The answer is `Collectors.toMap(s -> s.substring(0,3), Function.identity(), (a,b) -> b)`. This collector is correct because the merge function `(a, b) -> b` resolves key conflicts by discarding the first value `a` and keeping the second value `b`, which represents the later string encountered in the stream order. The `toMap` merge function is invoked automatically whenever duplicate keys arise from the key mapper—here, when two strings share the same three-character prefix. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this pattern tests your understanding of how to handle key collisions in stream collectors, a common scenario when transforming lists to maps. A frequent trap is confusing the merge function’s parameter order: `(a, b) -> b` keeps the later element, while `(a, b) -> a` keeps the earlier one. Memory tip: think “b for later” to remember that returning `b` preserves the subsequent value in the stream.
1Z0-829 Working with Streams and Lambda Expressions Practice Question
This 1Z0-829 practice question tests your understanding of working with streams and lambda expressions. 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 method receives a List<String> and needs to transform it to a Map where the key is the first three characters of each string and the value is the string itself. If two strings have the same prefix, the later one should override the earlier. Which collector achieves this?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Collectors.toMap(s -> s.substring(0,3), Function.identity(), (a,b) -> b)
Option D is correct because `Collectors.toMap` with a merge function `(a, b) -> b` ensures that when duplicate keys (first three characters) occur, the later string in the stream order replaces the earlier one. The merge function is invoked on key collision, and returning `b` (the second argument) keeps the later value, satisfying the requirement.
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.
✗
Collectors.groupingBy(s -> s.substring(0,3))
Why it's wrong here
Returns Map<String, List<String>>, not single values.
Collectors.toMap(s -> s.substring(0,3), Function.identity(), (a,b) -> b)
Why this is correct
Correct: The merge function (a,b) -> b keeps the later value.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often pick `toMap` without a merge function (Option B) or `groupingBy` (Option A), forgetting that duplicate keys cause exceptions or produce the wrong map type, respectively.
Detailed technical explanation
How to think about this question
Under the hood, `Collectors.toMap` uses a `HashMap` internally (or `ConcurrentHashMap` for `toConcurrentMap`) and the merge function is applied via `Map.merge(key, value, remappingFunction)`. The merge function `(a, b) -> b` effectively replaces the old value with the new one, which is a common pattern for 'last-wins' semantics. In real-world scenarios, this is useful when processing log entries or configuration overrides where later entries should take precedence.
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 Streams and Lambda Expressions — This question tests Working with Streams and Lambda Expressions — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Collectors.toMap(s -> s.substring(0,3), Function.identity(), (a,b) -> b) — Option D is correct because `Collectors.toMap` with a merge function `(a, b) -> b` ensures that when duplicate keys (first three characters) occur, the later string in the stream order replaces the earlier one. The merge function is invoked on key collision, and returning `b` (the second argument) keeps the later value, satisfying the requirement.
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: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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