This week's giveaway is in the OCPJP forum. We're giving away four copies of Practice Tests for OCP Java 17 Certification Exam (1Z0-829) and have Paul Anilprem on-line! See this thread for details.
Hashtable is thread-safe Abhi, and it was there in java since 1.0, you can see its naming convention "t" in Hashtable is written in small. It was designed such that it will not allow any null key or null value.
But HashMap is added in java 1.2 version, so designer thought null should be allowed. Reason I do not know, any sheriff or knowledgeable guy could say the reason.