If-else语句去了错误的条件语句java？

Question

我的程序输入似乎正在旅行到我的if-else语句中的错误else语句。

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，如果我输入c，并在不到4000000加仑所使用的总的水，它的执行代码时所使用的总加仑多于4000000加仑线。为什么？

Answer 1

它，因为或子句||放在（）周围。

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

Answer 2

由于操作的顺序与您的条件句

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

customerCode.equals（ “C”）|| customerCode.equals（ “C”）& & gallonsUsed < = 4000000

Ť|| F & & F

这相当于true，所以billingAmount = 1000.00;

但很下一个语句是

customerCode.equals（ “C”）|| customerCode.equals（ “C”）& & gallonsUsed> 4000000）

Ť|| F & & F

这也等同于true，因此billingAmount被覆盖 - billingAmount = 1000.00 +（（gallonsUsed-4000000）* 0.00025）;

这两个if语句是你的条件真的。

要修复，使用括号。并且还使用else语句。当第一个条件为真时，没有理由通过大量的条件检查。

Answer 3

的& &在第二和第三测试之间得到第一评价，然后||与第一次测试（当你输入'c'时是这样的）。

使用括号，来表示if语句的部分应首先评估的读者（和编译器）。在你的情况，你的if语句应该是：

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

一个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) 

相当于

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

因为

运算符优先级。具体而言，&&比||具有更高的优先级，因此如果两者具有相同的表达式，则它将&&的左侧和右侧的表达式作为&&的操作数，并将&&的结果作为||的操作数。因此，如果客户代码为小写c，则不会查看gallonsUsed。

您需要周围的||部分括号：

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

或者使用equalsIgnoreCase和避免整个运营商的优先问题：

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

Answer 5

，具有较高的preceence然后或者

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); 
} 

但是我把它改写为

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); 
    } 
} 

而对于customerCode “我”，可能看起来像，

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