Java Testing Interview Prep

Java testing interview questions cover the full testing pyramid — unit testing with JUnit 5 (lifecycle, parameterized tests, extensions), mocking with Mockito (verify, argument matchers, when not to mock), Spring Boot test slices, integration testing with Testcontainers, contract testing (Pact, Spring Cloud Contract), TDD discipline, performance testing, chaos testing, and the trade-offs between mocks vs real dependencies. This page covers JUnit, Mockito, Testcontainers and the testing-strategy questions that come up at every level.

Pyramid order: unit > integration > E2E.

• Unit — JUnit 5; isolate with Mockito.
• Integration — @SpringBootTest + Testcontainers (real DB, real Kafka).
• E2E — slow, flaky; reserve for the critical user journeys.
• Contract — Pact / Spring Cloud Contract; the seam between services.
• Mutation (PIT) — find what your tests don't cover.

The interview surface for this module breaks into these focus areas:

• Chaos testing — drill the named patterns, the failure modes, and the recommended Java / framework idiom.
• Comparisons — drill the named patterns, the failure modes, and the recommended Java / framework idiom.
• Contract testing — drill the named patterns, the failure modes, and the recommended Java / framework idiom.
• Integration testing — drill the named patterns, the failure modes, and the recommended Java / framework idiom.
• Mocking with mockito — drill the named patterns, the failure modes, and the recommended Java / framework idiom.
• Performance testing — drill the named patterns, the failure modes, and the recommended Java / framework idiom.
• Spring boot testing — drill the named patterns, the failure modes, and the recommended Java / framework idiom.
• Tdd practices — drill the named patterns, the failure modes, and the recommended Java / framework idiom.
• Test automation — drill the named patterns, the failure modes, and the recommended Java / framework idiom.
• Testcontainers — drill the named patterns, the failure modes, and the recommended Java / framework idiom.
• Testing strategies — drill the named patterns, the failure modes, and the recommended Java / framework idiom.
• Unit testing advanced — drill the named patterns, the failure modes, and the recommended Java / framework idiom.
• Unit testing basics — drill the named patterns, the failure modes, and the recommended Java / framework idiom.
• Scenario-based — applied questions tying multiple topics together. Drill these last, after the fundamentals are sharp.

• Don't mock what you don't own — tests become coupled to upstream APIs.
• Testcontainers > H2 — same DB engine as prod or your test is lying.
• `@SpyBean` is a smell — usually means the seam is wrong.
• Flakiness — almost always async timing or shared state; fix the cause, don't Thread.sleep.
• TDD vs test-after — TDD when the design is unclear; test-after when it's a refactor.

"I'd unit-test the pure logic, integration-test the wiring with Testcontainers, contract-test the public API, and reserve E2E for the smoke-critical paths. Coverage is a signal, not a goal — mutation testing tells me whether tests *assert* anything."