@Nested JUnit 5 Tests | Java Fresher Interview

Direct Answer

@Nested lets you put related tests inside an inner, non-static class so the report reads like an outline — e.g. a WhenAccountIsEmpty group under BankAccountTest. Each nested class can have its own @BeforeEach, and outer @BeforeEach methods run before the nested ones, so you build context incrementally. It groups scenarios for the same unit; use separate top-level classes for genuinely different units.

Code Example

class BankAccountTest {
    BankAccount account;
    @BeforeEach void createAccount() { account = new BankAccount(); } // runs first

    @Nested
    @DisplayName("when the account is empty")
    class WhenEmpty {
        @Test void balanceIsZero() { assertEquals(0, account.balance()); }
        @Test void withdrawThrows() {
            assertThrows(InsufficientFundsException.class, () -> account.withdraw(10));
        }
    }

    @Nested
    @DisplayName("when the account has 100")
    class WhenFunded {
        @BeforeEach void deposit() { account.deposit(100); } // runs AFTER createAccount
        @Test void canWithdrawWithinBalance() {
            account.withdraw(40);
            assertEquals(60, account.balance());
        }
    }
}

Common Mistakes & Pitfalls

• Making the nested class static. @Nested requires a non-static inner class so it can access the outer instance; static breaks it.
• Expecting @BeforeAll inside a nested class. It needs @TestInstance(PER_CLASS) because @BeforeAll is static by default.
• Over-nesting. Three or four levels deep hurts readability; two levels is usually plenty.
• Using @Nested for unrelated units. Different classes under test belong in different top-level test classes.