A small question in java

package com.newer.cjl.api;  
  
public class zuoye1 {  
  
      
    public static void main(String[] args) {  
        String s="abcdefg";  
        String s1="";  
        for(int i=s.length()-1;i>=0;i--){  
            char c = s.charAt(i);  
            s1=s1+c;  
        }  
        System.out.println(s1);  
          
          
    }  
}  
package com.newer.cjl.api;  
  
public class zuoye1 {  
  
      
    public static void main(String[] args) {  
        String s="abcdefg";  
        String s1="";  
        for(int i=0;i<=s.length()-1;i++){  
            char c = s.charAt(i);  
            s1=c+s1;  
        }  
        System.out.println(s1);  
          
          
    }  
}

import java.util.Scanner;  
  
public class zuoye2 {  
    public static void main(String[] args) {  
          
        Scanner sc = new Scanner(System.in);  
        System.out.println("请输入字符串：");  
        String str = sc.next();  
        boolean isOK=true;  
        for(int i=0;i<str.length()/2;i++){  
            if(str.charAt(i)!=str.charAt(str.length()-1-i)){  
                System.out.println("不是回文字符串！");  
                isOK=false;  
                break;  
            }  
        }  
          
        if(isOK){  
            System.out.println("是一个回文字符串");  
        }  
          
  
    }  
  
}