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Interfaces (in Java and Android — interfaces, in Kotlin — interfaces or abstract classes) serve to define a contract, that is, a set of methods that a class must implement. They allow:

• Standardization of behavior across different classes.
• Implementing polymorphism — working with objects of different classes through a common interface.
• Separating declaration and implementation, which improves modularity and testability.

For example, the OnClickListener interface defines the onClick() method, which must be implemented by all classes handling button clicks. This allows the system to invoke the handler without knowing the specific class.