- A
Keep List<Transaction> and add a separate HashSet<Long> of ids for fast lookup. Use the list for iteration.
Why wrong: Maintains order but requires two collections; can work but option A is cleaner and more efficient.
- B
Replace List<Transaction> with LinkedHashMap<Long, Transaction> mapping id to transaction. Iterate using values() which preserves insertion order.
LinkedHashMap provides O(1) lookup and maintains insertion order.
- C
Keep List<Transaction> and sort it by id. Use Collections.binarySearch() for lookup.
Why wrong: Sorting by id loses insertion order; binary search requires sorted list.
- D
Replace List<Transaction> with TreeMap<Long, Transaction> mapping id to transaction. Iterate using values() which returns in natural key order.
Why wrong: TreeMap sorts by key, not insertion order.
Quick Answer
The answer is to replace the List<Transaction> with a LinkedHashMap<Long, Transaction>, mapping each transaction ID to its object. This is correct because LinkedHashMap provides O(1) key-based lookup for fast existence checks, directly solving the performance bottleneck of linear search in a list, while its values() view preserves insertion order for chronological iteration. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your understanding of Map implementations and their iteration guarantees—a common trap is choosing HashMap for speed but forgetting it does not maintain order, or TreeMap for sorted order when only insertion order is needed. Remember the memory tip: “LinkedHashMap links order to insertion, giving you both speed and sequence.”
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 in batches. Each transaction is represented as a Transaction object with fields: long id, BigDecimal amount, LocalDateTime timestamp. Transactions are stored in a List<Transaction> in the order they arrive. The system needs to frequently check if a transaction with a specific id exists, and also needs to iterate through transactions in chronological order. The list currently contains millions of transactions, and the existence check is becoming a performance bottleneck because it currently uses a linear search. The system must also maintain insertion order for iteration. Which approach best improves the performance of the existence check while maintaining the required iteration order?
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 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
Replace List<Transaction> with LinkedHashMap<Long, Transaction> mapping id to transaction. Iterate using values() which preserves insertion order.
Option B is correct because LinkedHashMap maintains insertion order for iteration via its values() view, while providing O(1) key-based lookup for the existence check. This directly addresses the performance bottleneck of linear search in a List without sacrificing the required chronological iteration order.
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.
- ✗
Keep List<Transaction> and add a separate HashSet<Long> of ids for fast lookup. Use the list for iteration.
Why it's wrong here
Maintains order but requires two collections; can work but option A is cleaner and more efficient.
- ✓
Replace List<Transaction> with LinkedHashMap<Long, Transaction> mapping id to transaction. Iterate using values() which preserves insertion order.
Why this is correct
LinkedHashMap provides O(1) lookup and maintains insertion order.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Keep List<Transaction> and sort it by id. Use Collections.binarySearch() for lookup.
Why it's wrong here
Sorting by id loses insertion order; binary search requires sorted list.
- ✗
Replace List<Transaction> with TreeMap<Long, Transaction> mapping id to transaction. Iterate using values() which returns in natural key order.
Why it's wrong here
TreeMap sorts by key, not insertion order.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may confuse TreeMap's natural ordering with insertion order, or assume that sorting a List and using binary search is the best optimization, overlooking the O(1) lookup advantage of hash-based structures like LinkedHashMap.
Detailed technical explanation
How to think about this question
LinkedHashMap extends HashMap and uses a doubly linked list to maintain the order of entries, either insertion order or access order. When iterating via values(), it returns elements in the order they were inserted, which matches the chronological requirement. The containsKey() method on LinkedHashMap runs in O(1) average time due to hash-based bucket lookup, making it ideal for frequent existence checks on millions of transactions.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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: Replace List<Transaction> with LinkedHashMap<Long, Transaction> mapping id to transaction. Iterate using values() which preserves insertion order. — Option B is correct because LinkedHashMap maintains insertion order for iteration via its values() view, while providing O(1) key-based lookup for the existence check. This directly addresses the performance bottleneck of linear search in a List without sacrificing the required chronological iteration order.
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.
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Last reviewed: Jun 11, 2026
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.
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