求字符串中出现最多次数的字母

//求字符串中出现最多次数的字母，如果有多个，则都求出。BY MYSELF 已测试
import java.util.*;
public class FindValueOfMaxTimes{
    public static void main(String args[]){
        String input = "asdddgghhh";
        doString(input);
    }
    public static void doString(String input){
        char[] chars = input.toCharArray();
        TreeSet set = new TreeSet();
        for(int i=0; i<chars.length; i++){
            set.add(String.valueOf(chars[i]));//String.valueOf
        }
        Arrays.sort(chars);
        input = new String(chars);
        int max = 0 ;
        String maxString="";
        ArrayList maxlist = new ArrayList();
        Iterator it = set.iterator();
        while(it.hasNext()){
            String os = (String)it.next();//String。返回Object，需要强制转换
            int i = input.indexOf(os);
            int j = input.lastIndexOf(os);
            int times = j-i+1;
            if(times>max){
                max=times;    
                maxString=os;
                maxlist.add(os);        
            }else if(times==max){
                maxlist.add(os);
            }
        }
        int index=0;
        for(int i=0; i<maxlist.size(); i++){
            if(maxlist.get(i).equals(maxString)){//list可以get(i),set不行?必须用equals比较，不能用==
                index=i;
                break;
            }
        }
        for(int i=index; i<maxlist.size(); i++){
            System.out.println(maxlist.get(i)+" ");
        }
        System.out.println(max);
    }
}