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.
Given a list of integers, a developer wants to compute the sum of squares of numbers greater than 10. The following code is written:
int sum = list.stream().filter(i -> i>10).mapToInt(i->i*i).sum();
But the sum is incorrect. What is the most likely reason?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The list might contain nulls causing NullPointerException.
Option C is correct because if the list contains null elements, the lambda expression `i -> i > 10` will attempt to unbox a null `Integer` to an `int`, causing a `NullPointerException` at runtime. The stream pipeline itself is syntactically valid, but the presence of nulls in the source list is a common pitfall when using primitive-specialized operations like `mapToInt` after a filter that does not guard against nulls.
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 mapToInt operation is not allowed after filter.
Why it's wrong here
filter returns a Stream<Integer> which can be mapped to an IntStream via mapToInt.
✗
The stream is not sorted.
Why it's wrong here
Sorting is irrelevant to the computation.
✓
The list might contain nulls causing NullPointerException.
Why this is correct
If any element is null, unboxing in i > 10 throws NPE, making the result incorrect if caught or halting execution.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The filter should be before mapToInt, but it is after.
Why it's wrong here
The filter is before mapToInt; that's correct.
✗
The sum() method returns an OptionalInt and must be orElse(0).
Why it's wrong here
IntStream.sum() returns int, not OptionalInt.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Oracle often tests the misconception that `sum()` returns an `OptionalInt` (like `reduce` does) or that the order of `filter` and `mapToInt` is reversed, but the real trap is the silent NullPointerException from unboxing nulls in a lambda.
Detailed technical explanation
How to think about this question
Under the hood, `mapToInt` applies a `ToIntFunction` that unboxes each `Integer` to an `int`. If the `Integer` is `null`, unboxing throws a `NullPointerException`. This is a common issue when streams are sourced from legacy collections that may contain nulls. In real-world scenarios, using `filter(Objects::nonNull)` before the numeric filter or using `mapToInt(i -> i == null ? 0 : i*i)` can prevent this, but the cleanest approach is to ensure the source list is null-free or to use `filter(i -> i != null && i > 10)`.
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.
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 list might contain nulls causing NullPointerException. — Option C is correct because if the list contains null elements, the lambda expression `i -> i > 10` will attempt to unbox a null `Integer` to an `int`, causing a `NullPointerException` at runtime. The stream pipeline itself is syntactically valid, but the presence of nulls in the source list is a common pitfall when using primitive-specialized operations like `mapToInt` after a filter that does not guard against nulls.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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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