- A
stream.collect(Collectors.groupingBy(Employee::getDepartment, Collectors.collectingAndThen(Collectors.toList(), list -> list.stream().sorted(Comparator.comparing(Employee::getSalary).reversed()).limit(3).collect(Collectors.toList()))))
This groups by department, then for each group sorts descending and limits to 3, exactly as required.
- B
stream.collect(Collectors.toMap(Employee::getDepartment, e -> { List<Employee> l = new ArrayList<>(); l.add(e); return l; }, (l1, l2) -> { l1.addAll(l2); l1.sort(Comparator.comparing(Employee::getSalary).reversed()); return l1.subList(0, Math.min(3, l1.size())); }))
Why wrong: toMap is intended for one-to-one mapping; merging lists is complex and error-prone, and does not guarantee correct per-department top 3.
- C
stream.sorted(Comparator.comparing(Employee::getSalary).reversed()).collect(Collectors.groupingBy(Employee::getDepartment, Collectors.toList())).values().stream().map(l -> l.subList(0, Math.min(3, l.size()))).collect(Collectors.toList())
Why wrong: Sorting globally before grouping loses per-department top 3; the top 3 overall may all be from one department.
- D
stream.collect(Collectors.groupingBy(Employee::getDepartment, Collectors.toList())).entrySet().stream().flatMap(e -> e.getValue().stream().sorted(Comparator.comparing(Employee::getSalary).reversed()).limit(3)).collect(Collectors.toList())
Why wrong: This collects all employees into a Map then flattens, but flatMap is unnecessary and order may not be maintained properly.
Quick Answer
The correct approach uses `Collectors.collectingAndThen` combined with `groupingBy` to group and sort top N per group with Java streams. This works because `collectingAndThen` allows you to post-process each group’s collected list—here, sorting by salary descending and applying `limit(3)`—before storing the result in the map. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this pattern tests your understanding of downstream collectors and the fact that `groupingBy` alone cannot truncate or reorder within groups. A common trap is trying to use `toMap` or chaining `sorted` before `groupingBy`, which would sort the entire stream rather than per department. Remember: `collectingAndThen` is your “finisher” for per-group transformations. Memory tip: think “group, then fix”—first collect the group, then apply a finishing operation inside `collectingAndThen`.
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 developer is designing a method that processes a stream of Employee objects. The method needs to group employees by department and then, within each department, sort by salary in descending order, and collect the top 3 highest-paid employees per department into a list. Which approach correctly accomplishes this using streams?
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
stream.collect(Collectors.groupingBy(Employee::getDepartment, Collectors.collectingAndThen(Collectors.toList(), list -> list.stream().sorted(Comparator.comparing(Employee::getSalary).reversed()).limit(3).collect(Collectors.toList()))))
Option A is correct because it uses `Collectors.groupingBy` to partition employees by department, then applies `Collectors.collectingAndThen` to post-process each department's list. Inside, it sorts the list by salary in descending order, limits to the top 3, and collects them into a new list. This ensures each department's result is independently sorted and truncated.
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.
- ✓
stream.collect(Collectors.groupingBy(Employee::getDepartment, Collectors.collectingAndThen(Collectors.toList(), list -> list.stream().sorted(Comparator.comparing(Employee::getSalary).reversed()).limit(3).collect(Collectors.toList()))))
Why this is correct
This groups by department, then for each group sorts descending and limits to 3, exactly as required.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
stream.collect(Collectors.toMap(Employee::getDepartment, e -> { List<Employee> l = new ArrayList<>(); l.add(e); return l; }, (l1, l2) -> { l1.addAll(l2); l1.sort(Comparator.comparing(Employee::getSalary).reversed()); return l1.subList(0, Math.min(3, l1.size())); }))
Why it's wrong here
toMap is intended for one-to-one mapping; merging lists is complex and error-prone, and does not guarantee correct per-department top 3.
- ✗
stream.sorted(Comparator.comparing(Employee::getSalary).reversed()).collect(Collectors.groupingBy(Employee::getDepartment, Collectors.toList())).values().stream().map(l -> l.subList(0, Math.min(3, l.size()))).collect(Collectors.toList())
Why it's wrong here
Sorting globally before grouping loses per-department top 3; the top 3 overall may all be from one department.
- ✗
stream.collect(Collectors.groupingBy(Employee::getDepartment, Collectors.toList())).entrySet().stream().flatMap(e -> e.getValue().stream().sorted(Comparator.comparing(Employee::getSalary).reversed()).limit(3)).collect(Collectors.toList())
Why it's wrong here
This collects all employees into a Map then flattens, but flatMap is unnecessary and order may not be maintained properly.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often sort globally before grouping (Option C) or flatten the result (Option D), failing to realize that per-group sorting and limiting must occur inside the downstream collector to maintain group isolation.
Detailed technical explanation
How to think about this question
Under the hood, `Collectors.collectingAndThen` acts as a downstream finisher: it first collects into a mutable container (here `toList()`), then applies a function to transform that container. This is essential when you need to perform additional stream operations (like sorting and limiting) on each group's collection. In real-world scenarios, this pattern is used for leaderboard generation, where each category (department) needs its own ranked subset without cross-category interference.
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
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FAQ
Questions learners often ask
What does this 1Z0-829 question test?
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: stream.collect(Collectors.groupingBy(Employee::getDepartment, Collectors.collectingAndThen(Collectors.toList(), list -> list.stream().sorted(Comparator.comparing(Employee::getSalary).reversed()).limit(3).collect(Collectors.toList())))) — Option A is correct because it uses `Collectors.groupingBy` to partition employees by department, then applies `Collectors.collectingAndThen` to post-process each department's list. Inside, it sorts the list by salary in descending order, limits to the top 3, and collects them into a new list. This ensures each department's result is independently sorted and truncated.
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.
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
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