If-else語句去了錯誤的條件語句java？

Question

我的程序輸入似乎正在旅行到我的if-else語句中的錯誤else語句。

import java.util.Scanner; 
import java.text.*; 

public class CSCD210Lab6 
{ 
    public static void main (String [] args) 
    { 
    Scanner waterInput = new Scanner(System.in); 
    DecimalFormat df = new DecimalFormat("$#,###.00"); 
    DecimalFormat zf = new DecimalFormat("#,###.0"); 

     //declare variables 
     int beginMeter, endMeter; 
     String customerCode; 
     double billingAmount,gallonsUsed; 


     billingAmount = 0; 

     System.out.print("Please Enter Your Customer Code: "); 
     customerCode = waterInput.next(); 
     System.out.print("Please Enter Your Beginning Meter Reading: "); 
     beginMeter = waterInput.nextInt(); 
     if(beginMeter < 0) 
     { 
      System.out.println(); 
      System.out.print("ERROR! You Have Entered A Negative Number. The Program Will Now Close."); 
      System.exit(0); 
     } 

     System.out.print("Please Enter Your Ending Meter Reading: "); 
     endMeter = waterInput.nextInt(); 
     if(endMeter < 0) 
     { 
      System.out.println(); 
      System.out.print("ERROR! You Have Entered A Negative Number. The Program Will Now Close."); 
      System.exit(0); 
     } 
     if(endMeter > beginMeter) 
     { 
     gallonsUsed = ((double)endMeter - beginMeter)/10; 
     } 
     else 
     { 
     gallonsUsed = (1000000000-((double)beginMeter - endMeter))/10; 
     } 
     if (customerCode.equals("r")||customerCode.equals("R")) 
     { 
     billingAmount = 5.00 + (.0005 * gallonsUsed); 
     } 

     if(customerCode.equals("c")||customerCode.equals("C") && gallonsUsed <= 4000000)  
     { 
     billingAmount = 1000.00; 
     } 

     if(customerCode.equals("c")||customerCode.equals("C") && gallonsUsed > 4000000) 
     { 
     billingAmount = 1000.00 + ((gallonsUsed-4000000) * 0.00025); 
     } 

     if(customerCode.equals("i")||customerCode.equals("I")&& gallonsUsed <= 4000000) 
     { 
     billingAmount = 1000.00; 
     } 

     if(customerCode.equals("i")||customerCode.equals("I")&& gallonsUsed > 4000000 && gallonsUsed < 10000000) 
     { 
     billingAmount = 2000.00; 
     } 
     if(customerCode.equals("i")||customerCode.equals("I")&& gallonsUsed >= 10000000) 
     { 
     billingAmount = 2000.00 +(gallonsUsed * .00025); 
     } 


     System.out.println(); 
     System.out.print("Your Customer Code is: "+customerCode); 
     System.out.println(); 
     System.out.print("Your Beginning Meter Reading is: "+beginMeter); 
     System.out.println(); 
     System.out.print("Your Ending Meter Reading is: "+endMeter); 
     System.out.println(); 
     System.out.print("You Have Used "+zf.format(gallonsUsed)+" Gallons During This Billing Period."); 
     System.out.println(); 
     System.out.print("Your Bill is: "+df.format(billingAmount)); 
     System.out.println(); 
     System.out.println(); 
     System.out.print("Please Pay Promptly. We Detest Late Accounts."); 



    } 
} 

例如，如果我輸入c，並在不到4000000加侖所使用的總的水，它的執行代碼時所使用的總加侖多於4000000加侖線。爲什麼？

Answer 1

它，因爲或子句||放在（）周圍。

if((customerCode.equals("c")||customerCode.equals("C")) && gallonsUsed > 4000000) 

