字符串單個字符出現次數最多算法

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"));
    }
}