🔸 TL;DR
▪️ Subclass when you truly need to extend an existing class (or you’re forced to).
▪️ Abstract class when you want a shared template + partial implementation for closely related types.
▪️ Interface when you want a capability contract across unrelated types (great for testing + flexibility). ✅
🔸 THE “ONE PARENT” RULE (WHY IT MATTERS)
▪️ In Java, a class can extend only one class ➜ choosing inheritance too early can paint you into a corner 🎨🚪
▪️ But a class can implement multiple interfaces ➜ composition-friendly 🔧
🔸 WHEN TO CHOOSE A SUBCLASS 👶➡️🧱
▪️ When you need to extend a class and add/override functionality (real “is-a” relationship).
▪️ When you are required to extend a given class (framework constraints, provided base types).
▪️ When you need access to protected hooks or lifecycle methods designed for extension 🪝
🔸 WHEN TO CHOOSE AN ABSTRACT CLASS 🧩🏗️
▪️ When you want to provide a template for a family of subclasses (common fields + shared behavior).
▪️ When you need shared state (fields) + some implemented methods + some abstract methods.
▪️ When the relationship is tight and hierarchical (same domain concept, same core invariants) 🔒
🔸 WHEN TO CHOOSE AN INTERFACE ✅🔌
▪️ When there’s no common parent class but you want the same behavior (“capability”) across types.
▪️ When you want better testability: mock/stub a contract without dragging a concrete base class 🧪
▪️ When you want to expose a public API boundary (implementation can change, contract stays).
▪️ When you need a marker interface (semantic tagging / classification) 🏷️
🔸 “BUT INTERFACES HAVE DEFAULT METHODS NOW…” 🤔
▪️ Yes: interfaces with default methods can cover some use cases that used to require abstract classes.
▪️ Still: interfaces can’t hold instance state like an abstract class, so they’re not always interchangeable.
🔸 TAKEAWAYS 🎯
▪️ Prefer interfaces for capabilities + flexibility + testing.
▪️ Use abstract classes for a shared template + shared state within a tight hierarchy.
▪️ Use subclassing when you must, or when the base class was designed to be extended.
▪️ If you hesitate: “interface first” is often the safer default in Java.
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