1.代码实现：

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
public class ListUtilEme {
    public static Map<String,Integer> frequencyOfListElements(List<String> items ) {
        if (items == null || items.size() == 0) return null;
        Map<String, Integer> map = new HashMap<String, Integer>();
        for (String temp : items) {
            Integer count = map.get(temp);
            map.put(temp, (count == null) ? 1 : count + 1);
        }
        return map;
    }
    public static void main(String[] args) {
        List<String>  list = new ArrayList<>();
        list.add("aa");
        list.add("bb");
        list.add("bb");
        Map<String,Integer> map = ListUtilEme.frequencyOfListElements(list);
        for(String key: map.keySet()){
            System.out.println(key+":"+map.get(key));
        }
    }
}

2.代码运行图：