public static void main(String[] args) {

        String s = "aaaa......bbbb....ccc...dddddd";

        String s2 = s.replaceAll("\\.+","");

        System.out.println(s2);                                     // s2:aaaabbbbcccdddddd

        String s3 = s2.replaceAll("(.)\\1+","$1");  // $1代表第一组中的内容

        System.out.println(s3);                                     //s3:abcd

    }