为什么即使定义了字符串，java也找不到符号？

Question

import java.util.Scanner;

public class DJNameCreator {
   public static void main(String[] args){
   
   //Variables to store inputted names
      String firstName;
      String lastName;
   
      //Scanner for user inputted names
      Scanner keyboard = new Scanner (System.in);
         
      System.out.println("Please enter your first name: ");
      firstName = keyboard.nextLine();
         
      System.out.println("Nice to meet you, "+firstName+"!");
         
      System.out.println("Please enter your last name: ");
      lastName = keyboard.nextLine();
         
      //Use substring and length method to get total length of string
         
      //First name section
      int firstLength = firstName.length();
            
      if (firstLength%2 == 0){
         String firstHalf = firstName.substring(0, firstLength/2);
      }
            
      //Last name section
      int lastLength = lastName.length();
            
      if (lastLength%2 == 0){
         String lastHalf = lastName.substring((lastLength/2), lastLength);
      }
            
      if (lastLength%2 == 1){
         String lastHalf = lastName.substring((lastLength/2), (lastLength)-1);
      }
      //Output the DJ name using println combined with the word "Jayster"
      System.out.println(firstHalf + "Jayster" + lastHalf);
   }
}

我不明白为什么java找不到符号。我的代码在最后一个println之前编译得很好。它特别指向firstHalf和lastHalf。我在if语句中定义了它们，但它不会编译

这是因为if语句与主代码是隔离的吗

Answer 1

在上面的代码段中，firstHalf和secondHalf仅在各自的If块中具有作用域。当您试图在Main的方法范围内打印它时

您可以找到有关作用域here的更多信息

检查下面的重构代码

import java.util.Scanner;

    public class DJNameCreator {
        public static void main(String[] args){
    
            String firstName;
            String lastName;
    
            Scanner keyboard = new Scanner (System.in);
    
            System.out.println("Please enter your first name: ");
            firstName = keyboard.nextLine();
    
            System.out.println("Nice to meet you, "+firstName+"!");
    
            System.out.println("Please enter your last name: ");
            lastName = keyboard.nextLine();
    
           
            int firstLength = firstName.length();

             // FirstHalf is in the scope of main method.
            String firstHalf = "";
            if (firstLength%2 == 0){
                 firstHalf= firstName.substring(0, firstLength/2);
            }
    
            
            int lastLength = lastName.length();
            // LastHalf is in the scope of main method.
            String lastHalf ="";
            if (lastLength%2 == 0){
                lastHalf = lastName.substring((lastLength/2), lastLength);
            }
    
            if (lastLength%2 == 1){
                lastHalf = lastName.substring((lastLength/2), (lastLength)-1);
            }
            //Output the DJ name using println combined with the word "Jayster"
            System.out.println(firstHalf + "Jayster" + lastHalf);
        }
    }