Java temp转换程序。正确的输出，但错误

Question

我不知道我在做什么错，分配是创建一个代码，将温度从C转换为F或从F转换为C，直到用户决定完成为止，我也是应该在出现无效字符时打印错误消息，并让用户在不再次询问数字部分的情况下更正错误消息。

该程序似乎运行良好，直到我输入'c''C''f'或'F'以外的值。在这一点上，我仍然得到所需的输出，但比我得到一个错误，这里是我的代码。

import java.util.Scanner; 

public class ProjectThree 
{ 
    public static void main(String[] args) 
    { 
     Scanner keyboard = new Scanner(System.in); 
     System.out.println("Please enter a temperature to be converted followed" 
      + "\nby a C or c for Celsius or an F or f for Fahrenheit. If " 
      + "\nfinished converting temperatures enter done."); 
     String userInput, intString; 
     userInput = keyboard.nextLine(); 

     while (!(userInput.equalsIgnoreCase("done"))) 
     { 
      int length, temp, degreesC, degreesF; 
      length = userInput.length(); 
      intString = userInput.substring(0,length - 1); 
      temp = Integer.parseInt(intString); 
      char scale = userInput.charAt(length - 1); 

      while (!((scale == 'c') || (scale == 'C') || (scale =='f') || 
       (scale == 'F'))) 
      { 
       System.out.println("Error: Invalid temperature unit. Enter a C or c" 
        + " for Celsius or an F or f for Fahrenheit."); 
       String errorInput = keyboard.next(); 
       scale = errorInput.charAt(0); 
       userInput = intString + errorInput; 
      } 
      switch (scale) 
      { 
       case 'C': 
       case 'c': 
        degreesF = (9 * (temp/5) + 32); 
        System.out.println(userInput + " is equal to " + degreesF 
         + "F"); 
        break; 
       case 'F': 
       case 'f': 
        degreesC = (5 * (temp - 32))/9; 
        System.out.println(userInput + " is equal to " + degreesC 
         + "C"); 
        break; 
      } 
      System.out.println("\nPlease enter a temperature to be converted followed" 
       + "\nby a C or c for Celsius or an F or f for Fahrenheit. If " 
       + "\nfinished converting temperatures enter done."); 
      userInput = keyboard.nextLine(); 
     } 
    } 
} 

，误差

Please enter a temperture to be converted followed 
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: -1 
by a C or c for Celsius or an F or f for Farenheit. If 
finished converting tempertures enter done. 
    at java.lang.String.substring(String.java:1955) 
    at ChapterFour.ProjectThree.main(ProjectThree.java:33) 
Java Result: 1 
BUILD SUCCESSFUL (total time: 5 seconds) 

Answer 1

一般情况下，有很多的事情，“可能”去错了错误的输入，因为你没有任何错误检查做。这里有一些我注意到的事情...

1）正如我认为别人提到的，你没有检查任何输入的长度，然后才行事。如果有人在没有其他事情的情况下进入，您可能会遇到异常情况。相同，如果他们输入无效输入。

2）switch语句缺少默认子句。你应该添加一些打印出“出错了，请再试一次”的东西，以便知道它发生了。

3）当你只询问字符时，我首先看到你使用keyboard.nextLine（）和keyboard.next（）。如果在字符后面输入回车，则换行符可能会被下一个nextLine（）拾取，导致打印错误，如上面1中所述。虽然谷歌搜索未来的）行为（以确认这一点，我碰到下面的（其他人似乎也有类似的问题）：

Issues with nextLine();

Answer 2

为空字符串或长度one你没有处理任何事情的串所以它throw和exception。你需要用户try catch块或做我喜欢的事情。我简单地忽略输入字符串的长度zero和one。

Scanner keyboard = new Scanner(System.in); 

     System.out.println("Please enter a temperature to be converted followed" 
       + "\nby a C or c for Celsius or an F or f for Fahrenheit. If " 
       + "\nfinished converting temperatures enter done."); 

     String userInput, intString; 
     userInput = keyboard.nextLine(); 

     while (!(userInput.equalsIgnoreCase("done"))) { 
      int length, temp, degreesC, degreesF; 
      length = userInput.length(); 
      System.out.println(length); 
      if (length > 1) { 
       intString = userInput.substring(0, length - 1); 
       temp = Integer.parseInt(intString); 
       // System.out.println("Temp = " + temp); 
       char scale = userInput.charAt(length - 1); 
       // System.out.println("scale" + scale); 

       while (!((scale == 'c') || (scale == 'C') || (scale == 'f') || (scale == 'F'))) { 
        System.out.println("Error: Invalid temperature unit. Enter a C or c" 
          + " for Celsius or an F or f for Fahrenheit."); 
        String errorInput = keyboard.next(); 
        scale = errorInput.charAt(0); 
        userInput = intString + errorInput; 
       } 
       switch (scale) { 
       case 'C': 
       case 'c': 
        degreesF = (9 * (temp/5) + 32); 
        System.out.println(userInput + " is equal to " + degreesF + "F"); 
        break; 
       case 'F': 
       case 'f': 
        degreesC = (5 * (temp - 32))/9; 
        System.out.println(userInput + " is equal to " + degreesC + "C"); 
        break; 
       } 
      } 

      System.out.println("\nPlease enter a temperature to be converted followed" 
        + "\nby a C or c for Celsius or an F or f for Fahrenheit. If " 
        + "\nfinished converting temperatures enter done."); 
      userInput = keyboard.nextLine(); 
     } 

注意String substring(int beginIndex, int endIndex)：返回substring从给定的index(beginIndex)直至成为规定index(endIndex)开始。对于例如“塔尼亚”。 substring(2,5)将返回“ait”。它抛出IndexOutOfBoundsException如果beginIndex是小于零或beginIndex > endIndex或endIndex大于String的length。

在你的情况，如果你只是erter c或C lenght-1成为zero这是你的last index。还有你的first index也是zero。那就是为什么你得到了IndexOutOfBoundsException。

希望你明白了。