Unit tests isolate a small behavior and should be fast and deterministic. Integration tests verify interactions with real components such as databases, message brokers, HTTP servers, or filesystems.

@Test
void calculatesTotal() {
    Cart cart = new Cart();
    cart.add(new Item("Book", 500));

    assertEquals(500, cart.total());
}

A healthy test strategy uses the cheapest test that provides useful confidence while maintaining enough realistic integration coverage to catch wiring and infrastructure failures.