1Z0-829 Working with Arrays and Collections Practice Question
This 1Z0-829 practice question tests your understanding of working with arrays and collections. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
A web server logs user sessions. Each session has a unique session ID (String) and a last access time (long). The system needs to evict sessions that have been inactive for more than 30 minutes. The current implementation uses a HashMap<String, Long> to store session IDs and last access times. A scheduled task iterates over all entries and removes those where currentTime - lastAccess > 30 minutes. However, this iteration is becoming slow as the number of sessions grows (millions). The developer wants to improve the eviction performance without affecting the O(1) put and get operations. Which approach should be taken?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use LinkedHashMap<String, Long> with access-order=true and override removeEldestEntry() to evict entries older than 30 minutes.
Option C is correct because LinkedHashMap with access-order=true and a custom removeEldestEntry() method provides O(1) amortized put/get operations while automatically evicting the least-recently accessed entry when a condition is met. By overriding removeEldestEntry() to check if the eldest entry's last access time is older than 30 minutes, the eviction happens during put operations without requiring a separate iteration over all entries, thus solving the performance issue.
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.
✗
Use PriorityQueue<Session> ordered by last access time and update the time when accessed.
Why it's wrong here
PriorityQueue does not support efficient lookup by session ID.
✗
Use ConcurrentHashMap<String, Long> and use parallel streams to remove expired entries.
Why it's wrong here
Still requires iteration over all entries.
✓
Use LinkedHashMap<String, Long> with access-order=true and override removeEldestEntry() to evict entries older than 30 minutes.
Why this is correct
Efficiently maintains access order and automatically evicts eldest.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use TreeMap<String, Long> sorted by last access time and poll the first entry if too old.
Why it's wrong here
TreeMap does not provide O(1) get/put; sorting by time is not straightforward.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may think a sorted structure like TreeMap or PriorityQueue is needed for time-based eviction, but they overlook the O(1) access-time requirement and the fact that LinkedHashMap's access-order mode provides automatic LRU eviction without explicit iteration.
Detailed technical explanation
How to think about this question
LinkedHashMap with access-order=true maintains a doubly linked list of entries in the order of last access, so the eldest entry is always the least-recently accessed. The removeEldestEntry() method is called after each put() and putAll() invocation, allowing conditional eviction in O(1) time. This pattern is a classic implementation of an LRU (Least Recently Used) cache, which is ideal for session eviction where the oldest accessed session should be removed first.
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 practitioner preparing for the 1Z0-829 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
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: Use LinkedHashMap<String, Long> with access-order=true and override removeEldestEntry() to evict entries older than 30 minutes. — Option C is correct because LinkedHashMap with access-order=true and a custom removeEldestEntry() method provides O(1) amortized put/get operations while automatically evicting the least-recently accessed entry when a condition is met. By overriding removeEldestEntry() to check if the eldest entry's last access time is older than 30 minutes, the eviction happens during put operations without requiring a separate iteration over all entries, thus solving the performance issue.
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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