java-tutorial

14) Testing with JUnit 5 (Basics)

Goal

Learn unit testing fundamentals: arrange/act/assert, naming, and running tests.

Why tests?

• Catch regressions
• Enable refactoring
• Document behavior

JUnit 5 example

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

class MathUtilTest {
    @Test
    void add_addsTwoNumbers() {
        assertEquals(5, MathUtil.add(2, 3));
    }
}

What to test

• Pure logic (easy)
• Edge cases
• Error cases

Avoid testing:

• private methods directly
• trivial getters/setters

Exercises

1. Write tests for isPrime.
2. Write tests for reverse(String) including empty and null handling (decide expected behavior).

Note: this chapter pairs best with the Maven chapter to run tests easily.

Table of contents

1. Getting Started: Install, run, and your first program
2. Java Basics: types, variables, operators, formatting
3. Control Flow: if/switch/loops
4. Methods: parameters, return values, overloading
5. OOP: classes, objects, encapsulation
6. Inheritance & Polymorphism (and when not to use them)
7. Interfaces, abstract classes, and design basics
8. Exceptions and error handling
9. Strings, files, and I/O basics
10. Collections: List/Set/Map and Big-O intuition
11. Generics (the useful parts)
12. Lambdas & Streams
13. Dates and time (java.time)
14. Testing with JUnit 5 (basics)
15. Concurrency: threads, executors, futures
16. JVM basics: memory, GC, performance habits
17. Build tools: Maven essentials (recommended)
18. Next steps: projects to build