JAVA如何看懂这个程序？

import java.util.*;
public class Main2{
public static void main(String[] args)
{

    int sum =0;   
    int numbers;
    Scanner sc =new Scanner(System.in);        
    while(sc.hasNextInt()){
          int a =sc.nextInt();
         
          for(;;a--){
          while(a!=0){
               
              numbers =sc.nextInt();     
               sum+=numbers;                                
                  
               }
          System.out.print(sum);
             }
          }
      }
   }
祝大家新年快乐，有谁能看懂这个程序吗？它是怎么运行的？求解释