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. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
Exhibit
Refer to the exhibit.
List<String> list = List.of("a", "b", "c");
Stream<String> stream = list.stream();
stream.filter(s -> s.startsWith("b")).forEach(System.out::print);
stream.forEach(System.out::print);
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
An exception is thrown at runtime.
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.
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.
✓
An exception is thrown at runtime.
Why this is correct
IllegalStateException is thrown when a terminal operation is called on an already-consumed stream.
Related concept
Read the scenario before looking for a memorised answer.
✗
bbc
Why it's wrong here
Incorrect output.
✗
Compilation error
Why it's wrong here
The code compiles; the error occurs at runtime.
✗
baaabc
Why it's wrong here
This would happen if the stream could be used twice, but it cannot.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often forget that streams are single-use and incorrectly assume that chaining multiple terminal operations on the same stream is allowed, leading them to pick a concatenated output like 'baaabc' instead of recognizing the runtime exception.
Trap categories for this question
Command / output trap
Incorrect output.
Detailed technical explanation
How to think about this question
Under the hood, `Stream` implementations maintain a `linkedOrConsumed` flag that is set to `true` after a terminal operation (like `forEach` or `findFirst`) is invoked. The stream pipeline's `spliterator` or `sourceStage` is then marked as consumed, and any subsequent terminal operation checks this flag and throws `IllegalStateException` with the message 'stream has already been operated upon or closed'. This design ensures deterministic behavior and prevents accidental reuse of stream resources.
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 junior network technician can log in to a core router but cannot reach the enable prompt or configuration mode. The AAA server is authenticating the login — but the authorisation policy only grants privilege level 1, not 15. Authentication (who you are) is working; authorisation (what you can do) is not.
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: An exception is thrown at runtime. — 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.
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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Question Discussion
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