Java OOP and Design: Classes, Interfaces, Abstraction, and Composition

Java OOP and Design

Master Java object-oriented programming principles for backend systems.

Class and Object

• A class defines state and behavior.
• An object is a runtime instance of a class.
• Backend code models domain entities, requests, and service components as classes.

Constructor

• Constructors initialize object state.
• Use constructor injection for dependencies and immutable fields.

Encapsulation

• Encapsulation protects object invariants through private fields and controlled accessors.
• Expose behavior, not internal state.

Inheritance and Polymorphism

• Use inheritance for is-a relationships.
• Avoid deep inheritance trees; prefer composition for flexibility.
• Polymorphism enables interchangeable implementations through interfaces and abstract classes.

Interfaces vs Abstract Classes

• Use interfaces for behavior contracts and loose coupling.
• Use abstract classes when you need shared implementation.
• Prefer interfaces for backend service APIs.

Composition over Inheritance

• Composition delegates work to contained objects and is usually safer than inheritance.
• Choose composition when behaviors can change at runtime.