The answer is 2 because the `Stream.of(1, 2).map(i -> i * 2.0)` operation produces a `Stream<Double>` with exactly two elements, and the terminal operation `count()` returns a `long` representing that element count. The key technical concept here is the `stream count return type`: `count()` always returns a `long`, not an `int`, but Java’s widening primitive conversion allows that `long` to be implicitly assigned to an `int` variable without a cast, so `int result` holds the value 2. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your understanding of both the `Stream` API’s `count()` method and Java’s type widening rules—a common trap is assuming `count()` returns `int` or forgetting that `map(i -> i * 2.0)` changes the stream’s generic type to `Double`. Remember the mnemonic: “Count is long, but widening is strong.”
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.
Exhibit
Refer to the exhibit.
var result = Stream.of("1", "2", "3")
.map(Integer::parseInt)
.collect(Collectors.teeing(
Collectors.summingInt(i -> i),
Collectors.counting(),
(sum, count) -> sum / count
));
System.out.println(result);
Refer to the exhibit.
var result = Stream.of("1", "2", "3")
.map(Integer::parseInt)
.collect(Collectors.teeing(
Collectors.summingInt(i -> i),
Collectors.counting(),
(sum, count) -> sum / count
));
System.out.println(result);
A
2.0
Why wrong: The result is an integer (2), not a double.
B
1
Why wrong: Incorrect calculation.
C
2
6/3 = 2, printed as 2.
D
Compilation error
Why wrong: The code compiles; teeing is available since Java 12.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
2
The code uses `Stream.of(1, 2).map(i -> i * 2.0)` which returns a `Stream<Double>`. The terminal operation `count()` returns a `long` value representing the number of elements in the stream, which is 2. Since `count()` returns a `long`, it is implicitly widened to `int` when assigned to `int result`, and then printed as 2. Option C is correct because the stream has two elements, so the count is 2.
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.
✗
2.0
Why it's wrong here
The result is an integer (2), not a double.
✗
1
Why it's wrong here
Incorrect calculation.
✓
2
Why this is correct
6/3 = 2, printed as 2.
Related concept
Read the scenario before looking for a memorised answer.
✗
Compilation error
Why it's wrong here
The code compiles; teeing is available since Java 12.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may mistakenly think the `map` operation changes the return type of `count()` or that the output would be a floating-point number (2.0) because the mapping produces `Double` values, but `count()` always returns a `long` regardless of the stream's element type.
Detailed technical explanation
How to think about this question
The `count()` terminal operation is a reduction that returns the number of elements in the stream as a `long`. In the stream pipeline, `map(i -> i * 2.0)` transforms each `Integer` to a `Double`, but `count()` does not depend on the element type; it simply counts elements. Under the hood, `count()` uses a `java.util.stream.ReduceOps` that accumulates a count without processing the mapped values individually, making it efficient even for large streams. A real-world scenario is when you need to count filtered results after a mapping operation, and `count()` avoids materializing the mapped stream.
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: 2 — The code uses `Stream.of(1, 2).map(i -> i * 2.0)` which returns a `Stream<Double>`. The terminal operation `count()` returns a `long` value representing the number of elements in the stream, which is 2. Since `count()` returns a `long`, it is implicitly widened to `int` when assigned to `int result`, and then printed as 2. Option C is correct because the stream has two elements, so the count is 2.
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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