Problem Statement
How does Kotlin support the delegation pattern with the by keyword for class and property delegation?
Explanation
Kotlin provides first-class support for the delegation pattern using the by keyword, allowing a class to delegate implementation of an interface to another object like class Derived open paren val base colon Base close paren colon Base by base where all interface methods are automatically forwarded to the base object. This eliminates boilerplate delegation code where you'd manually forward each method call, and you can still override specific methods if needed while delegating the rest.
Property delegation using by allows delegating getter and setter implementation to another object following the getValue and setValue operator conventions, enabling patterns like lazy initialization with lazy, observable properties with Delegates dot observable, storing properties in a map with map delegation, or custom delegates. Built-in delegates include lazy for thread-safe lazy initialization computed only on first access, observable for property observers notified on changes, and vetoable for validation before allowing changes.
Class delegation is useful for the decorator pattern or when you want to expose an interface but delegate implementation to a contained object, while property delegation reduces boilerplate for common patterns like lazy initialization or validation. Understanding delegation helps you write more maintainable code by separating concerns and reusing delegation logic across properties or classes.
