Lectures

We break down an interface in Kotlin as a contract: what exactly a class “promises” when it writes : SomeInterface, how fun methods work without a body and with a default implementation, why val/ var properties in an interface are not “storage fields”, and how to write reusable functions that accept an interface type (for example, CliPrintable) instead of concrete classes. Inside: practical examples with Identifiable, Greeter, RenderableLine, and HasTitle, plus a walkthrough of typical mistakes.

We break down multiple interface implementation in Kotlin: why it’s better to keep interfaces narrow, how Kotlin forbids ambiguity when implementations conflict, when override is mandatory, and how to explicitly choose the needed implementation via super<...>. Using examples, we’ll show conflicts of default methods, a combination of default + abstract declaration, conflicts between a base class and an interface, as well as similar situations with properties ( val).

We break down why Kotlin needs fun interface, what SAM conversion is, and how the compiler “turns” a lambda into an interface object. We will compare a SAM interface with function types ( ( Int ) -> Boolean) and typealias, build a registry of CLI commands via CommandHandler, discuss closures and common mistakes (two abstract functions, signature mismatch, hidden mutability).

We break down how sealed interface helps model a “closed set of variants” instead of strings, flags, and null. We learn to parse input into the Command type, write an exhaustive when without else, introduce an execution result via CommandResult ( Ok/ Error), and also look at the nuances of choosing between enum, sealed class, and sealed interface.

We will cover three forms of objects in Kotlin: a named singleton via object, an “almost static” API via companion object, and one-off implementations via object : Interface { ... }. Along the way, we will discuss where to store constants and factories, how not to turn a singleton into a “global junk drawer,” and why an anonymous object may “lose” its unique methods because of the variable’s static type.