Kotlin Playground

• Main.kt - no package; no semicolon

Basics

• Hello1, Hello2 - top-level function; main with args & no args; string templates, Array syntax & wrappers; if expression
• Variables - read-only & mutable variables; type inference
• Functions1, Functions2, FunctionsJ - expression body; Unit; top-level, member, local; calling from Java; named & default args; @JvmOverloads

Control Structures

• Conditionals - if & when; is & smart cast;
• Loops - for, in, ranges
• InChecksAndRanges - in use cases; ranges
• CheckingIdentifier - small task to practice ranges
• Exceptions - no checked/unchecked; throw & try expressions; @Throws

Extensions

• ExtensionFunctions - extension functions, examples, standard library
• SumExtensionFunction - small task to create extension function
• CallingExtensions - how extensions interact with inheritance and can extensions hide members
• With extensions, we can keep our classes and interfaces APIs minimal and write everything else as extension, but keep them close together.
• We can use extensions when we don't control some class or interface, many existing Java libraries have companion libraries with a number of extensions.

Nullability

• NullableTypes - nullable types; nullability operators
• EmptyOrNull - small task to check if string is null or empty
• SafeCasts - a safe way to cast an expression to a type
• Nullability is very important

Functional Programming

• Lambdas - lambda syntax
• CollectionsOps - common operations on collections
• OperationsQuiz - practice common operations on collections
• InterchangeablePredicates - small task to practice none, any, all
• FunctionTypes - function types, nullability
• MemberReferences - member references, function references, bound, non-bound
• ReturnFromLambda - how return behaves with lambdas
• Kotlin is not purely functional language, but rather combines OOP and functional styles

Properties

• Properties - properties, fields, accessors
• InterfaceAndExtProperties - properties in interfaces, extension properties
• LazyAndLateInit - lazy properties, late initialization

OOP

• Defaults are different in Kotlin - public and final by default
• If you want to make it non-final, you have to set it explicitly as open
• No package private visibility in Kotlin, use internal for that - visible in module
• Module is a set of Kotlin files compiled together (IntelliJ IDEA, Maven project, Gradle source set)
• So we have there modifiers: public, protected private, internal. Then: final, open, abstract. Also override.
• In Kotlin we can have multiple classes in the same file, while in Java we prefer one class per file. One file can additionally contain multiple top-level functions, properties, etc.
• In Kotlin, package doesn't have to correspond to directory structure - we can have org.company.store package, but it can be physically located in top level store folder for example. Good recommendatioon is to simply omit org.company or org.company.project from directory structure.
• CtxInherit - constructors, inheritance syntax
• ClassModifiers - class modifiers, enum, data, equality, sealed, inner, nested, class delegation
• Objects - objects, object declaration, object expression, companion object, "static" members
• Constants
• Generics

Conventions

• OperatorOverloading - operator overloading
• Conventions

Inline Functions

• LibraryFunctions - by inlining, library functions look like built-in language construct
• Inline - the power of inline

Mix

• Sequences - collection vs sequences, how to create mix...
• LambdaWithReceiver - lambda with receiver
• Types - types, type hierarchy, Any, Unit, Nothing, nullable types, collection types

Other

• Kotlin
• Gradle
• Ktor
• Kodein