Question 488 of 513
1Z0-829 Working with Arrays and Collections Practice Question
Which interface provides the ability to store key-value pairs and allows null keys?
⚠ Common exam trap
Many candidates confuse HashMap's allowance of null keys with Hashtable's historical prohibition, or assume that all Map implementations support null keys, when in fact only HashMap (and LinkedHashMap) permit null keys among the commonly used implementations.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
HashMap
HashMap is the correct answer because it implements the Map interface, allows null keys (only one null key is permitted), and does not synchronize its operations, making it suitable for unsynchronized key-value storage. In contrast, ConcurrentHashMap, TreeMap, and Hashtable all prohibit null keys for various reasons, such as thread-safety guarantees or sorting requirements.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
ConcurrentHashMap
Why it's wrong here
Does not allow null keys.
- ✗
TreeMap
Why it's wrong here
Does not allow null keys.
- ✓
HashMap
Why this is correct
Allows null keys.
- ✗
Hashtable
Why it's wrong here
Does not allow null keys.
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Last reviewed: Jun 11, 2026
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