我想让打印机打印，直到帐户中的余额不足以完成打印作业

Question

问题是我一直让余额为负，它执行需要取出钱的部分，但是我已经实施的"if"语句应该可以防止这种情况。我是编程新手，如果这很愚蠢，我很抱歉。

这是我的主要：

public class PrinterAccount
{
    private int balance;
    public void topUp(int amount)
    { 
        balance += amount;
    }
    public int getBalance()
    { 
        return balance;
    }
    public boolean printDocument(int numPages, boolean isDoubleSided)
    {
        if (isDoubleSided)
        {
            if(numPages % 2 == 0)
            {
                int b = 0;
                b = b - ((numPages/2)*5);
                    if (balance > b)
                    {
                        balance -= ((numPages/2) * 5);
                        return true;
                    }
                    else
                    {
                        return false;
                    }
            }
            else
            {
                numPages = numPages + 1;
                int a = 0;
                a = a - ((numPages/2)*5);
                    if (balance > a)
                    {
                        balance -= ((numPages/2) * 5);
                        return true;
                    }
                    else
                    {
                        return false;
                    }
            }
        }
        else
        {
            int c = 0;
            c = c - (numPages*5);
                if (balance > c)
                {
                    balance -= (numPages*5);
                    return true;
                }
                else
                {
                    return false;
                }
        }   
    }
}

这是我的测试：

public class PrinterAccountTest
{
    public static void main(String[] args)
    {
        // Create an object of class PrinterAccount
        PrinterAccount myprinterAccount = new PrinterAccount();
        // Add 10 pounds credit (1000 pence)
        myprinterAccount.topUp(1000);
        // **************TEST 1************************
        // Print a 15-page document, single-sided, should cost 75p
        boolean res = myprinterAccount.printDocument(15, false);
        // The document should have printed successfully
        if (res)
            System.out.println("The document printed successfully");
        else
            System.out.println("The document failed to print: not enough credit.");
        // Display the remaining balance: should be 925
        System.out.println("Remaining balance is " + myprinterAccount.getBalance());
        // **************TEST 2************************
        // Print a 20-page document (even number of pages), double-sided, should cost 50p
        res = myprinterAccount.printDocument(20, true);
        // The document should have printed successfully
        if (res)
            System.out.println("The document printed successfully");
        else
            System.out.println("The document failed to print: not enough credit.");
        // Display the remaining balance: should be 875
        System.out.println("Remaining balance is " + myprinterAccount.getBalance());
        // **************TEST 3************************
        // Print a 7-page document (odd number of pages), double-sided, should cost 20p
        res = myprinterAccount.printDocument(7, true);
        // The document should have printed successfully
        if (res)
            System.out.println("The document printed successfully");
        else
            System.out.println("The document failed to print: not enough credit.");
        // Display the remaining balance: should be 855
        System.out.println("Remaining balance is " + myprinterAccount.getBalance());
        // **************TEST 4************************
        // Print a 200-page document, single-sided, should cost 1000p
        res = myprinterAccount.printDocument(200, false);
        // Should get a message saying the document failed to print
        if (res)
            System.out.println("The document printed successfully");
        else
            System.out.println("The document failed to print: not enough credit.");
        // Display the remaining balance: should be 855
        System.out.println("Remaining balance is " + myprinterAccount.getBalance());

    }
}

这是我运行代码时得到的结果：

The document printed successfully
Remaining balance is 925
The document printed successfully
Remaining balance is 875
The document printed successfully
Remaining balance is 855
The document printed successfully
Remaining balance is -145

但我需要它来显示：

The document printed successfully
Remaining balance is 925
The document printed successfully
Remaining balance is 875
The document printed successfully
Remaining balance is 855
The document failed to print: not enough credit.
Remaining balance is 855

Answer 1

打印文档方法中的以下行：

    int c = 0;
    c = c - (numPages*5);
    if (balance > c) ...

变量 c 将始终具有负值。我想你想加（numPages*5）而不是减去它。

Answer 2

因为您的balance将始终大于 a，b，c，因为 c 将为负值，余额将为正值balance>c将始终为真。尝试更改逻辑