怎么用java实现把一个字符串按照字符出现个数排序

你可以使用HashMap来统计每个字符出现的次数，然后将其放入一个List中，再根据出现次数进行排序。以下是示例代码：

import java.util.*;

public class SortStringByFrequency {
    public static void main(String[] args) {
        String str = "hello world";
        List<Map.Entry<Character, Integer>> list = new ArrayList<>();
        Map<Character, Integer> map = new HashMap<>();

        for (int i = ; i < str.length(); i++) {
            char c = str.charAt(i);
            map.put(c, map.getOrDefault(c, ) + 1);
        }

        list.addAll(map.entrySet());
        Collections.sort(list, (a, b) -> b.getValue() - a.getValue());

        StringBuilder sb = new StringBuilder();
        for (Map.Entry<Character, Integer> entry : list) {
            char c = entry.getKey();
            int count = entry.getValue();
            for (int i = ; i < count; i++) {
                sb.append(c);
            }
        }

        System.out.println(sb.toString());
    }
}