Answer 2

由於操作的順序與您的條件句

http://docs.oracle.com/javase/tutorial/java/nutsandbolts/operators.html

customerCode.equals（ 「C」）|| customerCode.equals（ 「C」）& & gallonsUsed < = 4000000

Ť|| F & & F

這相當於true，所以billingAmount = 1000.00;

但很下一個語句是

customerCode.equals（ 「C」）|| customerCode.equals（ 「C」）& & gallonsUsed> 4000000）

Ť|| F & & F

這也等同於true，因此billingAmount被覆蓋 - billingAmount = 1000.00 +（（gallonsUsed-4000000）* 0.00025）;

這兩個if語句是你的條件真的。

要修復，使用括號。並且還使用else語句。當第一個條件爲真時，沒有理由通過大量的條件檢查。

Answer 3

的& &在第二和第三測試之間得到第一評價，然後||與第一次測試（當你輸入'c'時是這樣的）。

使用括號，來表示if語句的部分應首先評估的讀者（和編譯器）。在你的情況，你的if語句應該是：

if((customerCode.equals("c")||customerCode.equals("C")) && gallonsUsed <= 4000000) 

一個更好的方法是使用下面的模式：

if(customerCode.equals("c")||customerCode.equals("C"))  
{ 
    if(gallonsUsed <= 4000000) 
    { 
     billingAmount = 1000.00; 
    } else { 
     billingAmount = 1000.00 + ((gallonsUsed-4000000) * 0.00025); 
    } 
} 

Answer 4

if(customerCode.equals("c")||customerCode.equals("C") && gallonsUsed > 4000000) 

相當於

if(customerCode.equals("c") || 
     (customerCode.equals("C") && gallonsUsed > 4000000)) 

因爲

運算符優先級。具體而言，&&比||具有更高的優先級，因此如果兩者具有相同的表達式，則它將&&的左側和右側的表達式作爲&&的操作數，並將&&的結果作爲||的操作數。因此，如果客戶代碼爲小寫c，則不會查看gallonsUsed。

您需要周圍的||部分括號：

if((customerCode.equals("c")||customerCode.equals("C")) && gallonsUsed > 4000000) 

或者使用equalsIgnoreCase和避免整個運營商的優先問題：

if (customerCode.equalsIgnoreCase("c") && gallonsUsed > 4000000) 

Answer 5

，具有較高的preceence然後或者

if(customerCode.equals("c")||customerCode.equals("C") && gallonsUsed <= 4000000)  
{ 
    billingAmount = 1000.00; 
} 
if(customerCode.equals("c")||customerCode.equals("C") && gallonsUsed > 4000000) 
{ 
    billingAmount = 1000.00 + ((gallonsUsed-4000000) * 0.00025); 
} 

所以，你想要

if((customerCode.equals("c")||customerCode.equals("C")) && gallonsUsed <= 4000000)  
{ 
    billingAmount = 1000.00; 
} 
if((customerCode.equals("c")||customerCode.equals("C")) && gallonsUsed > 4000000) 
{ 
    billingAmount = 1000.00 + ((gallonsUsed-4000000) * 0.00025); 
} 

但是我把它改寫爲

if (customerCode.equalsIgnoreCase("c")) { 
    billingAmount = 1000.00 + (gallonsUsed <= 4000000) ? 0 : 
     ((gallonsUsed-4000000) * 0.00025); 
} 

，或者

if (customerCode.equalsIgnoreCase("c")) { 
    if (gallonsUsed <= 4000000) { 
    billingAmount = 1000.00; 
    } else { 
    billingAmount = 1000.00 + ((gallonsUsed-4000000) * 0.00025); 
    } 
} 

而對於customerCode 「我」，可能看起來像，

if (customerCode.equalsIgnoreCase("i")) { 
    if (gallonsUsed <= 4000000) { 
     billingAmount = 1000.00; 
    } else { 
     billingAmount = 2000.00 + ((gallonsUsed < 10000000) ? 0 
       : (gallonsUsed * 0.00025)); 
    } 
}