str.substring(beginIndex,endIndex)-008

// 将字符串str前n位放在后面，返回新的字符串
    public String headToTail(String str,int n){
        if(n==0){
            System.out.println(str);
            return str;
        } else{
            System.out.println(str.substring(n, str.length()) + str.substring(,n));
            return str.substring(n, str.length()) + str.substring(0,n);
        }
    }
//  The substring begins at the specified beginIndex and extends to the character at index endIndex - 1.
// Thus the length of the substring is endIndex-beginIndex.
// 0 --> n-1  + n -->str.length()