JUnit 5 Basics Java | Fresher Interview

Direct Answer

JUnit 5 is a widely used Java testing generation built around the JUnit Platform launcher and test-engine API, the JUnit Jupiter programming and extension model, and the optional Vintage engine for JUnit 3/4 tests. A basic Jupiter test is a non-private method annotated with @Test that arranges a scenario, calls the code, and verifies observable behavior with an assertion.

Code Example

import org.junit.jupiter.api.Test;                 // Jupiter API
import static org.junit.jupiter.api.Assertions.*;  // static assertions

class CalculatorTest {              // no 'public' needed in JUnit 5
    @Test
    void shouldAddTwoNumbers() {
        // Arrange
        var calc = new Calculator();
        // Act
        int result = calc.add(2, 3);
        // Assert
        assertEquals(5, result, "2 + 3 should be 5"); // expected, actual, message
    }
}

Note the imports come from org.junit.jupiter.api — that's the JUnit 5 package, not JUnit 4's org.junit.

Common Mistakes & Pitfalls

• Wrong generation for the intended engine. org.junit.Test is JUnit 4; it runs on the Platform only when the Vintage engine is present. Jupiter tests import org.junit.jupiter.api.Test.
• Making everything public. Jupiter does not require public test classes or methods, though they must not be private.
• Returning a value. Ordinary Jupiter test methods must return void.
• Assuming every test needs an assertion call. A test may verify through assertThrows, mocks, or another assertion library, but a method with no observable check usually proves little.