- A
Use a TreeMap<LocalDateTime, List<Transaction>> to group by timestamp, then flatten on iteration.
Why wrong: This increases memory due to Map and List overhead, and grouping duplicates adds complexity.
- B
Use a SortedSet<Transaction> (java.util.TreeSet) with a comparator to store transactions in sorted order.
Why wrong: TreeSet does not allow duplicate elements, but transactions may have identical timestamps but different amounts/categories, so duplicates could be lost.
- C
Use a parallel stream with .sorted() and collect to a ConcurrentLinkedDeque.
Why wrong: Parallel streams introduce overhead for thread management and concurrent collections have additional memory overhead, increasing memory usage.
- D
Collect transactions into an array (Transaction[]) and sort using Arrays.sort().
An array avoids the overhead of ArrayList's internal array expansion (which may overallocate up to 50%) and uses contiguous memory with no wrapper objects, reducing memory footprint.
Quick Answer
The answer is collecting transactions into an array (Transaction[]) and sorting with Arrays.sort(). This is correct because ArrayList internally allocates a backing array that can be up to 50% larger than the actual element count due to its growth strategy, wasting precious heap space when handling over 10 million objects. An array, by contrast, uses exactly the required contiguous memory with no wrapper overhead, directly reducing peak memory footprint for large data sets. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your understanding of memory optimization for large collections, a common trap where candidates overlook ArrayList's capacity overhead and default to list-based solutions. Remember the memory tip: "Array for exact fit, ArrayList for flexibility"—when heap is tight and data size is known, raw arrays beat collections every time.
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 financial application processes transactions as List<Transaction> objects. The application runs on a server with limited memory (2 GB heap). The development team observes that after processing a large number of transactions (over 10 million), heap usage spikes to near 1.8 GB and garbage collection pauses become frequent (over 5 seconds). The Transaction class is defined as public record Transaction(LocalDateTime timestamp, double amount, String category) {}. The current processing code reads all transactions from a database result set into an ArrayList<Transaction> using a loop with list.add(). Then the list is sorted by timestamp using Collections.sort(list, Comparator.comparing(Transaction::timestamp)). The sorted list is then iterated multiple times to generate various reports. The code runs in a single-threaded context. Which change would most effectively reduce peak memory usage while preserving the sorted report output?
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
Collect transactions into an array (Transaction[]) and sort using Arrays.sort().
Option C is correct because using an array (Transaction[]) avoids the overhead of ArrayList's internal array expansion (which may overallocate up to 50% additional capacity) and uses contiguous memory with no wrapper objects, reducing memory footprint. Option A increases memory due to Map and List overhead. Option B loses duplicate transactions. Option D adds overhead from parallel streams and concurrent collections.
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 a TreeMap<LocalDateTime, List<Transaction>> to group by timestamp, then flatten on iteration.
Why it's wrong here
This increases memory due to Map and List overhead, and grouping duplicates adds complexity.
- ✗
Use a SortedSet<Transaction> (java.util.TreeSet) with a comparator to store transactions in sorted order.
Why it's wrong here
TreeSet does not allow duplicate elements, but transactions may have identical timestamps but different amounts/categories, so duplicates could be lost.
- ✗
Use a parallel stream with .sorted() and collect to a ConcurrentLinkedDeque.
Why it's wrong here
Parallel streams introduce overhead for thread management and concurrent collections have additional memory overhead, increasing memory usage.
- ✓
Collect transactions into an array (Transaction[]) and sort using Arrays.sort().
Why this is correct
An array avoids the overhead of ArrayList's internal array expansion (which may overallocate up to 50%) and uses contiguous memory with no wrapper objects, reducing memory footprint.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
- Use explanations to understand the rule behind the answer.
TExam Day Tips
- Underline the problem statement mentally.
- 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.
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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: Collect transactions into an array (Transaction[]) and sort using Arrays.sort(). — Option C is correct because using an array (Transaction[]) avoids the overhead of ArrayList's internal array expansion (which may overallocate up to 50% additional capacity) and uses contiguous memory with no wrapper objects, reducing memory footprint. Option A increases memory due to Map and List overhead. Option B loses duplicate transactions. Option D adds overhead from parallel streams and concurrent collections.
What should I do if I get this 1Z0-829 question wrong?
Identify which 1Z0-829 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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 →
Same concept, more angles
1 more ways this is tested on 1Z0-829
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A Java 17 application uses a HashMap<Integer, List<String>> to group error messages by error code. The map may contain up to 5000 error codes, and each list may contain up to 1000 messages. The application frequently retrieves the list for a given error code and iterates over it. The lists are rarely modified after initial population. The current performance is acceptable, but the team wants to reduce memory footprint. The developer suggests replacing the inner List<String> with an array (String[]), but then iteration would require conversion. Another developer suggests using a TreeMap instead of HashMap to save memory because TreeMap uses less memory per entry? Actually TreeMap has more overhead. Actually HashMap typically has lower overhead than TreeMap. The correct approach: Since lists are rarely modified, consider using List.of to create immutable lists that can be shared? Or use ArrayList with initial capacity to reduce resizing. But the stem says 'reduce memory footprint'. Option C: Use ArrayList with initial capacity to reduce internal array resizing overhead. Option A: Use TreeMap - wrong because TreeMap uses more memory per entry due to tree nodes. Option B: Use array of arrays - not type-safe. Option D: Use LinkedList - higher memory overhead per element. So best is to use ArrayList with proper sizing to minimize wasted space.
hard- A.Replace ArrayList<String> with LinkedList<String>
- B.Replace HashMap with TreeMap
- C.Replace List<String> with String[]
- ✓ D.Use ArrayList<String> with initial capacity set to expected list size
Why D: Option D is correct because using an ArrayList with an initial capacity set to the expected list size eliminates the overhead of repeated internal array resizing (which wastes memory by leaving unused capacity). Since the lists are rarely modified after initial population, this approach minimizes wasted space without sacrificing iteration performance, unlike the other options.
Last reviewed: Jun 24, 2026
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