OOP

Core OOP Concepts

Concept	Description	Key Feature
Encapsulation	Bundling state and behaviour, exposing only what's needed via access modifiers	Private fields + public getters/setters (or records for immutable data)
Inheritance	A class (extends) reuses and specialises another class's members	Single inheritance for classes; super accesses parent members
Polymorphism	Same method signature, different runtime behaviour	Method overriding (runtime) vs overloading (compile-time)
Abstraction	Expose behaviour contracts without exposing implementation details	Abstract classes and interfaces

Interface vs Abstract Class

Aspect	Interface	Abstract Class
Multiple inheritance	Yes — a class can implement several interfaces	No — a class extends only one abstract class
State (instance fields)	No instance fields (only static final constants)	Can hold instance fields
Constructors	Not allowed	Allowed
Method bodies	default and static methods can have bodies (Java 8+)	Any method can have a body
When to choose	Define a capability/contract shared across unrelated types	Share common state and partial implementation among closely related types

Exam tip: Prefer interfaces for defining a contract implementable by unrelated classes; prefer an abstract class when subclasses share common state or a partial implementation.