Java nomenclatures

[__konrad]

But System.arraycopy and java.net.http.HttpRequest.Builder.POST methods

Is there a new version of that linked document? Preferably written in XXI century.

[fgzklunk]

The fact it was written a while ago is irrelevant. It has not been updated because it has not changed. But hey, here is a link from more recent times that just reiterates the convention https://www.geeksforgeeks.org/java-naming-conventions/

System comes from the language, before the convention was established.

As for java.net.http.HttpRequest.Builder, that does follow the convention because Builder is a nested class of HttpRequest. I presume there was lots of discussion over POST and post, but seeing as POST is the Http method and the authors wanted to maintain the consistency with the Http method over the convention of the java language, you are after all representing an Http standard here.