JUnit 5 Fundamentals

JUnit 5 is the industry-standard framework for writing developer-led unit tests in Java. It is composed of three sub-modules: JUnit Platform (launcher engine), JUnit Jupiter (new programming and extension model), and JUnit Vintage (backward compatibility engine).

Test Lifecycles

By default, JUnit creates a new instance of the test class for every single test method execution (Lifecycle.PER_METHOD). This isolates tests and prevents mutable instance state from bleeding between test executions. You can override this behavior using @TestInstance(Lifecycle.PER_CLASS), which executes all tests in the same instance, allowing @BeforeAll and @AfterAll hooks to be non-static.

Code Walkthrough

This class demonstrates assertions, parameter inputs, and exception expectations using modern JUnit Jupiter features.

import org.junit.jupiter.api.*;
import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.ValueSource;
import static org.junit.jupiter.api.Assertions.*;
@DisplayName("Standard Calculator Suite")
class CalculatorTest {
private Calculator calculator;
@BeforeEach
    void init() {
        calculator = new Calculator();
    }
@Test
    @DisplayName("Basic Addition Test")
    void testAddition() {
        assertAll("Grouped Assertions",
            () -> assertEquals(5, calculator.add(2, 3)),
            () -> assertEquals(0, calculator.add(-1, 1))
        );
    }
@Test
    @DisplayName("Exception Propagation Test")
    void testDivisionByZero() {
        ArithmeticException exception = assertThrows(
            ArithmeticException.class, 
            () -> calculator.divide(10, 0),
            "Should throw division by zero exception"
        );
        assertEquals("/ by zero", exception.getMessage());
    }
@ParameterizedTest
    @ValueSource(ints = {2, 4, 6, 8, 10})
    @DisplayName("Verification of Even Numbers")
    void testIsEven(int number) {
        assertTrue(calculator.isEven(number));
    }
}

Interview-Relevant Information