- A
logs.parallelStream().filter(l -> l.timestamp() > System.currentTimeMillis() - 3600000).collect(Collectors.groupingBy(LogRecord::userId, Collectors.counting())).entrySet().stream().sorted(Map.Entry.<Long, Long>comparingByValue().reversed()).limit(3).collect(Collectors.toList())
Correct.
- B
logs.parallelStream().filter(l -> l.timestamp() > System.currentTimeMillis() - 3600000).collect(Collectors.groupingBy(LogRecord::userId, Collectors.counting())).entrySet().stream().sorted(Map.Entry.comparingByKey()).limit(3).collect(Collectors.toList())
Why wrong: Sorted by userId, not count.
- C
logs.parallelStream().collect(Collectors.groupingBy(LogRecord::userId, Collectors.counting())).entrySet().stream().sorted(Map.Entry.comparingByValue().reversed()).limit(3).collect(Collectors.toList())
Why wrong: No time filter.
- D
logs.parallelStream().filter(l -> l.timestamp() > System.currentTimeMillis() - 3600000).collect(Collectors.groupingBy(LogRecord::action, Collectors.counting())).entrySet().stream().sorted(Map.Entry.comparingByValue().reversed()).limit(3).collect(Collectors.toList())
Why wrong: Grouped by action, not userId.
Quick Answer
The answer is the code that uses `parallelStream()` with `filter`, `groupingBy`, `counting`, and `sorted` in a single pipeline. This solution is correct because it leverages parallel processing to efficiently handle large log datasets while applying a time-based filter for the last hour, then groups by userId and counts actions, sorts the map entries by count in descending order, and limits to the top 3—all without manual sorting. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your understanding of parallel streams, the `Collectors.groupingBy` and `Collectors.counting` combo, and chaining terminal operations like `sorted` and `limit`. A common trap is forgetting that `parallelStream()` requires stateless, non-interfering operations; using `sorted` after `collect` is safe because it operates on a new stream. Memory tip: think “filter, group, count, sort, limit” as the five-step pipeline for any top N users example.
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 data analytics platform processes user activity logs. Each log entry is a LogRecord with fields: userId (int), action (String), timestamp (long). The requirement is to find the top 3 most active users (by count of actions) in the last hour. The logs are stored in a List<LogRecord> logs. The current solution sorts all records by userId and counts manually, but it's slow. The team decides to use streams with parallel processing. Which code correctly identifies the top 3 users?
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
logs.parallelStream().filter(l -> l.timestamp() > System.currentTimeMillis() - 3600000).collect(Collectors.groupingBy(LogRecord::userId, Collectors.counting())).entrySet().stream().sorted(Map.Entry.<Long, Long>comparingByValue().reversed()).limit(3).collect(Collectors.toList())
Option A is correct because it first filters logs to only those within the last hour, then groups by userId and counts actions, sorts the resulting map entries by count in descending order, limits to the top 3, and collects the result. This uses parallelStream() for performance and correctly applies the required logic.
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.
- ✓
logs.parallelStream().filter(l -> l.timestamp() > System.currentTimeMillis() - 3600000).collect(Collectors.groupingBy(LogRecord::userId, Collectors.counting())).entrySet().stream().sorted(Map.Entry.<Long, Long>comparingByValue().reversed()).limit(3).collect(Collectors.toList())
Why this is correct
Correct.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
logs.parallelStream().filter(l -> l.timestamp() > System.currentTimeMillis() - 3600000).collect(Collectors.groupingBy(LogRecord::userId, Collectors.counting())).entrySet().stream().sorted(Map.Entry.comparingByKey()).limit(3).collect(Collectors.toList())
Why it's wrong here
Sorted by userId, not count.
- ✗
logs.parallelStream().collect(Collectors.groupingBy(LogRecord::userId, Collectors.counting())).entrySet().stream().sorted(Map.Entry.comparingByValue().reversed()).limit(3).collect(Collectors.toList())
Why it's wrong here
No time filter.
- ✗
logs.parallelStream().filter(l -> l.timestamp() > System.currentTimeMillis() - 3600000).collect(Collectors.groupingBy(LogRecord::action, Collectors.counting())).entrySet().stream().sorted(Map.Entry.comparingByValue().reversed()).limit(3).collect(Collectors.toList())
Why it's wrong here
Grouped by action, not userId.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may forget to filter by timestamp (as in option C) or group by the wrong field (as in option D), or sort by the wrong comparator (as in option B), leading to incorrect results that still compile and run.
Detailed technical explanation
How to think about this question
The groupingBy collector with counting() returns a Map<UserId, Long>. Sorting by value in reverse order uses Map.Entry.comparingByValue().reversed(), which requires a Comparator that reverses natural order. The filter condition l.timestamp() > System.currentTimeMillis() - 3600000 ensures only logs from the last 3600 seconds (1 hour) are considered. Parallel streams can improve performance on large datasets but require thread-safe collectors.
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: logs.parallelStream().filter(l -> l.timestamp() > System.currentTimeMillis() - 3600000).collect(Collectors.groupingBy(LogRecord::userId, Collectors.counting())).entrySet().stream().sorted(Map.Entry.<Long, Long>comparingByValue().reversed()).limit(3).collect(Collectors.toList()) — Option A is correct because it first filters logs to only those within the last hour, then groups by userId and counts actions, sorts the resulting map entries by count in descending order, limits to the top 3, and collects the result. This uses parallelStream() for performance and correctly applies the required logic.
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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