java hashtable 数据统计，计数功能

package test;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Hashtable;
import java.util.List;

class Counter{
    int i = 1;

    @Override
    public String toString(){
        return Integer.toString(i);
    }
}
public class Statistics {
    public static void main(String[] args) {
        List counter = new ArrayList(Arrays.asList(1, 2, 3, 1, 3, "b", "aa", "b", "b"));
        Hashtable ht =new Hashtable();
        // 统计数组中每个元素出现的次数
        for (int i = 0; i < counter.size(); i++) {
            if(ht.containsKey(counter.get(i))){
                ((Counter)ht.get(counter.get(i))).i++;
            }else{
                ht.put(counter.get(i), new Counter());
            }
        }

        // 生成20个30以内的随机数统计出现频次
        for (int i = 0; i < 20; i++) {
            int r = (int)(Math.random() * 30);
            if(ht.containsKey(r)){
                ((Counter)ht.get(r)).i++;
            }else{
                ht.put(r, new Counter());
            }
        }
        System.out.println(ht);
    }
}