public int compare(User u1, User u2) {    if (u1.number.charAt(0) == u2.number.charAt(0)) {        // 如果两人的号都是A开头或者都是V开头,比较号的大小:    if (u1.number.length()==u2.number.length()){        return u1.number.compareTo(u2.number);}    else {        String a= u1.number.substring(1,u1.number.length());        String b= u2.number.substring(1,u2.number.length());        return b.compareTo(a);    }    }    if (u1.number.charAt(0) == 'V') {        // u1的号码是V开头,优先级高:        return -1;    } else {        return 1;    }}