Can you explain what test-driven development (TDD) is and how it is implemented in practice?

Test-Driven Development (TDD) is a methodology where automated tests describing functionality are written first, and then code is written to pass these tests. The process is cyclical and consists of three steps:

1. Write a test that checks new functionality (the test fails).
2. Write the minimal code to pass the test.
3. Refactor the code without changing behavior to improve quality.

In practice, this helps create more reliable and maintainable code, as requirements are immediately formalized in tests. For example, when developing an addition function:

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

First, write this test, which fails, then implement the add method so that the test becomes successful.