Test Java behavior at unit, web, repository, and integration boundaries.
Testing · test pyramid
Use the strong answer as your reference, explain it aloud in your own words, then choose how soon you want to review it again.
Interview prompt
Strong answer
A healthy Java test suite has many fast unit tests for domain rules, focused slice tests for framework boundaries, and a smaller number of integration tests that prove the wiring works. JUnit 5 provides the lifecycle, assertions, and parameterized tests; Mockito replaces collaborators you do not own or that are slow, but mocks should verify behavior at a boundary rather than restate the implementation. Spring slices such as @WebMvcTest and @DataJpaTest load only the relevant part of the context.
Key points
Interviewer looks for
Explain which test type you choose for a given risk, how you keep tests deterministic (clocks, randomness, time zones), and when a mock gives false confidence.
Java example
1@WebMvcTest(OrderController.class)
2class OrderControllerTest {
3
4 @Autowired MockMvc mvc;
5 @MockitoBean OrderService service;
6
7 @Test
8 void rejectsInvalidQuantity() throws Exception {
9 mvc.perform(post("/orders")
10 .contentType(MediaType.APPLICATION_JSON)
11 .content("{\"sku\":\"A-1\",\"quantity\":0}"))
12 .andExpect(status().isBadRequest())
13 .andExpect(jsonPath("$.errors[0].field").value("quantity"));
14
15 verifyNoInteractions(service);
16 }
17}
18
19@ParameterizedTest
20@CsvSource({"100, 0", "1000, 50", "5000, 500"})
21void appliesDiscountTiers(int subtotal, int expectedDiscount) {
22 assertEquals(expectedDiscount, pricing.discountFor(subtotal));
23}Topic interview questions
These questions are tied to this lesson. Read each question aloud, then compare your explanation with the strong answer.
Strong answer
A unit test exercises a class directly without Spring. A slice test such as @WebMvcTest or @DataJpaTest starts only one layer of the context. An integration test, often @SpringBootTest, starts the full application with real infrastructure to verify wiring and end-to-end behavior.
Strong answer
Use a real object for value types and pure domain logic. Mock or fake collaborators that are slow, nondeterministic, or external, such as payment gateways or clocks. Prefer fakes or real infrastructure when the interaction itself, like a SQL query, is what you need to prove.
Strong answer
@Mock creates a Mockito mock with default stubbed returns. @Spy wraps a real object so unstubbed calls run real code. @MockitoBean, the successor to @MockBean, replaces a bean in the Spring application context with a mock for that test.
Strong answer
Inject a java.time.Clock instead of calling Instant.now() directly, then pass Clock.fixed or a mutable test clock in tests. This makes expiry, scheduling, and date-boundary behavior deterministic.
Strong answer
H2 differs from PostgreSQL or MySQL in SQL dialect, locking, JSON types, sequences, and constraint behavior, so native queries and edge cases may behave differently. Run repository tests against the same engine and version as production, typically via Testcontainers.
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