JUnit Testing

To say that code "works" it needs to be verified. JUnit 5 is Java's de facto standard testing framework, providing a short and clear annotation-based API.

Learning Objectives

• Know the location of src/test/java in the Maven project
• Use @Test, @BeforeEach, @AfterEach, @DisplayName
• Know the difference between assertEquals / assertTrue / assertThrows
• Run the test with mvn test

Key concepts

1) Maven dependencies

<dependency>
    <groupId>org.junit.jupiter</groupId>
    <artifactId>junit-jupiter</artifactId>
    <version>5.10.5</version>
    <scope>test</scope>
</dependency>

scope=test is not included in the operating code.

2) Basic testing

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

class CalculatorTest {
    @Test
void add returns the sum of two numbers () {
        assertEquals(5, new Calculator().add(2, 3));
    }
}

3) Life cycle

@BeforeEach void setUp() { ... }    //Run before each @Test
@AfterEach  void tearDown() { ... } //Run after each @Test
@BeforeAll  static void all() { ... }  //Once at the start of the entire class
@AfterAll   static void clean() { ... } //Once at the end of the entire class

4) Exception testing

@Test
void zero_division() {
    assertThrows(ArithmeticException.class,
        () -> new Calculator().divide(10, 0));
}

5) mvn test

cd 22_JUnit_test
mvn test

JUnit 5 automatically discovers and executes it and outputs the results.

See as example

Example 1 — Calculator.java: Operational code

package com.codingnow.lecture.modern22;

public class Calculator {
    public int add(int a, int b) { return a + b; }
    public int subtract(int a, int b) { return a - b; }
    public int divide(int a, int b) {
        if (b == 0) throw new ArithmeticException("/ zero");
        return a / b;
    }
}

Example 2 — CalculatorTest.java: The most basic test

package com.codingnow.lecture.modern22;

import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

class CalculatorTest {
    Calculator calc = new Calculator();

    @Test
    @DisplayName("add returns the sum of two numbers")
void add returns_sum() {
        assertEquals(5, calc.add(2, 3));
        assertEquals(0, calc.add(-1, 1));
    }

    @Test
    void divide_by_zero() {
        assertThrows(ArithmeticException.class, () -> calc.divide(10, 0));
    }
}

mvn test execution results (partial)

[INFO] Tests run: 2, Failures: 0, Errors: 0, Skipped: 0
[INFO] BUILD SUCCESS

Example 3 — FixtureTest.java: Lifecycle

package com.codingnow.lecture.modern22;

import org.junit.jupiter.api.AfterEach;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

class FixtureTest {
    Calculator calc;

    @BeforeEach
    void setUp() {
        calc = new Calculator();
        System.out.println("(setUp)");
    }

    @AfterEach
    void tearDown() {
        System.out.println("(tearDown)");
    }

    @Test
    void test_add() { assertEquals(7, calc.add(3, 4)); }

    @Test
    void test_sub() { assertEquals(1, calc.subtract(4, 3)); }
}

(setUp) and (tearDown) are displayed on the console for each test.

Example 4 — AssertionsTest.java: Various assertions

package com.codingnow.lecture.modern22;

import org.junit.jupiter.api.Test;
import java.util.List;
import static org.junit.jupiter.api.Assertions.*;

class AssertionsTest {
    @Test
    void variety() {
        assertTrue(2 + 2 == 4);
        assertFalse("hi".isEmpty());
        assertNotNull("");
        assertNull(null);
        assertArrayEquals(new int[]{1, 2, 3}, new int[]{1, 2, 3});
        assertIterableEquals(List.of("a"), List.of("a"));
    }
}

Common Mistakes

1. Add JUnit dependency to compile scope → operation jar becomes bloated
2. Write the @Test method only as public (in 5, both package-private or public are OK, only private is not OK)
3. Dependency between tests (results of one test affect other tests)
4. Change the argument order like assertEquals(actual, expected) (correct answer first)
5. Satisfaction by directly catching exceptions with try/catch → Use assertThrows

Cleanup

• Start automated verification with JUnit 5 + Maven
• The @Test method is small and only verifies one thing.
• Separate common initialization with @BeforeEach
• Verification of exception flow diagram with assertThrows

Try it yourself

cd 05_Modern_Java/22_JUnit_Test
mvn test