用五个线程求和1-100用synchronized应该怎样上锁？

Question

package javaapplication31;




public class JavaApplication31 {

    
    public static void main(String[] args) {
       QiuheThread q[]=new QiuheThread[5];
       for(int i=0;i<5;i++)
       {
           q[i]=new QiuheThread(i);
           q[i].start();
       }
       int he=q[0].sum+q[1].sum+q[2].sum+q[3].sum+q[4].sum;
       System.out.print("1-100求和为:"+he+"\n");
    }
    
}
class QiuheThread extends Thread

{
    int index;
    int sum;
    public QiuheThread(int nindex)
    {
        index=nindex;
    }
    
    public  void run()
    {
        synchronized(this)
        {
            for(int i=0;i<20;i++)
            {
                sum+=index*20+i+1;  
            }
        }
    }
   
}```
代码如上，但并没有上锁成功，还是多线程同时运行？应该怎么改！🙏

Answer 1

对结果的写入进行上锁