The answer is to keep the HashMap and sort with Collections.sort when a report is needed. This is correct because HashMap offers O(1) insertion performance, which is critical when handling 10 million entries, while TreeMap’s O(log n) insertion cost per entry creates unacceptable overhead for continuous writes. Sorting on demand with Collections.sort, which uses TimSort (O(n log n)), is efficient for infrequent reports and avoids the constant sorting penalty of a TreeMap. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this scenario tests your understanding of choosing data structures based on access patterns—specifically, that TreeMap is ideal for always-sorted data, but HashMap with deferred sorting is better when writes dominate and reads are rare. A common trap is assuming TreeMap is always faster for sorted output; remember that insertion cost compounds with volume. Memory tip: “Sort only when you report—HashMap for the sport, TreeMap for the court.”
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.
A company's Java 17 application processes large log files and stores word counts. Initially, they used a TreeMap<String, Integer> to maintain sorted word counts. After adding 10 million entries, insertion performance became unacceptably slow. The team switched to a HashMap<String, Integer> for fast insertions, but now they need to produce sorted reports. They are considering two approaches: (1) Keep the HashMap and, when a sorted report is needed, extract all entries into an ArrayList<Map.Entry<String, Integer>> and sort it using Collections.sort with a comparator, or (2) Use a ConcurrentSkipListMap instead. The application is single-threaded, and reports are requested infrequently. What is the best course of action?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use HashMap and sort with Collections.sort when needed
Option C is correct because the application is single-threaded and reports are infrequent, so the overhead of sorting a HashMap's entries only when needed is acceptable and avoids the continuous insertion cost of a TreeMap. HashMap provides O(1) insertions, and sorting an ArrayList of 10 million entries with Collections.sort (which uses TimSort, O(n log n)) is efficient for occasional use, making this the best balance of performance and simplicity.
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 ConcurrentSkipListMap
Why it's wrong here
Overkill for single-threaded; higher overhead than TreeMap.
✗
Use LinkedHashMap and convert to sorted list when needed
Why it's wrong here
LinkedHashMap preserves insertion order, not sorted; still need sort.
✓
Use HashMap and sort with Collections.sort when needed
Why this is correct
Optimal balance: fast inserts, sorting only on demand.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Revert to TreeMap because it always keeps data sorted
Why it's wrong here
TreeMap insertions are O(log n) each, slower for high volume.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume sorted data must always be maintained in a sorted structure, overlooking the performance trade-off between continuous sorting overhead (TreeMap) and on-demand sorting (HashMap) when insertions are frequent and reads are rare.
Detailed technical explanation
How to think about this question
Under the hood, HashMap uses an array of buckets with linked lists or trees (when collisions exceed a threshold) for O(1) average-time insertions, while TreeMap uses a Red-Black tree with O(log n) operations. Collections.sort on an ArrayList uses TimSort, a hybrid stable sorting algorithm that performs well on partially sorted data and has O(n log n) worst-case time, which is acceptable for infrequent reports. The key trade-off is amortizing the sorting cost over many insertions: with 10 million entries, the cost of maintaining sorted order continuously (TreeMap) far exceeds the occasional O(n log n) sort of a HashMap.
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 HashMap and sort with Collections.sort when needed — Option C is correct because the application is single-threaded and reports are infrequent, so the overhead of sorting a HashMap's entries only when needed is acceptable and avoids the continuous insertion cost of a TreeMap. HashMap provides O(1) insertions, and sorting an ArrayList of 10 million entries with Collections.sort (which uses TimSort, O(n log n)) is efficient for occasional use, making this the best balance of performance and simplicity.
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.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.