k2datas 基础编程题，判断字符串是否有重复串

package String;

public class DuplicateString {
    public static  boolean isDup(String s) throws Exception {
        if(s==null) {
            return false;
        }
        if(s.length() >10000){
            throw new Exception("字符串长度超过10000");
        }
        for(int k =0;k<s.length();k++){
            char c =s.charAt(k);
            if(!Character.isLowerCase(c)){
                throw new Exception("字符串必须全是小写字母");
            }
        }

        int len = s.length();
        for(int i =0 ;i<len;i++){
            int j = i+1;
            String sI = s.substring(i,i+1);
            for(;j< len;j++){
               String sJ = s.substring(j,j+1);
               if(sI.equals(sJ)){
                   break;
               }
            }
            if(j< len){
                String s1 = s.substring(i,j);
                String s2 = s.substring(j,2*j-i>len?len:2*j-i);
                if (s1.equals(s2)) {
                    return true;
                }
            }
        }
        return false;
    }

    public static void main(String[] args) throws Exception {
        String s = "abc";
        System.out.println(isDup(s));

    }

}