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NOTE

Object-Oriented Programming in Java

Encapsulation, abstraction, inheritance, composition, and polymorphism with practical Java design guidance.

JavaCreated Updated 1 min readhistorical

This is a historical learning note and may contain outdated or incomplete understanding.

Object-oriented design groups state and behavior behind abstractions.

  • Encapsulation protects invariants behind an API.
  • Abstraction exposes what callers need while hiding implementation choices.
  • Polymorphism lets code depend on interfaces/contracts rather than concrete implementations.
  • Inheritance can reuse/extend behavior but tightly couples subclasses to superclass design.

Prefer composition when the relationship is “has/use” rather than a true substitutable “is-a”. Good OOP is mainly about maintaining clear invariants and dependency boundaries, not maximizing the number of classes or inheritance levels.