Stage 2 · Lesson 5 of 8
Abstraction, interfaces and abstract classes
means depending on what something does, not on how it does it. An interface names a set of operations, such as save(order), without saying how they work. An abstract class can do the same.
The rest of the program then works the same way whether orders go to a database, a file or a fake store used in tests. An interface is a pure contract: a promise that certain operations exist. An abstract class can also share some code and data with its subclasses.
This is the tool that makes and testing practical. It leads straight into the in SOLID.
Common trapcreating an interface for every class "just in case", which adds files and indirection without any real second implementation.
A question to check yourself, an AI practice session and 1 cheat sheet entry come with the full roadmap. Unlock
Resources for this stage
Start with· follow one of these
- CoursePaidGrokking SOLID Design Principles
Design Gurus · Beginner · $50 one-time
Each SOLID principle with a broken example and its fix, at a beginner-friendly pace
- CourseFreeJava Programming MOOC
University of Helsinki · Beginner
Free, exercise-heavy course whose second half is a thorough, tested introduction to OOP