Question 1easymultiple choice
Full question →- A
ConcurrentHashMap
Why wrong: Does not allow null keys.
- B
TreeMap
Why wrong: Does not allow null keys.
- C
HashMap
Allows null keys.
- D
Hashtable
Why wrong: Does not allow null keys.
Question 479 of 509
1Z0-829 Working with Arrays and Collections Practice Question
This 1Z0-829 practice question tests your understanding of working with arrays and collections. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Which interface provides the ability to store key-value pairs and allows null keys?
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Hashtable
Why it's wrong here
Does not allow null keys.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often 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.
Detailed technical explanation
How to think about this question
Under the hood, HashMap uses an array of Node<K,V> entries and a hash function to compute bucket indices; null keys are stored in bucket 0 with a special hash value of 0. This design allows HashMap to handle null keys efficiently, but it also means that if you need thread-safe operations or sorted keys, you must use ConcurrentHashMap or TreeMap respectively, which enforce non-null keys to maintain their invariants. In real-world scenarios, using null keys can simplify data modeling (e.g., representing a default or unknown entry), but it can also lead to subtle bugs if the map is later passed to a concurrent context.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this 1Z0-829 question test?
Working with Arrays and Collections — This question tests Working with Arrays and Collections — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 1Z0-829 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
This 1Z0-829 practice question is part of Courseiva's free Oracle certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 1Z0-829 exam.
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