Testing with JUnit 5 and Mockito

Code that is not tested is code you cannot change with confidence, and automated tests are how professional Java is verified continuously rather than by hopeful manual checking. JUnit 5 is the standard testing framework: you write test methods that exercise your code and assert expected outcomes, and the framework runs them, reports passes and failures, and integrates with build tools and IDEs so the whole suite runs on every change.

Real code, though, has dependencies, a service that calls a database, a component that talks to a remote API, and testing it in isolation requires standing in for those collaborators. Mockito is the standard mocking library: it creates fake implementations of dependencies whose behaviour you program (return this when called with that) and whose interactions you can verify, so you can test a unit's logic without its real, slow, or unavailable collaborators.

Understanding testing with JUnit 5 and Mockito matters because tests are what make a codebase safe to evolve: a good suite catches regressions the moment they are introduced, documents how code is meant to behave, and enables confident refactoring. Grasping how to structure tests, write clear assertions, and isolate a unit with mocks is a core professional skill, the difference between code you can change fearlessly and code everyone is afraid to touch.

Analogy🏏Cricket

🏏 Think of it like cricket: a team sheet does not just list players, it assigns each to a precise, declared role, opener, spinner, wicketkeeper, and the laws and the captain enforce that a player operates within their declared role: you cannot send a designated bowler to keep wicket without an official change. Just as each player's role is fixed and checked before play, each Java variable's type is fixed at compile time and checked by the compiler. Just as trying to use a player outside their role is caught by the officials before it disrupts the match, using a variable in a type-incompatible way is caught by the compiler before the program runs. Just as clear role assignments prevent on-field confusion, clear type declarations prevent runtime errors. The insight is that declaring and enforcing roles up front, for players or for data, catches mistakes early rather than mid-match.