Java JUnit

Introduced in 2026-03-26-testing (Lecture, Week 5). See java-testing for the broader testing concepts JUnit is used to implement.

What is JUnit?

An open-source test automation framework for Java — one of many xUnit frameworks (JUnit, NUnit, CPPUnit, PyUnit…). This course uses JUnit 4. Depending on how software is structured, unit-testing frameworks like JUnit can also be used for other kinds of automated testing (the term “unit testing” is sometimes loosely used to mean any automated test).

Writing tests

Tests are defined in classes — conventionally one test class per real class. @Test marks a method as a test JUnit should run; test methods take no parameters and return void, and should be named for what they check.

public class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() { return name; }
    public int getAge() { return age; }
}

import org.junit.Test;
import static org.junit.Assert.assertEquals;

public class PersonTest {
    @Test
    public void testGetAge() {
        Person p1 = new Person("Jack", 10);
        assertEquals(10, p1.getAge());
    }

    @Test
    public void testGetName() {
        Person p1 = new Person("Jack", 10);
        assertEquals("Jack", p1.getName());
    }
}

@Before and @After

Each test method should be independent, but common setup (e.g. object creation) can be factored out:

public class PersonTest {
    private Person person;

    @Before
    public void setUp() {
        person = new Person("Jack", 10);
    }

    @After
    public void tearDown() {
        person = null;
    }
    // ...
}

@Before runs before each test; @After runs after each test.

Assert

Some static methods on Assert:

• assertEquals — asserts two objects are equal.
• assertArrayEquals — asserts two object arrays are equal.
• assertFalse / assertTrue — asserts a condition is false/true.
• assertSame / assertNotSame — asserts two objects do/don’t refer to the same object.

Checking for exceptions

Don’t catch the exception yourself — tell the test runner which exception type to expect via @Test(expected = ...), and let it catch it:

@Test(expected = EOFException.class)
public void testExceptions() throws EOFException {
    callThatShouldThrowEOFException();
}

Note the .class after the exception type is required.

Black box vs white box JUnit tests

• Black box: write tests purely from the specification/Javadoc, without looking at the implementation, then apply smoke tests / boundary tests / equivalence classes (see java-testing) to decide what to check — tests must not violate the method’s preconditions.
• White box: design tests using knowledge of the internal logic, aiming to cover branches, loops, and edge cases (e.g. targeting 100% branch coverage) — e.g. for a calculateDiscount(amount, isMember) method with nested if/else branches on both parameters, white-box tests are written to exercise every branch combination (amount ≤ 0; member above/at-or-below 100; non-member above/at-or-below 100).