Java is built around objects, so thinking in objects is the biggest step from writing small programs to writing real ones. Object-oriented programming (OOP) lets you model a problem as a set of things that hold data and know how to act on it, which keeps larger codebases organized and easier to change.
You'll start with classes and objects, then work through the four pillars: encapsulation to protect data, inheritance to reuse behaviour, polymorphism to treat related types uniformly, and abstraction to hide detail behind a clear contract. Along the way you'll meet interfaces and enums, two tools you'll use constantly in real Java code.
With a solid grasp of objects, you'll be ready for the next module, Collections & Generics, which builds on classes to store and process groups of objects in a type-safe way.
A language-neutral introduction to the four pillars of OOP: encapsulation, abstraction, inheritance, and polymorphism.
A code-focused walk through classes, objects, and the core OOP ideas in Java, with working examples.
Classes, objects, inheritance, polymorphism, encapsulation, and abstraction explained with real-life examples.
A closer look at how subclasses extend a parent class and how one type can take many forms at runtime.