List of lectures for the course Kotlin, 41 quest level on JavaRush

Nullable as a contract: where null is justified and where it gets in the way
Kotlin SELF
41 level,0 lecture
We break down nullable types in Kotlin as a contract: what String ? really means, when the safe call ?. and the Elvis operator ?: are the norm at system boundaries (input, parsing, lookup), and when null starts to “leak” into business logic. Using examples with toIntOrNull(), lookups in Map, and a mini app ExpenseTracker, we learn to localize nullable usage and avoid anti-patterns like !! and sentinels ( 0, - 1, "UNKNOWN").
Open
What to use instead of null: preconditions, sealed results, Result
Kotlin SELF
41 level,1 lecture
We break down why null often turns into a “foggy contract”, and what to replace it with in Kotlin in a practical way: preconditions ( require/ check and the *NotNull variants), meaningful results via sealed classes (an exhaustive when), and also the standard container Result<T> for “error as a value”. Using a CLI expense tracker as an example, we show how to distribute these approaches across layers and avoid common pitfalls.
Open
Smart cast and how to use it
Kotlin SELF
41 level,2 lecture
Let’s break down how smart cast works in Kotlin: when the compiler “promises” you a narrower type without as and without !!, why it sometimes refuses (instability of var, properties like obj . prop, lambdas, repeated reads), and which techniques help: taking a snapshot into a local val, guard clauses with return, and also an explicit conversion of T? -> T via requireNotNull/ checkNotNull. At the end — a practical example with searching for an Expense? and a cheat-sheet table.
Open
Removing null from collections and transformation pipelines
Kotlin SELF
41 level,3 lecture
We’ll go over practical “sanitation” for Kotlin collections: why List<T?> quickly infects a pipeline with checks, how to “kill” null early with filterNotNull() and mapNotNull(), how to build an input-normalization gateway, carefully handle search results ( find/ firstOrNull), and transform Map<K, V?> into Map<K, V> without !!. At the end—nuances of takeIf/ takeUnless and common mistakes.
Open
Designing a model without nullable fields
Kotlin SELF
41 level,4 lecture
We break down a practical approach to modeling in Kotlin: keep null at the boundary (input/external data), normalize strings via trim(), parse numbers via toIntOrNull(), return errors explicitly via a sealed class ( Ok/ Error), and in the domain model use only strict types without ? and protect invariants with require(...).
Open