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.
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
int result = numbers.stream()
.reduce(0, (a, b) -> a - b);
System.out.println(result);Refer to the exhibit. What is the output?
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
int result = numbers.stream()
.reduce(0, (a, b) -> a - b);
System.out.println(result);1
Why wrong: Incorrect reduction.
15
Why wrong: Incorrect sign.
-15
reduce(0, (a,b)->a-b) computes 0-1-2-3-4-5 = -15.
0
Why wrong: Incorrect reduction.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
-15
The code uses `reduce(0, (a, b) -> a - b)` on a stream of integers. This operation starts with an identity value of 0 and applies the subtraction lambda cumulatively: 0 - 1 = -1, then -1 - 2 = -3, then -3 - 3 = -6, then -6 - 4 = -10, then -10 - 5 = -15. Therefore, the output is -15, making option C correct.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
1
Why it's wrong here
Incorrect reduction.
15
Why it's wrong here
Incorrect sign.
-15
Why this is correct
reduce(0, (a,b)->a-b) computes 0-1-2-3-4-5 = -15.
Related concept
Read the scenario before looking for a memorised answer.
0
Why it's wrong here
Incorrect reduction.
Common exam traps
Candidates often confuse the behavior of `reduce` with subtraction, mistakenly thinking it sums the elements or that the identity is ignored. The correct cumulative subtraction starting from identity 0 yields -15.
Detailed technical explanation
The `reduce` method in Java Streams is a terminal operation that performs a reduction on the elements using an associative accumulation function and an identity value. The identity must be an identity for the accumulator function (i.e., `accumulator.apply(identity, x) == x`), but subtraction is not associative, so using `reduce` with subtraction can produce unexpected results if parallelism is involved. In a real-world scenario, this matters when performing cumulative operations like calculating running balances or differences, where associativity and identity correctness are critical for parallel stream correctness.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
Quick reference
| Model | You Manage | Provider Manages | Examples |
|---|---|---|---|
| IaaS | OS, runtime, apps, data | Hardware, hypervisor, networking | EC2, Azure VMs, GCP Compute Engine |
| PaaS | Apps and data | OS, runtime, middleware, hardware | Elastic Beanstalk, Azure App Service |
| SaaS | Data and settings only | Everything else | Microsoft 365, Salesforce, Workday |
| FaaS / Serverless | Function code only | Infra, scaling, runtime | Lambda, Azure Functions, Cloud Run |
| CaaS | Containers and apps | Kubernetes, OS, hardware | EKS, AKS, GKE |
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FAQ
Working with Streams and Lambda Expressions — This question tests Working with Streams and Lambda Expressions — Read the scenario before looking for a memorised answer..
The correct answer is: -15 — The code uses `reduce(0, (a, b) -> a - b)` on a stream of integers. This operation starts with an identity value of 0 and applies the subtraction lambda cumulatively: 0 - 1 = -1, then -1 - 2 = -3, then -3 - 3 = -6, then -6 - 4 = -10, then -10 - 5 = -15. Therefore, the output is -15, making option C correct.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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.
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