找出字符串中最长的对称字符串

 public static String reverseStr(String str) {
        char[] chars = str.toCharArray();
        StringBuffer sb = new StringBuffer();
        for (int i = chars.length - 1; i >= 0; i--) {
            sb.append(chars[i]);
        }
        return sb.toString();
    }

    public static String test() {
        String str = "qwertyuiop1234567890987654321zxcvbnm,";
        //System.out.println(str);
        String max = "";
        int count = 0;
        //起始位置开始扫描
        for (int i = 0; i < str.length(); i++) {
            //结束位置开始扫描
            for (int j = str.length(); (j >= i)&(j-i>=max.length()); j--) {
                count++;
                String substr = str.substring(i, j);
                //截取一个字符串
                int length = substr.length();
                //判断是否对称
                int middle = 0;
                if (substr.length() % 2 == 0) {
                    middle = substr.length() / 2;
                } else {
                    middle = substr.length() / 2 + 1;
                }
//如果对称赋值给max
                if (substr.substring(0, substr.length() / 2).equals(
                        reverseStr(substr.substring(middle)))) {
                    if(max.length()<substr.length()){
                        max = substr;
                    }
                }
            }
        }
        System.out.println(count);
        return max;
    }