Question 176 of 513
1Z0-829 Working with Arrays and Collections Practice Question
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?
⚠ Common exam trap
A common mix-up: candidates 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.
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
✓
Use LinkedHashMap<String, Long> with access-order=true and override removeEldestEntry() to evict entries older than 30 minutes.
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.
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.
- ✗
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.
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Last reviewed: Jun 11, 2026
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