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Working with Streams and Lambda Expressions →hardMultiple ChoiceObjective-mapped

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

List<String> words = Arrays.asList("apple", "banana", "cherry", "date", "elderberry");
Stream<String> stream = words.stream();
stream.filter(w -> w.startsWith("a"))
      .map(String::toUpperCase)
      .forEach(System.out::println);
// Output: APPLE
// Then:
stream.forEach(System.out::println); // Throws IllegalStateException

Refer to the exhibit. A developer runs the code and gets an IllegalStateException on the second forEach. Which statement explains why?

Exhibit

List<String> words = Arrays.asList("apple", "banana", "cherry", "date", "elderberry");
Stream<String> stream = words.stream();
stream.filter(w -> w.startsWith("a"))
      .map(String::toUpperCase)
      .forEach(System.out::println);
// Output: APPLE
// Then:
stream.forEach(System.out::println); // Throws IllegalStateException

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

✓

The stream has already been operated upon or closed.

Option D is correct because a Stream in Java cannot be reused after a terminal operation has been executed. Once the first forEach terminal operation completes, the stream is consumed and closed. Attempting to call another terminal operation (the second forEach) on the same stream reference throws an IllegalStateException with the message 'stream has already been operated upon or closed'.

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

A common misconception in OCP Java exam questions is that streams can be reused like collections, or that the exception is due to resource leaks or modification of the source.

Detailed technical explanation

How to think about this question

Under the hood, a Java Stream pipeline is a one-shot construct. The stream source (e.g., a collection) is wrapped in a pipeline head, and each intermediate operation adds a stage. When a terminal operation like forEach is invoked, the pipeline is traversed and the stream is marked as 'operated upon'. Any subsequent attempt to invoke a terminal operation on the same stream reference triggers an IllegalStateException because the internal linked list of stages has been consumed. This design ensures that stream pipelines are used in a single pass, preventing unintended side effects and enabling optimizations like short-circuiting.

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

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.

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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: The stream has already been operated upon or closed. — Option D is correct because a Stream in Java cannot be reused after a terminal operation has been executed. Once the first forEach terminal operation completes, the stream is consumed and closed. Attempting to call another terminal operation (the second forEach) on the same stream reference throws an IllegalStateException with the message 'stream has already been operated upon or closed'.

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: Jul 4, 2026

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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.