利用集合類，其中按照map中value排列需要考慮一下具體思路，最后結合list完成排列

/** * Created by changqing on 2017/3/4. */import java.io.*;import java.util.*;public class test {    public static void main(String [] args) throws Exception {        System.out.PRintln("starting to read file");        File file=new File("test.txt");        BufferedReader bf=new BufferedReader(new FileReader(file));        String line;        String reg="//s+";//正則表達式，把句子劃分為一個個單詞        TreeMap<String,Integer> tm= new TreeMap<String, Integer>();        while((line=bf.readLine())!=null) {           String []strs=line.split(reg);            for(String str:strs)            {                str=str.replaceAll("[//p{Punct}//pP]", ""); //去掉單詞中含有的標點符號                if(!tm.containsKey(str))                {                    tm.put(str,1);                }                else                    tm.put(str,tm.get(str)+1);            }        }        List<Map.Entry<String,Integer>> list = new ArrayList<Map.Entry<String,Integer>>(tm.entrySet());        Collections.sort(list, new Comparator<Map.Entry<String, Integer>>() {            public int compare(Map.Entry<String, Integer> o1,                               Map.Entry<String, Integer> o2) {                return (o2.getValue() - o1.getValue());            }        });        for (int i = 0; i <20 ; i++) {            System.out.println(list.get(i).getKey());        }    }}