Lectures

Let’s break down why the primary constructor in Kotlin is the class’s public contract: how a constructor parameter differs from a property (when you need val/ var), how to design a convenient object-creation API using Expense as an example, why to use default values and named arguments, and how to choose mutability consciously so that types remain clear and safe.

We break down how the init-block works in Kotlin: when it runs, how to use it to normalize input ( trim(), lowercase()), prepare derived properties (for example, summary), enforce invariants via require(...), and how to read the execution order of multiple init-blocks. At the end: typical design mistakes and practical advice for console applications.

We break down why Kotlin needs secondary constructors, how delegation via this (...) works, why it is important to keep a single initialization path through the primary constructor and init, and how to turn secondary constructors into “adapters” for raw input (strings, CSV, partial data) in a practical case with Expense.

We break down the order in which Kotlin creates an object: first, property initializers ( val/ var) run, then init blocks (top to bottom), and only after that does the body of a secondary constructor run. Using examples with println(), require(), and delegation : this(...), we learn how to correctly “fix” input data before validations and avoid hidden expensive work inside initializers.

We break down the fail-fast approach in Kotlin and two key tools for validation: require(...) for input arguments (with IllegalArgumentException) and check(...) for state correctness (with IllegalStateException). Using practical examples (the Expense and ExpenseTracker classes), we show where to place checks, how to write useful “expected → got” messages, and how to carefully catch these errors at the main level.