Unit Testing

Analogy: concept

Unit Testing is like Quality Control in a Factory:

Before a car leaves the factory, you test each part individually:

Does the door lock? (Pass/Fail)
Does the window roll down? (Pass/Fail)
Does the engine start? (Pass/Fail)

If one test fails, you don't ship the car.

The Test Runner

Run a suite of automated tests. Introduce a "bug" to see what happens when a test fails.

Unit Testing (Test Runner)

Test Suite: CalculatorService

should_add_numbers

should_handle_null

should_throw_error

should_save_user

Key Concepts

Unit Test: A small piece of code that tests a specific function or method in isolation.
Assertion: A statement that checks if a result matches the expectation (e.g., assert(2 + 2 == 4)).
Mocking: Creating fake objects (like a fake Database) so you can test your code without relying on external systems.

The Code

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

public class CalculatorTest {
    
    @Test
    public void shouldAddNumbers() {
        Calculator calc = new Calculator();
        int result = calc.add(2, 3);
        
        // Assertion: Expect 5, get result
        assertEquals(5, result); 
    }
}