Java OOP Principles

Encapsulation in Java requires declaring all fields private and exposing them only through getters or setters when truly needed. Return unmodifiable copies of collections, prefer final fields for immutability, and use Java Records for DTOs instead of hand-written getters.

Use composition whenever the relationship is not a clear IS-A relationship, such as an OrderService using a PaymentProcessor. Reserve inheritance for template method patterns, framework contracts, and tightly related subclasses, and mark classes final when they are not designed for extension.

Use an abstract class when subclasses share state or behavior, such as a BaseEntity with id and audit timestamps. Use an interface when defining a pure contract, and consider sealed interfaces in Java 17+ when the set of implementations is known in advance.

Instanceof chains violate polymorphism because every new type forces you to modify the checking method, breaking the open-closed principle. The correct approach is defining a common interface with implementations like EmailNotificationSender and SmsNotificationSender, then injecting the interface type.

Yes, the rules align directly with Spring Boot practices. Dependencies should be declared as interface types and injected via constructors, using @Qualifier to select among multiple implementations such as emailSender or smsSender beans.