java - 线程 "main"java.lang.NullPointerException-part2 除外

Question

我按照 Dan 的建议更改了代码我现在可以编译该程序，但是，无论输入是什么，结果始终是 2。我将此程序的第二部分放在新代码下方。请帮忙。

这是新代码。

    public class VowelCons 
        {
    private final String str;
    private final int totalConsonants;
    private final int totalVowels;

        public VowelCons(final String s) 
    {
             this.str = s;
                int totalConsonants = 0;
                int totalVowels = 0;
                if (null != s) 
        {
                    for (final char c : s.toCharArray()) 
            {
                                switch (c) 
                    {
                                        case 'A':
                                        case 'a':
                                        case 'E':
                                        case 'e':
                                        case 'I':
                                        case 'i':
                                        case 'O':
                                        case 'o':
                                        case 'U':
                                        case 'u':

                        totalVowels++;
                                            break;

                        default:

                        if (Character.isLetter(c)) 
                        {
                                                    totalConsonants++;
                                            }
                                            break;
                                    }
                        }
                }
            this.totalConsonants = totalConsonants;
            this.totalVowels = totalVowels;
        }


    public String getString() 
    {
             return str;
    }

    public int getNumConsonants() 
    {
                return this.totalConsonants;
    }

    public int getNumVowels() 
    {
                return this.totalConsonants;
    }
}

该程序的另一部分获取用户的输入并将其传递给此类。这是代码。 [根据规定此部分不能更改]

    import java.util.Scanner;

    public class VowelConsCounter
    {
        public static void main(String[] args)
        {
         String input;        // User input
         char selection;      // Menu selection

         Scanner keyboard = new Scanner(System.in);

         System.out.print("Enter a string: ");
         input = keyboard.nextLine();

         VowelCons vc = new VowelCons(input);

        do
        {
          selection = getMenuSelection();

          switch(Character.toLowerCase(selection))
          {
            case 'a' :  System.out.println("\nNumber of vowels: " +
                        vc.getNumVowels());
                        break;
            case 'b' :  System.out.println("\nNumber of consonants: " +
                        vc.getNumConsonants());
                        break;
            case 'c' :  System.out.println("\nNumber of vowels: " +
                        vc.getNumVowels());
                        System.out.println("Number of consonants: " +
                        vc.getNumConsonants());
                        break;
            case 'd' :  System.out.print("Enter a string: ");
                        input = keyboard.nextLine();
                        vc = new VowelCons(input);
         }

      } while (Character.toLowerCase(selection) != 'e');

   }

   public static char getMenuSelection()
   {
      String input;     
      char selection;   

      Scanner keyboard = new Scanner(System.in);

      System.out.println("a) Count the number of vowels in the string.");
      System.out.println("b) Count the number of consonants in the string.");
      System.out.println("c) Count both the vowels and consonants in the string.");
      System.out.println("d) Enter another string.");
      System.out.println("e) Exit the program.");

      input = keyboard.nextLine();
      selection = input.charAt(0);

      while (Character.toLowerCase(selection) < 'a' || Character.toLowerCase(selection) > 'e')
      {
         System.out.print("Only enter a, b, c, d, or e: ");
         input = keyboard.nextLine();
         selection = input.charAt(0);
      }

      return selection;
   }
}

Answer 1

看起来您正在初始化本地数组 result，但随后尝试从成员数组 result 中读取。由于您尚未初始化成员一，它仍然是 null，因此您会看到 java.lang.NullPointerException。

您可能希望将 countVowelsAndCons 更改为 void 返回类型，并删除本地结果。然后，您需要确保在尝试调用 getNumVowels 或 getNumConsonants 之前调用它。顺便说一句，像 index 这样的东西应该是成员函数中的局部变量 - 它们不属于类范围。

但更重要的是，这可能根本不应该是一个类。您可能想要类似的东西:

private static boolean isVowel(char c)
{
    return c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u';
}

public static int countConsonants(String s)
{
    int count = 0;
    for(int i=0, len=s.length(); i<len; ++i)
    {
        if(!isVowel(s.charAt(i))) ++count;
    }
    return count;
}

public static int countVowels(String s)
{
    int count = 0;
    for(int i=0, len=s.length(); i<len; ++i)
    {
        if(isVowel(s.charAt(i))) ++count;
    }
    return count;
}