Q:

Java api public-suffix-list returns a null domain

I use an java API to extract the second domain name based on the given URLs. 
https://github.com/whois-server-list/public-suffix-list
It based on the PUBLIC SUFFIX LIST of Mozilla.
Given codes below:
PublicSuffixListFactory factory = new PublicSuffixListFactory();
    PublicSuffixList suffixList = factory.build();
    String[] domainArray = new String[10];
    domainArray[0] = "swcdn.apple.com";
    domainArray[1] = "production-pdt.siriusxm.com";
    domainArray[2] = "audio2.spotify.com";
    domainArray[3] = "a1935.phobos.apple.com";
    domainArray[4] = "r13---sn-ab5l6n7r.c.pack.google.com";
    domainArray[5] = "a132.phobos.apple.com";
    domainArray[6] = "a1736.phobos.apple.com";
    domainArray[7] = "r17---sn-vgqs7n7l.c.android.clients.google.com";
    domainArray[8] = "s4.amazonaws.com";
    domainArray[9] = "s3.amazonaws.com";

    for(String string : domainArray){
        String domain = suffixList.getRegistrableDomain(string);
        System.out.println(domain);
    }

I got the following output:
apple.com
siriusxm.com
spotify.com
apple.com
google.com
apple.com
apple.com
google.com
amazonaws.com
null

It works well for most domains, but for the s3.amazonaws.com, it returns null. However for a similar one: s4.amazonaws.com, it give the right result. What's the issue? 

A:

s3.amazonaws.com is included in the public suffix list and therefore considered as not registrable. PublicSuffixList.getRegistrableDomain() returns null in this case:

Returns:
      the registrable domain, null if the domain is not registrable at all

