40. Interfaces and Polymorphism

Interfaces provide another powerful way to enforce structure. They define a contract: any class implementing an interface must implement all the methods listed in that interface.

Defining an Interface

• Defined using the interface keyword.
• All methods in an interface must be public.
• Interfaces contain only method signatures, no logic or properties (except constants).

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Implementing an Interface

A class uses the implements keyword to adopt an interface. If it fails to implement all required methods, it results in a fatal error.

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Polymorphism in Action

Since both classes implement the same interface, we can use them interchangeably, relying on the common method signatures (processPayment). This is polymorphism.

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