The answer is that the stream has already been operated upon or closed, which directly causes the IllegalStateException on the second forEach call. In Java, streams are designed as single-use pipelines; once a terminal operation like forEach executes, the stream is consumed and closed, making any further attempt to operate on the same stream reference illegal. This concept tests your understanding of the stream lifecycle, a frequent trap on the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam where candidates mistakenly assume streams behave like collections that can be iterated multiple times. The exam often presents code where a stream variable is reused after a terminal operation, and the correct response hinges on recognizing that a stream cannot be reopened. A simple memory tip: think of a stream like a one-way ticket—once you’ve taken the trip (terminal operation), the ticket is invalid for reuse.
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The stream has already been operated upon or closed.
Streams can only be consumed once. After the terminal operation forEach is called on stream, the stream is closed. Any subsequent operation on the same stream reference throws IllegalStateException.
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.
✗
The filter operation is not lazy.
Why it's wrong here
Filter is lazy, but that is not the cause of the exception.
✗
The map operation modifies the source list.
Why it's wrong here
Map is non-interfering and does not modify the source.
✗
The stream is not closed after the first terminal operation.
Why it's wrong here
Incorrect; the stream is indeed closed.
✓
The stream has already been operated upon or closed.
Why this is correct
Correct. A stream cannot be reused after a terminal operation.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 1Z0-829 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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: The stream has already been operated upon or closed. — Streams can only be consumed once. After the terminal operation forEach is called on stream, the stream is closed. Any subsequent operation on the same stream reference throws IllegalStateException.
What should I do if I get this 1Z0-829 question wrong?
Identify which 1Z0-829 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. What is the result of executing this code?
medium
✓ A.An exception is thrown at runtime.
B.bbc
C.Compilation error
D.baaabc
Why A: Option A is correct because the code attempts to call `findFirst()` on a `Stream<String>` that has already been consumed by a previous terminal operation (`forEach`). In Java, streams are single-use; after a terminal operation is executed, the stream is closed and cannot be reused. Any attempt to perform another terminal operation on the same stream throws an `IllegalStateException` at runtime.
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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.
Question Discussion
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