Welcome to the Klassbook!

283 Kotlin Programming Lessons, In Your Browser

CommonsWare's Klassbook is a place for you to learn Kotlin by running and tinkering with Kotlin code directly in your Web browser.

The Klassbook exists to support CommonsWare's books:

• Elements of Kotlin

• Elements of Kotlin Coroutines

The Lessons

• Hello World
  • Hello, World! (In a Function)
  • Hello, World! (In a Class)
• Basic Types
  • Printing the Class of Numbers
  • Using Underscores for Thousands Separators
  • Basic Math
  • Raw Strings
  • trimMargin()
  • String Concatenation
  • Characters
  • Content and Instance Equality
• Variables
  • Variables
  • Typed Variables
  • Read-Only Variables
  • You Can Modify Normal Variables
  • You Cannot Modify Read-Only Variables
  • Variables and Operators
  • String Interpolation
  • String Interpolation with Expressions
  • Increment and Decrement Operators
  • Augmented Assignments
  • Unary Operators
  • Type Conversions
• Functions
  • Functions with Parameters
  • Functions with Return Types
  • Assigning Function Results
  • Local Variables
  • Expression Bodies
  • Expression Bodies and Return Types
  • Default Parameter Values
  • Named Parameter Values
• Collections
  • Arrays
  • Typed Arrays
  • Lists
  • Sets
  • Maps
  • Combining Collections
  • Accessing Elements via []
  • Array Modification
  • Mutable Lists
  • Lambda Expressions
  • Lambda Expressions and Parameters
  • forEach()
  • forEach() is a Function
  • for Loop
  • filter()
  • map()
  • map() Type Conversions
  • Varargs
  • Values for Varargs
  • Spread Operator
  • Spread Operator with Primitive Types
• Branches and Loops
  • if
  • if Without else
  • Single-Line if
  • else if
  • when
  • when Works with Any Type
  • Capturing Values in when
  • when Without an Expression
  • when with Commas
  • when with Ranges
  • when with Expressions for Branches
  • while
  • if as an Expression
  • if as an Expression, Using Blocks
  • when as an Expression
  • break Statements
  • continue Statements
• Classes
  • Classes and Instances
  • Adding Functions to Classes
  • Adding Properties to Classes
  • The this Pseudo-Property
  • Passing this as a Parameter
  • Constructors
  • Formal Constructor Declaration
  • Init Block
  • Constructor Init-Only Parameters
  • Secondary Constructors
  • Inheritance
  • Inheritance and Constructors
  • Inheritance and Functions
  • Inheritance and Overriding Functions
  • override is Transitive
  • Overriding Properties
  • Chaining to Superclass Functions
  • Checking Inheritance Via is
  • is and when
  • Smart Casts
  • Manual Casts
  • Data Class
• Properties
  • Property Initialization in Declaration
  • Property Initialization in Init Block
  • lateinit Properties
  • lazy Properties
• Visibility
  • private Members
  • protected Members
• Scope
  • Instance Scope
  • Function-Local Scope
  • Block-Local Scope
• Abstract Classes and Interfaces
  • Abstract Classes
  • Interfaces
  • Multiple Interfaces
• Object
  • Manual Singletons
  • Object Singletons
  • Objects and Superclasses
  • Companion Objects
  • Companion Objects and Private Members
  • Named Companion Objects
• Nested Objects and Classes
  • Named Nested Objects
  • Object Properties
  • Nested Classes
  • Nested Classes and Private Access
  • Inner Classes
  • Inner Classes and this
  • Inner Classes and Outer this
• Enums and Sealed Classes
  • enum Class
  • enum and Functions
  • enum name and ordinal
  • enum valueOf()
  • enum values()
  • enum and when()
  • Sealed Classes
  • Sealed Classes and when
  • Non-Nested Sealed Classes
• Generics
  • Generics
  • Implied Generic Type
  • Your Own Generic Classes
  • As Generic As You Want
  • Generics and Supertypes
  • Generics and Upper Bounds
• Exceptions
  • Exceptions
  • try and catch
  • Multiple catch Blocks
  • Exceptions and the Call Stack
