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 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.
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.
Clue: "minimum / minimize"
Why it matters: Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
Replace ArrayList<String> with LinkedList<String>
Why wrong: LinkedList has higher per-element memory overhead.
Replace HashMap with TreeMap
Why wrong: TreeMap uses more memory per entry.
Replace List<String> with String[]
Why wrong: Reduces flexibility; iteration requires conversion; may not save much memory.
Use ArrayList<String> with initial capacity set to expected list size
Minimizes internal array resizing and waste.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Use ArrayList<String> with initial capacity set to expected list size
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Replace ArrayList<String> with LinkedList<String>
Why it's wrong here
LinkedList has higher per-element memory overhead.
Replace HashMap with TreeMap
Why it's wrong here
TreeMap uses more memory per entry.
Replace List<String> with String[]
Why it's wrong here
Reduces flexibility; iteration requires conversion; may not save much memory.
Use ArrayList<String> with initial capacity set to expected list size
Why this is correct
Minimizes internal array resizing and waste.
Clue confirmation
The clue words "best", "minimum / minimize" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
The trap here is that candidates assume switching to a different collection type (like TreeMap or LinkedList) will save memory, when in fact the most effective memory optimization is to eliminate wasted internal capacity by using ArrayList with a proper initial capacity.
Detailed technical explanation
HashMap internally uses an array of Node objects, each storing key, value, hash, and next pointer; the load factor (default 0.75) causes the array to be larger than the entry count, wasting memory. ArrayList uses an Object[] array that grows by 50% when full; setting initial capacity to the exact expected size avoids this growth overhead and unused tail slots. In Java 17, immutable lists from List.of() are compact but cannot be modified, which is acceptable here since lists are rarely modified after population, but the question's focus on 'reducing memory footprint' is best addressed by sizing the ArrayList correctly.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
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FAQ
Working with Arrays and Collections — This question tests Working with Arrays and Collections — Read the scenario before looking for a memorised answer..
The correct answer is: Use ArrayList<String> with initial capacity set to expected list size — 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "best", "minimum / minimize". 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.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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