Testing with JUnit

In plain English

JUnit is a testing framework for Java that turns small methods into automated checks for your code. The @Test annotation marks a method as a test, so JUnit runs it and reports whether it passed. An assertion is a statement that compares an actual value to an expected value, and it fails the test when the two do not match. A parameterised test runs the same test body several times with different inputs supplied by a source, so you do not copy one test per example. Test structure means arranging input, acting on the code and asserting the result, and test names should describe the behaviour under a condition, such as shouldFailForEmptyTitle.

Step by step

1. 1Add the JUnit 5 dependency to your build file.
2. 2Create a test class named after the class under test.
3. 3Write an @Test method that arranges, acts and asserts.
4. 4Use assertEquals with the expected value first.
5. 5Convert repeated examples into a parameterised test.

import org.junit.jupiter.api.Test;
import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.ValueSource;

import static org.junit.jupiter.api.Assertions.assertEquals;

class LessonTest {
    @Test
    void returnsTitle() {
        Lesson lesson = new Lesson("Java", 12);

        String title = lesson.title();

        assertEquals("Java", title);
    }

    @ParameterizedTest
    @ValueSource(ints = {1, 5, 120})
    void storesMinutes(int minutes) {
        Lesson lesson = new Lesson("Java", minutes);

        assertEquals(minutes, lesson.minutes());
    }
}

Common mistakes

• Naming a test after the method instead of the behaviour hides what the test promises, so a reader must open the body to learn it.
• Writing assertEquals(actual, expected) swaps the arguments and produces confusing failure messages that mislabel the values.
• A parameterised test that asserts the same thing for every input proves little, so make the expected value vary with the input.
• Forgetting to import JUnit annotations fails to compile, because the test runner cannot discover methods without them.