Question

我在此行double dollar = Double.parseDouble(input)

收到错误

java.lang.NumberFormatException:
For input string: "q"(in sun.misc.FloatingDecimal)

代码：

   /**
   Converts money using CurrencyConverter
   */

   import java.util.*;

   public class CurrencyConverterTester {
     public static void main(String[] args) {
       //Normally the scanner is based on System.in,
       //Scanner scanner = new Scanner(System.in) ;
       //but for predictability we set the input to a fixed sequence:
       Scanner scanner = new Scanner("0.79447 100 20 88.88 q");

       System.out.print("Conversion factor (euros per dollar): ");
       //We could use scanner.nextDouble() here, but this is an example
       //of using parseDouble, which you need in the next loop.
       String input = scanner.next();
       double rate = Double.parseDouble(input);
       System.out.println(rate);
       //----------------Start below here. To do: approximate lines of code = 1
       // 1. make the CurrencyConverter object based on the rate
       CurrencyConverter converter = new CurrencyConverter(rate);
       //----------------------End here. Please do not remove this comment. Reminder: no changes outside the todo regions.

       System.out.print("Dollar value (Q to quit): ");
       input = scanner.next();
       System.out.println(input); //echo the input
       //----------------Start below here. To do: approximate lines of code = 8
       // 1. write a while loop where the condition is that input is not "Q" or "q" ; 2. use parseDouble to get the dollars amount ; 3.  use the converter object to convert the dollars to euros ; 4.  print the dollars and euros in the style shown in the Expected region (Hint: use printf); 5. prompt for the next input ; 6. read the next input ; 7. echo the input (i.e., print it out)

       while (input != "q" && input != "Q") {
         double dollar = Double.parseDouble(input);
         double euros = converter.convert(dollar);
         System.out.printf("%.2f dollars = %.2f euros\n", dollar, euros);
         System.out.print("Dollar value (Q to quit): ");
         input = scanner.next();
         System.out.println(input);
       }
     }
   }

   Conversion factor(euros per dollar): 0.79447
   Dollar value(Q to quit): 100
   100.00 dollars = 79.45 euros
   Dollar value(Q to quit): 20
   20.00 dollars = 15.89 euros
   Dollar value(Q to quit): 88.88
   88.88 dollars = 70.61 euros
   Dollar value(Q to quit): q

Answer 1

您正在使用等于运算符==作为String输入。对于字符串，您必须使用他们的.equals()方法。

因此：

while (input != "q" && input != "Q")

应该是：

while (!(input.equals("q") || input.equals("Q")))

或

while (!"q".equalsIgnoreCase(input)) // While input not q or Q

使用scanner.next()时要小心，最好使用scanner.nextLine()，否则你会在输入变量中得到换行符。因此，当您键入“q”时，输入变量可能是String input = "q\n";

Answer 2

input是一个字符串，因此您必须使用.equals()，而不是==。等于运算符==确定两个引用是否指向同一个对象。 .equals()方法检查它们在逻辑上是否相等的地方。

您还想使用||而不是&&。更好的方法是使用String#equalsIgnoreCase()

Answer 3

请勿将字符串值与==或!=进行比较。请改用String.equals。

您对==所做的是比较对象地址。这不是您想要的，因为您在内存中的某个位置包含"q"的String对象，并且您在内存中有另一个包含"q"的对象 else 。它们具有不同的地址，这导致使用==进行比较返回false，而内容在文本上是相同的。

Java currencyConverter java.lang.NumberFormatException：

3 个答案: