java - 如何解决 "error: bad operand types for binary operator"问题？

Question

该程序旨在检查任何用户字符串的第一个元素然后，检查它是否是“大写字母”或“2到9的数字”

import java.util.*;
public class test1
{
   public static void main (String [] args)
   {
      Scanner mykey= new Scanner (System.in);
      System.out.println("Enter your first sentence");
      String firstSen= mykey.nextLine();
      String firstChar= firstSen.substring(0,1);
      if ((firstChar <='Z') && (firstChar >= 'A'))
         {System.out.println("Its a letter");}
      else if ((firstChar>='2') && (firstChar<='9'))
         {System.out.println("Its a number");}
   }
}

错误

test1.java:10: error: bad operand types for binary operator '<='
      if ((firstChar <='Z') && (firstChar >= 'A'))
                     ^
  first type:  String
  second type: char
test1.java:10: error: bad operand types for binary operator '>='
      if ((firstChar <='Z') && (firstChar >= 'A'))
                                          ^
  first type:  String
  second type: char
test1.java:12: error: bad operand types for binary operator '>='
      else if ((firstChar>='2') && (firstChar<='9'))
                         ^
  first type:  String
  second type: char
test1.java:12: error: bad operand types for binary operator '<='
      else if ((firstChar>='2') && (firstChar<='9'))

Answer 1

substring(0, 1)返回 String你的错误是比较 String到 char使用>=和<= .

您应该获取字符串的第一个字母，而不是 String ，但作为 char ，通过使用charAt方法:

  Scanner mykey= new Scanner (System.in);
  System.out.println("Enter your first sentence");
  String firstSen= mykey.nextLine();
  char firstChar= firstSen.charAt(0); // note this line!
  if ((firstChar <='Z') && (firstChar >= 'A'))
     {System.out.println("Its a letter");}
  else if ((firstChar>='2') && (firstChar<='9'))
     {System.out.println("Its a number");}

两个 char s 可以与 <= 进行比较和>= 。一个String和一个 char不能。