我想打印機打印，直到帳戶餘額不足以完成打印任務

Question

問題是我一直將餘額減去，它執行需要將錢取出的部分，但是我已經實施的'if'聲明應該可以避免這種情況。我是編程新手，如果這很愚蠢，我很抱歉。

這裏是我的主：

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; 
       } 

     } 
    } 
} 

，這裏是我的測試：

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()); 


    } 
} 

這是結果我得到的，當我運行代碼：

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 

但我需要它來顯示：

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

此線路中的打印文檔的方法：

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

變量C將總是具有負值。我想你想添加（numPages * 5）而不是減去它。

Answer 2

因爲你的balance將總是大於a，b，c，因爲c將是負值，餘額將是正值balance>c將始終爲真。嘗試改變邏輯