字符串单个字符出现次数最多算法

public class CountStringMaxSign {    public String getMaxCountSign(String str){    if(str==null || str.length()==0)return null;    String result = null;    char chars[] = str.toCharArray();    int length = chars.length;    HashMap map = new HashMap();    int max = 0;    String key = null;    for(int i=0;i<length;i++){     String sign = String.valueOf(chars[i]);     //System.out.println(sign);     if(map.containsKey(sign)){      int value = Integer.parseInt(map.get(String.valueOf(sign)).toString());      value++;      map.put(sign, value);      if(value>max){       max = value;       key = String.valueOf(sign);      }     }else{      map.put(sign, 1);     }    }    result = (key==null?"":"["+key+"][").concat(String.valueOf(max)).concat("]");    return result;   }   public static void main(String[] args) {    CountStringMaxSign test = new CountStringMaxSign();    System.out.println(test.getMaxCountSign("abcdefgabcbbcdd"));   }  }