00-introduction.md

This material was written by Aasmund Eldhuset; it is owned by Khan Academy and is licensed for use under CC BY-NC-SA 3.0 US. Please note that this is not a part of Khan Academy's official product offering.

---

Kotlin is a compiled, statically typed language, which might provide some initial hurdles for people who are used to the interpreted, dynamically typed Python. This document aims to explain a substantial portion of Kotlin's syntax and concepts in terms of how they compare to corresponding concepts in Python.

Kotlin can be compiled for several different platforms. In this document, we assume that the target platform is the Java virtual machine, which grants some extra capabilities - in particular, your code will be compiled to Java bytecode and will therefore be interoperable with the large ecosystem of Java libraries.

Even if you don't know Python, this document should hopefully be a useful introduction to Kotlin, in particular if you are used to other dynamically typed languages. However, if you're coming from a Java background, you're probably better off diving directly into the excellent official docs (from which this doc has drawn a lot of inspiration). To some extent, you can try to write Java code and look stuff up whenever what you're trying to do doesn't work - and some IDEs can even automatically convert Java code to Kotlin.

Contents

• Hello World
• Compiling and running
• Declaring variables
  • Read-only variables
  • Constants
  • Specifying the type explicitly
  • Scopes and naming
• Primitive data types and their limitations
  • Integer types
  • Floating-point and other types
• Strings
• Conditionals
  • if/else
  • Comparisons
  • when
• Collections
• Loops
  • for
  • while
  • continue and break
• Functions
  • Declaration
  • Calling
  • Returning
  • Overloading
  • Varargs and optional/named parameters
• Classes
  • Declaration and instantiation
  • Inherited built-in functions
  • Properties
  • Constructors and initializer blocks
  • Setters and getters
  • Member functions
  • Lateinit
  • Infix functions
  • Operators
  • Enum classes
  • Data classes
• Exceptions
  • Throwing and catching
  • Nothing
• Null safety
  • Working with nulls
  • Safe call operator
  • Elvis operator
  • Not-null assertion operator
• Functional programming
  • Function types
  • Function literals: lambda expressions and anonymous functions
  • Comprehensions
  • Receivers
  • Inline functions
  • Nice utility functions
    • run(), let(), and with()
    • apply() and also()
    • takeIf() and takeUnless()
• Packages and imports
  • Packages
  • Imports
• Visibility modifiers
• Inheritance
  • Subclassing
  • Overriding
  • Interfaces
  • Abstract classes
  • Polymorphism
  • Casting and type testing
  • Delegation
  • Delegated properties
  • Sealed classes
• Objects and companion objects
  • Object declarations
  • Companion objects
  • Object expressions
• Generics
  • Generic type parameters
  • Constraints
  • Variance
    • Introduction
    • Declaration-site covariance and contravariance
    • Variance directions
    • Type projections (use-site covariance and contravariance)
  • Reified type parameters
• Extension functions/properties
• Member references and reflection
  • Property references
  • Function references
  • Obtaining member references from a class reference
  • Java-style reflection
• Annotations
• File I/O
• Scoped resource usage
• Documentation

---

Next: Hello World →