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 TWO statements about HashMap are true? (Select two.)
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.
Option A is correct because 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.
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.
✓
It allows null values.
Why this is correct
HashMap allows null values for any key.
Related concept
Read the scenario before looking for a memorised answer.
✗
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.
Related concept
Read the scenario before looking for a memorised answer.
✗
It is thread-safe.
Why it's wrong here
HashMap is not thread-safe; ConcurrentHashMap or synchronized map is needed for concurrent access.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often 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.
Detailed technical explanation
How to think about this question
HashMap stores entries in an array of buckets, using the key's hashCode() to determine the bucket index. When collisions occur, entries are stored in a linked list within the bucket; if the list length exceeds 8 and the array size is at least 64, the list is converted to a red-black tree for performance. The ability to store null keys is handled by a special sentinel value (a static final Object called NULL_KEY) that maps to bucket index 0.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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: It allows null values. — Option A is correct because 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.
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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Question Discussion
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