BNSF·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

A technical round at BNSF for a Software Engineer role where the main prompt was to write a full unit test suite from a spec alone, no implementation provided. Pretty interesting exercise, more design-heavy than I expected.

Questions Asked (1)

You are given only a function's specification, not its code. Write a comprehensive unit test suite covering happy paths, boundary conditions, invalid inputs, equivalence partitioning, idempotency, and any concurrency or stateful effects implied by the spec. For each test case, state the input, expected behavior, and which requirement it covers. How would you prioritize if time-boxed?

Algorithms & Data StructuresTechnical Trade-offsSystem Design

Author's notes

This is a meaty prompt and I spent the first few minutes just trying to figure out where to start.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the spec and identifying all inputs, outputs, and implied behaviors. Then systematically derive test cases using equivalence partitioning and boundary value analysis, covering happy paths, edge cases, invalid inputs, and stateful/concurrency aspects. Finally, discuss prioritization based on risk and business impact, emphasizing critical paths first.

Pro tip: Demonstrate maturity by acknowledging that you would first ask clarifying questions about the spec and its context (e.g., expected usage, performance requirements) before writing tests, and that you would prioritize tests that cover the most likely failure modes and critical functionality.

1. Clarify the Specification

Ask questions to resolve ambiguities: What are the input types and ranges? What are the expected outputs? Are there side effects, state, or concurrency concerns? What are the non-functional requirements?

2. Identify Test Categories

Map out the categories: happy paths (typical valid inputs), boundary conditions (min/max, empty, null), invalid inputs (wrong types, out-of-range), equivalence partitions (grouping inputs that should behave similarly), idempotency (repeated calls), and concurrency/stateful effects (if applicable).

3. Design Test Cases

For each category, create specific test cases. For each, define the input, expected behavior (including exceptions or errors), and which requirement it covers. Ensure coverage of all partitions and boundaries.

4. Prioritize Under Time Constraints

If time-boxed, prioritize tests that cover critical functionality, high-risk areas, and common failure modes. Use risk-based prioritization: focus on happy paths and critical boundaries first, then invalid inputs, then concurrency/stateful tests if time permits.

5. Summarize and Justify

Present the test suite structure, explain the rationale for prioritization, and discuss any trade-offs made. Highlight how the tests align with the specification and business needs.

Key Points to Mention

Equivalence partitioning and boundary value analysis to reduce redundant tests while maintaining coverage.
Importance of testing invalid inputs and error handling to ensure robustness.
Idempotency: verifying that repeated calls with the same input produce the same result without unintended side effects.
Concurrency and stateful effects: if the function modifies shared state or is not thread-safe, design tests to expose race conditions or state leakage.
Prioritization based on risk and business impact: critical paths first, then edge cases, then less likely scenarios.
Clear documentation of each test case: input, expected output, and requirement covered, to facilitate maintenance and traceability.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.