The answer is IllegalStateException because a stream in Java cannot be reused after a terminal operation has been executed. When you call a terminal operation like forEach, the stream is consumed and closed, meaning its internal state is marked as used. Any subsequent attempt to call another operation—terminal or intermediate—on the same stream reference will throw an IllegalStateException with the message "stream has already been operated upon or closed." This concept is a frequent trap on the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, where questions often present code that chains a terminal operation and then tries to reuse the same stream variable. The exam tests your understanding that streams are single-use pipelines, not reusable collections. A common memory tip is to think of a stream like a one-way conveyor belt: once the items have passed through to the terminal operation, the belt stops and cannot be restarted. Always create a new stream from the source if you need to perform multiple operations.
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
✓
IllegalStateException
After the first terminal operation (forEach), the stream is consumed. Any further operation on the same stream 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.
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abc
Why it's wrong here
Only the first forEach prints abc, but the second forEach will cause an exception.
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NullPointerException
Why it's wrong here
No null pointer issue here.
✓
IllegalStateException
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.
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abcabc
Why it's wrong here
The stream is already consumed; the second forEach will not execute.
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: IllegalStateException — After the first terminal operation (forEach), the stream is consumed. Any further operation on the same stream 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.
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Question Discussion
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