllegalArgumentException("Cannot withdraw a negative amount"); } if (amount > balance) { throw new IllegalStateException("Cannot overdraw an account"); } balance = balance - amount; }

当然，这段代码可以作为一个爱好示例，但它不是线程安全的。

假设您不想抛出异常，您也可以提前退出＆＃34;

if (amount > balance) { System.out.println("Withdrawl amount exceeded account balance"); return; } balance = balance - amount;

Answer 2

根据OP的评论，该问题实际上是Else，而不是If：

public void withdraw(double withdrawAmount)
{
    if (withdrawAmount > 0.0 && withdrawAmount > balance)
    {
        System.out.println("Withdrawal amount exceeded account balance");
        return;
    }

    balance = balance - withdrawAmount;
}

Answer 3

只是肛门，避免任何if或布尔运算符：

String msg = (withdrawAmount > balance) ? "Withdrawl amount exceeded account balance\n" : "";
double delta = (withdrawAmount > balance) ? 0.0d : Math.max(withdrawAmount,0.0d);
System.out.print( msg ); // prints nothing if balance high enough
balance -= delta;        // delta is 0 unless balance is high enough & wA is positive

Answer 4

您可以通过真正的无分支代码来实现：

private double balance = -10;

private String[] negativeWaring = { "", "Withdrawing negative amnt!\n" };
private String[] exceedWaring = { "", "Withdrawing exceeds balance!\n" };

public static int isNegative(double d) {
    long bits = Double.doubleToLongBits(d);
    bits = bits >>> 63;
    return (int) bits;
}

void withdraw(double amnt) {
    int negative = isNegative(amnt);
    int exceed = isNegative(balance - amnt);

    exceed *= 1 - negative; // only apply exceed flag if amnt is positive
    System.out.print(negativeWaring[negative]);
    System.out.print(exceedWaring[exceed]);
    balance += amnt * negative * exceed;
}

对于负数，isNegative()方法将返回1，对于正数，0方法将返回System.out.print()。

修改

我不确定{{1}}究竟是如何工作的，如果有某些条件，那么你的代码就不会真正无分支。

如果用Java中的语句撤销方法

4 个答案: