每次都出现这个问题Exception in thread "main" java.lang.NoSuchMethodError:main

语法没问题 ，就是两个类的问题，麻烦帮我改一下 并解释一下。（在class QSDemo前面家public也有错误）


 class Quicksort
{
    static void qsort(char items[])
    {
        qs(items,0,items.length-1);
    }
    private static void qs(char items[],int left,int right)
    {
        int i,j;
        char x,y;
        i=left;j=right;
        x=items[(left+right)/2];
        
        do
        {
            while((items[i] < x) && (i < right))i++;
            while((x < items[j]) && (j > left))j++;

            if(i<=j)
            {
                y=items[i];
                items[i]=items[j];
                items[j]=y;
                i++;j--;
            }
        }while(i<=j);

        if(left < j)qs(items,left,j);
        if(i < right)qs(items,i,right);
    }
}   
 class QSDemo
    {   
        public static void main(String args[])
        {
            char a[]={'b','d','b','z','m','g'};
            int i;
            System.out.print("Original array:");
            for(i=0;i<a.length;i++)
                System.out.print(a[i]);
            System.out.println();
            Quicksort.qsort(a);
            
            System.out.print("Sorted array:");
            for(i=0;i<a.length;i++)
                System.out.print(a[i]);
        }
    }