Question 356 of 513
1Z0-829 Working with Arrays and Collections Practice Question
Which TWO statements about HashMap are true? (Select two.)
⚠ Common exam trap
Many exam-takers confuse HashMap with TreeMap (which does not allow null keys) or with ConcurrentHashMap (which does not allow null keys or values), and may incorrectly assume HashMap is thread-safe because it is widely used in single-threaded contexts.
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
✓
It allows null values.
HashMap allows null values; you can store null as a value for any key, including null keys. This is explicitly permitted by the HashMap specification, which does not restrict value types.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
It allows null values.
Why this is correct
HashMap allows null values for any key.
- ✗
It uses red-black trees for collision resolution.
Why it's wrong here
HashMap uses linked lists and trees only when collisions exceed threshold (TREEIFY_THRESHOLD), but it's not always trees.
- ✗
It guarantees insertion order.
Why it's wrong here
LinkedHashMap guarantees insertion order; HashMap does not.
- ✓
It allows null keys.
Why this is correct
HashMap permits one null key and multiple null values.
- ✗
It is thread-safe.
Why it's wrong here
HashMap is not thread-safe; ConcurrentHashMap or synchronized map is needed for concurrent access.
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Last reviewed: Jun 25, 2026
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