Lectures

We break down why Kotlin needs enum class instead of “magic strings”: type-safe values (for example, Status.DONE), standard properties name and ordinal (and why you must not use ordinal as a stable identifier), getting all options via entries, strict parsing via valueOf, as well as enums with parameters and methods (and the mandatory ; before the body). At the end — a practical example for console expense tracking and common mistakes.

We break down how to use when with enum in Kotlin so that the compiler checks branch completeness: why an exhaustive when is better written without else, how to turn branching into a “mapping table”, how when works as an expression (returns a value), what happens to the result type, and how to correctly handle null for a nullable enum ( E?).

We break down how sealed hierarchies differ from enum class, and why sealed class is an ideal tool for modeling an “operation result” without null and “magic strings”. We will assemble a minimal CommandResult model with Ok/ Error variants, apply it in a CLI expense-tracker example, and discuss common design mistakes when modeling sealed results.

We break down how to safely work with “general” types in Kotlin: checking a type with is/ !is, getting a smart cast (including in Kotlin 2.x with the K2 compiler), and also how unsafe casting as differs from safe casting as? when paired with the Elvis operator ?:. At the end: a practical example of parsing an amount from Any?, a mermaid diagram, and typical mistakes.

We break down a practical pattern for a Kotlin CLI application: a single command result via a sealed hierarchy CommandResult ( Ok/ Error), returning results from business functions without println (), and a message “dispatcher” via an exhaustive when without else. Separately, we show separation of responsibilities: handleCommand does the work, render formats, main outputs.