What types of tests exist and how do they differ?

There are several main types of tests, which differ by level and purpose:

1. Unit tests — test individual components or methods in isolation from the rest of the system. The goal is to ensure that a specific piece of code works correctly.

2. Integration tests — verify the interaction between multiple components or modules, such as working with a database or external services.

3. Functional/End-to-End tests — test the entire system from the user's perspective, simulating real usage scenarios.

4. Smoke tests — quick checks of the main functions of the application to ensure it launches and core features work.

5. Regression tests — re-run tests after code changes to ensure nothing is broken.

In Java, JUnit is often used for writing unit tests, Spring Test for integration tests, and Selenium or other tools for functional tests.

Example of a simple unit test in JUnit:

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

class CalculatorTest {
    @Test
    void testAdd() {
        Calculator calc = new Calculator();
        assertEquals(5, calc.add(2, 3));
    }
}

Each type of test is important for ensuring the quality and stability of the application.