仅供参考，勿要模仿

import java.io.BufferedReader;
import java.io.FileReader;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.Map;
import java.util.TreeMap;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class WordsCounter {
	public static void main(String[] args) throws Exception {

		BufferedReader reader = new BufferedReader(new FileReader(
				"main.txt"));
		StringBuffer buffer = new StringBuffer();
		String line = null;
		while ((line = reader.readLine()) != null) {
			buffer.append(line);
		}
		reader.close();
		Pattern expression = Pattern.compile("[a-zA-Z]+");// 定义正则表达式匹配单词
		String string = buffer.toString();
		Matcher matcher = expression.matcher(string);//
		Map<String, Integer> map = new TreeMap<String, Integer>();
		String word = "";
		int times = 0;
		// 是否匹配单词
		while (matcher.find()) {
			// 得到一个单词-树映射的键
			word = matcher.group();
			// 如果包含该键，单词出现过
			if (map.containsKey(word)) {
				// 得到单词出现的次数
				times = map.get(word);
				map.put(word, times + 1);
			} else {
				// 否则单词第一次出现，添加到映射中
				map.put(word, 1);
			}
		}
		
		/*
		 * 排序
		 */
		List<Map.Entry<String, Integer>> list = new ArrayList<Map.Entry<String, Integer>>(
				map.entrySet());
		/*
		 * 定义排序规则，按照Entry的Value排序
		 */
		Comparator<Map.Entry<String, Integer>> comparator = new Comparator<Map.Entry<String, Integer>>() {
			public int compare(Map.Entry<String, Integer> left,
					Map.Entry<String, Integer> right) {
				return (left.getValue()).compareTo(right.getValue());
			}
		};
		// 利用Collections类的sort方法来排序，按照自定义的规则排序
		Collections.sort(list, comparator);
		// 打印最多五个
		int last = list.size() - 1;
		for (int i = last; i > last - 5; i--) {
			String key = list.get(i).getKey();
			Integer value = list.get(i).getValue();
			System.out.println(key + " :" + value);
		}
	}
}