@ParameterizedTest JUnit 5 | Java Fresher Interview

Direct Answer

@ParameterizedTest runs the same test body once per input set, replacing copy-pasted near-identical tests. You feed data with a source: @ValueSource (single literals), @CsvSource (input→expected rows), @MethodSource (a Stream<Arguments> for complex objects), plus @EnumSource, @NullSource, and @CsvFileSource. It needs the junit-jupiter-params module.

Deep Dive & Architecture

@ParameterizedTest marks a reusable test template. An argument-source annotation supplies the values, and JUnit runs the template once for every supplied case. This keeps the behavior in one place while showing the set of examples clearly in the test report.

For example, @ValueSource(ints = {2, 4, 8}) is suitable for one integer parameter. Use @CsvSource when each case has several simple columns, @MethodSource for objects or computed data, @EnumSource for enum values, and @NullSource or @EmptySource for those edge cases. The source is part of the test design: choosing a complex factory for two simple values makes the test harder to read, while forcing objects into CSV strings makes it fragile.

Code Example

import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.*;
import java.util.stream.Stream;

@ParameterizedTest
@ValueSource(strings = {"racecar", "level", "noon"})
void detectsPalindromes(String word) {
    assertTrue(StringUtils.isPalindrome(word));
}

@ParameterizedTest
@CsvSource({"2, 3, 5", "0, 0, 0", "-1, 1, 0"})
void addsNumbers(int a, int b, int expected) {
    assertEquals(expected, calc.add(a, b));
}

@ParameterizedTest
@MethodSource("orders")
void pricesOrders(Order order, double expected) {
    assertEquals(expected, service.price(order));
}
static Stream<Arguments> orders() {
    return Stream.of(Arguments.of(new Order(100.0, PREMIUM), 90.0),
                     Arguments.of(new Order(100.0, REGULAR), 100.0));
}

Common Mistakes & Pitfalls

• Using @Test. Parameters won't be injected; you need @ParameterizedTest.
• Wrong source. @ValueSource carries only one argument; use @CsvSource for pairs.
• Non-static @MethodSource. Factory must be static unless using @TestInstance(PER_CLASS).
• Missing module. Needs junit-jupiter-params (already in spring-boot-starter-test).