🧱 Abstract Class
An abstract class is like a partially built house. It can have:
- Both completed rooms (methods with code) and unfinished rooms (abstract methods).
- Variables, constructors, and non-abstract methods.
- Only one abstract class can be inherited due to single inheritance in Java.
🌐 Interface
An interface is like a blueprint. It says:
“This is what you must build—but I’m not doing any of it for you.”
- All methods are abstract by default (until Java 8).
- No constructors or instance variables (only constants).
- A class can implement multiple interfaces (this is how Java supports multiple inheritance).
🧠 Quick Comparison Table
| Feature | Abstract Class | Interface |
|---|---|---|
| Contains concrete methods | Yes | Only from Java 8 (default/static) |
| Constructors | Yes | ❌ No |
| Variables | Instance + static | Only static + final (constants) |
| Multiple Inheritance | ❌ Not supported | ✅ Yes (can implement many) |
| Use Case | “Is-a” relationship with shared code | “Can-do” behavior or capability |
🎭 Real-world Analogy
- Abstract class: A general job role like Musician—they all share a few skills (e.g., read music), but each musician plays a different instrument (implemented by subclasses).
- Interface: A skill like Driver’s License—you can have one even if your job is something else. It just means you can drive.