  • try as an Expression
  • Throwing Exceptions
  • Custom Exception Classes
• Nullability
  • Nullable Types
  • Functions on Nullable Types
  • Safe Calls
  • Safe Call Chains
  • The Elvis Operator
  • Elvis Returns!
  • Smart Contracts
  • Nullable Generic Types
  • Nullable Generic Types, Continued
  • Nullable Casts
  • Nullable Casts, Continued
• Scope Functions
  • let()
  • let() Return Value
  • apply()
  • apply() Return Value
  • run()
  • run() and Safe Calls
  • with()
  • also()
• Functional Programming
  • first()
  • first() Sans Lambda
  • firstOrNull()
  • firstOrNull() and the Elvis Operator
  • Chaining Higher-Order Functions
  • any()
  • any() and Short Circuits
  • fold()
  • joinToString()
  • Function Types
  • Function Reference
  • Function Reference for Member Functions
  • Function References and Receivers
  • Function Types and Direct Invocation
  • Function Types and Parameters
  • Type Aliases
  • Functional Interfaces
• Sequence
  • Sequences and sequenceOf()
  • generateSequence()
• Extension Functions
  • Extension Functions
  • Extension Functions on Nullable Types
  • Private Extension Functions
• Delegates
  • Map as a Property Delegate
  • Delegates.observable()
  • Delegates.vetoable()
• Coroutine Basics
  • A Simple Coroutine Sample
  • Suspending main()
  • A Sequential Coroutine Sample
  • A Parallel Coroutine Sample
  • Massively Parallel Coroutines
  • async()
  • runBlocking()
  • promise()
  • Contexts and Jobs
  • Jobs and Parents
  • Lazy Coroutines
  • Starting Lazy Coroutines
  • join()
  • Canceling a Job
  • cancelAndJoin()
  • Canceling a Parent Cancels Its Children
  • Jobs Cancel When They Crash
  • Parent Jobs Cancel When a Child Crashes
  • Timeouts
  • TimeoutCancellationException
  • withTimeoutOrNull()
  • Canceling and Cooperation
  • Cooperation by Yielding
  • Cooperation by Checking isActive
  • Non-Cancellable Coroutines
  • invokeOnCompletion()
• Flows and Channels
  • A Simple Flow
  • flowOf()
  • single() and singleOrNull()
  • first() and Flow
  • toList() and toSet()
  • suspend Functions and Flows
  • flowOn()
  • Flow Actions
  • Flow onStart()
  • Flows and Exceptions
  • retry()
  • retryWhen()
  • catch()
  • A Simple Channel
  • Hot Channels Via offer()
  • The Channel() Function
  • toList() for Channel
  • Channel and receive()
  • A Zero-Buffer Channel
  • A Fixed-Buffer Channel
  • A Conflated Channel
  • Channels and Fan-out
  • BroadcastChannel
  • A Simple SharedFlow
  • MutableSharedFlow Replay Option
  • A Simple StateFlow
  • StateFlow and Content Equality
  • combine() Operator
  • distinctUntilChanged() Operator
  • distinctUntilChangedBy() Operator
  • debounce() Operator
  • drop() Operator
  • dropWhile() Operator
  • filter() Operator
  • filterIsInstance() Operator
  • filterNot() Operator
  • filterNotNull() Operator
  • flattenConcat() Operator
  • flattenMerge() Operator
  • fold() Operator
  • map() Operator
  • mapLatest() Operator
  • mapNotNull() Operator
  • sample() Operator
  • scan() Operator
  • take() Operator
  • takeWhile() Operator
  • withIndex() Operator
  • zip() Operator
• Advanced Syntax
  • Custom Getter Functions
  • Custom Setter Functions
  • Custom Accessors and the Field
  • Inline Custom Getter Functions
  • Custom Accessor Parallelism
  • Extension val Property
  • Extension var Property
  • Destructuring Declarations
  • Destructuring Declarations of Maps
  • Destructuring Declarations and Lambdas
  • Operator Overloading
  • invoke() Operator
  • Infix Functions
  • Anonymous Functions
  • Anonymous Functions as Parameters
  • Inline Functions
  • Inline Properties
  • Abstract val Properties
  • Abstract val Custom Getter
  • Abstract var
  • Default return Behavior
  • return and Anonymous Functions
  • Labeled Returns
  • Custom Labels
  • Nothing As a Return Type
  • Reified Types
  • Class Delegates