Easy Collections Question 20 of 226

Why can a HashMap have one null key but a Hashtable cannot?

Java Specialist · Speak this in 60–90 seconds · Faridabad & Delhi NCR

PICTURE THIS: HASH MAP

Key"user_id"

Hashslot 17

Valuethe record

Simple meaning

HashMap treats a null key as a special case and stores it in a dedicated bucket.

WHY — Collections instead of guessing?

Why interviewers care about Collections:

They are checking judgment

on Collections.

A good answer names

the situation, the default choice, and one exception - that reads as experience.

Stay structured

Name the idea, why it exists, then one short example.

Close cleanly

End with when you use it and one common pitfall.

STEPS — What happens step by step?

Before you speak the answer, walk the interviewer through these steps:

1
HashMap treats a null
key as a special case and stores it in a dedicated bucket.
2
Hashtable was written to
be synchronized and simply rejects nulls to avoid extra null checks inside locked code.
3
ConcurrentHashMap also disallows nulls
because null would be ambiguous with a missing key during concurrent reads.
4
Give an example
One tiny concrete case you can say aloud.
5
Common mistake
What juniors usually get wrong.
6
Close
When you pick this over the alternative.

EXAMPLE — See it in action

Here's a short line you can speak, broken into clear beats:

Say this line

“Hashtable was written to be synchronized and simply rejects nulls to avoid extra”

Break into beats

Hashtablewaswrittentobesynchronized

Speaking order

2987408337471632900

Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.

Key takeaway

HashMap treats a null key as a special case and stores it in a dedicated bucket. Hashtable was written to be synchronized and simply rejects nulls to avoid extra null checks inside locked code.