An interface defines a contract. It declares which public methods a class must implement, but it does not provide any implementation.

Interfaces are declared with the interface keyword. All methods inside an interface are implicitly abstract and must be public.

When a class uses implements, it agrees to fulfill the contract. It must define every method declared in the interface.

Unlike abstract classes:

• Interfaces cannot have properties.
• Interfaces cannot contain implemented methods.
• A class can implement multiple interfaces.
• A class can extend a parent class and implement an interface at the same time.

Conceptually:
Abstract class → “is a” relationship with shared structure.
Interface → “can do” capability contract.

Interfaces enable polymorphism across unrelated classes. If multiple classes implement the same interface, they can be treated uniformly based on shared behavior.