Step-by-Step Explanation: Intermediate Classes & Interfaces

Kotlin provides a robust hierarchy for building complex systems while maintaining strict rules to prevent common OOP pitfalls.

1. Controlled Inheritance

Unlike Java, where classes are open by default, Kotlin classes are closed by default. To allow inheritance, you must explicitly mark a class as open.

open class Shape(val name: String)
class Circle(...) : Shape("Circle")

2. Abstract Classes

Abstract classes are inherently "open" but cannot be instantiated. They are perfect for defining a base type that has some shared logic but requires sub-classes to fill in specific details (abstract members).

3. Interfaces and Multiple Inheritance

Interfaces are contracts. A class can implement as many interfaces as it needs. - Default Methods: Kotlin interfaces can contain logic (though the "Tour" focused on pure contracts). - Properties: Interfaces can define properties that must be overridden in the implementing class.

4. Delegation with by

Delegation is a first-class citizen in Kotlin. Instead of writing wrapper methods for every function in an interface, you can delegate the entire implementation to a field.

class CanvasSession(val tool: DrawingTool) : DrawingTool by tool

This tells the compiler: "Any call to DrawingTool methods on CanvasSession should be forwarded to tool."

5. The Any Class

Every class in Kotlin inherits from Any. This is the root of the hierarchy and provides methods like toString(), equals(), and hashCode().

6. Verification

Verified by building the project. The Java code calls the top-level demonstration function, which exercises inheritance, abstraction, and delegation patterns.