问题： 出现ClassCastException
见下图 ：
图片描述

程序代码如下：

import java.util.*;
import net.mindview.util.*;

class Words 
{
        String s ; 
        int i ; 
        public Words (String s , int i )
        {
                this . s = s ;
                this . i = i ;
        }
}
public class HoldingEx22 {
        public static void main(String[] args)
        {
                Set<String> words = new TreeSet<String>(
                        new TextFile("SetOperations.java", "\\W+"));
                System.out.println(words);
                
                Set <Words> wordSet = new TreeSet <Words> ( ) ;
                System.out.println( );
                for (String s : words)
                {
                        int count  = 0; 
                        for (String m : words)
                        {
                                if (s . equals (m) )
                                {
                                        count ++ ;
                                }
                        }
                        Words w = new Words (s , count - 1);
                        wordSet . add (w ) ; 
                }
                System.out.println(wordSet);
        }
}