笔试题：输入一个字符串，输出出现次数最多的前2个字符及出现次数

参考答案： 

package com.qzcsbj;

import java.util.*;

public class Test {
    public static void main(String[] args) {
        System.out.print("请输入一个字符串：");
        Scanner sc = new Scanner(System.in);
        String str = sc.nextLine();

        Map<String, Integer> map = new HashMap<String, Integer>();

        for (int i = 0; i < str.length(); i++) {
            String c = str.charAt(i) + "";
            map.put(c, map.containsKey(c) ? map.get(c) + 1 : 1);
        }


        Set<Map.Entry<String, Integer>> entries = map.entrySet();
        ArrayList<Map.Entry<String, Integer>> list = new ArrayList<Map.Entry<String, Integer>>(entries);
        Collections.sort(list,new Comparator<Map.Entry<String, Integer>>() {  // 匿名内部类
            @Override
            public int compare(Map.Entry<String, Integer> obj1,
                               Map.Entry<String, Integer> obj2) {
                if(obj1.getValue() > obj2.getValue()){
                    return -1;
                }else if(obj1.getValue() < obj2.getValue()){
                    return 1;
                }
                return 0;
            }
        });

        int len = list.size();
        if(len<2){
            System.out.println("字符串中只出现了" + len + "种字符");
            for(int i=0;i<len;i++){
                Map.Entry<String, Integer> entry = list.get(i);
                if (" ".equals(entry.getKey())){
                    System.out.println("空格="+entry.getValue());
                }else {
                    System.out.println(entry.getKey()+"="+entry.getValue());
                }
            }
        }
        else {
            System.out.println("出现次数最多的前2：");
            for(int i=0;i<2;i++){
                Map.Entry<String, Integer> entry = list.get(i);
                if (" ".equals(entry.getKey())){
                    System.out.println("空格="+entry.getValue());
                }else {
                    System.out.println(entry.getKey()+"="+entry.getValue());
                }
            }
        }

    }
}