The general format for a reference citation in Harvard style is:
Last name, First Initial. (Year published). Title. City: Publisher, Page(s).
For a web site, this guide gives the following format:
Last name, First initial (Year published). Page title. [online] Website name. Available at: URL [Accessed Day Mo. Year].
If there is no author, you substitute the name of the site:
Website name, (Year published). Page title. [online] Available at: URL [Accessed Day Mo. Year].
(This is different from other styles where you would use something like "____" in place of an author or just omit it.)
For your example, then, the citation would be:
Java™ Platform, Standard Edition 7 API Specification, (2017). Interface Map<K,V>. [online] Available at: https://docs.oracle.com/javase/7/docs/api/java/util/Map.html [Accessed Dec 3, 2017].
I didn't find a clearly-authoritative source for Harvard Style; the site I used seems to be pretty popular.