Kotlin Notes for Professionals (Mike Stebbins) (Z-Library)

Contents About ............................................................................................................. ...................................................................... 1 Chapter 1: Getting started with Kotlin ............................................................................................................. .. 2 Section 1.1: Hello World ........................................................................................................................... ...................... 2 Section 1.2: Hello World using a Companion Object .................................................................................................. 2 Section 1.3: Hello World using an Object Declaration ................................................................................................ 3 Section 1.4: Main methods using varargs .................................................................................................................... 4 Section 1.5: Compile and Run Kotlin Code in Command Line ................................................................................... 4 Section 1.6: Reading input from Command Line ........................................................................................................ 4 Chapter 2: Basics of Kotlin ............................................................................................................. .......................... 6 Section 2.1: Basic examples ........................................................................................................................... ............... 6 Chapter 3: Strings .............................................................................................................

............................................ 7 Section 3.1: String Equality ........................................................................................................................... ................. 7 Section 3.2: String Literals ........................................................................................................................... ................. 7 Section 3.3: Elements of String ........................................................................................................................... .......... 8 Section 3.4: String Templates ........................................................................................................................... ............ 8 Chapter 4: Arrays ............................................................................................................. ............................................ 9 Section 4.1: Generic Arrays ........................................................................................................................... ................ 9 Section 4.2: Arrays of Primitives ........................................................................................................................... ....... 9 Section 4.3: Create an array ........................................................................................................................... ............. 9 Section 4.4: Create an array using a closure ............................................................................................................. 9 Section 4.5: Create an uninitialized array ................................................................................................................... 9 Section 4.6: Extensions ........................................................................................................................... ..................... 10 Section 4.7: Iterate Array

........................................................................................................................... ................. 10 Chapter 5: Collections ............................................................................................................. ................................. 11 Section 5.1: Using list ........................................................................................................................... ........................ 11 Section 5.2: Using map ........................................................................................................................... .................... 11 Section 5.3: Using set ........................................................................................................................... ....................... 11 Chapter 6: Enum ............................................................................................................. ............................................. 12 Section 6.1: Initialization ........................................................................................................................... ................... 12 Section 6.2: Functions and Properties in enums ...................................................................................................... 12 Section 6.3: Simple enum ........................................................................................................................... ................. 12 Section 6.4: Mutability ........................................................................................................................... ...................... 12 Chapter 7: Functions ............................................................................................................. .................................... 14

Section 7.1: Function References ........................................................................................................................... .... 14 Section 7.2: Basic Functions ........................................................................................................................... ............ 15 Section 7.3: Inline Functions ........................................................................................................................... ............ 16 Section 7.4: Lambda Functions ........................................................................................................................... ....... 16 Section 7.5: Operator functions ........................................................................................................................... ...... 16 Section 7.6: Functions Taking Other Functions ........................................................................................................ 17 Section 7.7: Shorthand Functions ........................................................................................................................... ... 17 Chapter 8: Vararg Parameters in Functions ................................................................................................ 18 Section 8.1: Basics: Using the vararg keyword ......................................................................................................... 18 Section 8.2: Spread Operator: Passing arrays into vararg functions .................................................................... 18 Chapter 9: Conditional Statements ............................................................................................................. ...... 19 Section 9.1: When-statement argument matching .................................................................................................. 19 Section 9.2: When-statement as expression ............................................................................................................ 19

Section 9.3: Standard if-statement ........................................................................................................................... . 19 Section 9.4: If-statement as an expression ............................................................................................................... 19 Section 9.5: When-statement instead of if-else-if chains ....................................................................................... 20 Section 9.6: When-statement with enums ................................................................................................................ 20 Chapter 10: Loops in Kotlin ............................................................................................................. ....................... 22 Section 10.1: Looping over iterables ........................................................................................................................... 22 Section 10.2: Repeat an action x times ...................................................................................................................... 22 Section 10.3: Break and continue ........................................................................................................................... .... 22 Section 10.4: Iterating over a Map in kotlin ............................................................................................................... 23 Section 10.5: Recursion ........................................................................................................................... .................... 23 Section 10.6: While Loops ........................................................................................................................... ................ 23 Section 10.7: Functional constructs for iteration ...................................................................................................... 23 Chapter 11: Ranges ............................................................................................................. ........................................ 25

Section 11.1: Integral Type Ranges ........................................................................................................................... .. 25 Section 11.2: downTo() function ........................................................................................................................... ....... 25 Section 11.3: step() function ........................................................................................................................... ............. 25 Section 11.4: until function ........................................................................................................................... ................ 25 Chapter 12: Regex ............................................................................................................. .......................................... 26 Section 12.1: Idioms for Regex Matching in When Expression ................................................................................ 26 Section 12.2: Introduction to regular expressions in Kotlin ..................................................................................... 27 Chapter 13: Basic Lambdas ............................................................................................................. ....................... 30 Section 13.1: Lambda as parameter to filter function .............................................................................................. 30 Section 13.2: Lambda for benchmarking a function call ......................................................................................... 30 Section 13.3: Lambda passed as a variable .............................................................................................................. 30 Chapter 14: Null Safety ............................................................................................................. .............................. 31

Section 14.1: Smart casts ........................................................................................................................... .................. 31 Section 14.2: Assertion ........................................................................................................................... ...................... 31 Section 14.3: Eliminate nulls from an Iterable and array ......................................................................................... 31 Section 14.4: Null Coalescing / Elvis Operator .......................................................................................................... 31 Section 14.5: Nullable and Non-Nullable types ......................................................................................................... 32 Section 14.6: Elvis Operator (?:) ........................................................................................................................... ....... 32 Section 14.7: Safe call operator ........................................................................................................................... ....... 32 Chapter 15: Class Delegation ............................................................................................................. ................... 34 Section 15.1: Delegate a method to another class ................................................................................................... 34 Chapter 16: Class Inheritance ............................................................................................................. .................. 35 Section 16.1: Basics: the 'open' keyword .................................................................................................................... 35 Section 16.2: Inheriting fields from a class ................................................................................................................ 35 Section 16.3: Inheriting methods from a class .......................................................................................................... 36

Section 16.4: Overriding properties and methods .................................................................................................... 36 Chapter 17: Visibility Modifiers ............................................................................................................. ................ 38 Section 17.1: Code Sample ........................................................................................................................... ................ 38 Chapter 18: Generics ............................................................................................................. .................................... 39 Section 18.1: Declaration-site variance ...................................................................................................................... 39 Section 18.2: Use-site variance ........................................................................................................................... ........ 39 Chapter 19: Interfaces ............................................................................................................. ................................. 41 Section 19.1: Interface with default implementations ............................................................................................... 41 Section 19.2: Properties in Interfaces ......................................................................................................................... 42 Section 19.3: super keyword ........................................................................................................................... ............ 42 Section 19.4: Basic Interface ........................................................................................................................... ............ 42

Section 19.5: Conflicts when Implementing Multiple Interfaces with Default Implementations .......................... 43 Chapter 20: Singleton objects ............................................................................................................. ................. 44 Section 20.1: Use as replacement of static methods/fields of java ....................................................................... 44 Section 20.2: Use as a singleton ........................................................................................................................... ..... 44 Chapter 21: coroutines ............................................................................................................. ................................ 45 Section 21.1: Simple coroutine which delay's 1 second but not blocks ................................................................... 45 Chapter 22: Annotations ............................................................................................................. ............................ 46 Section 22.1: Meta-annotations ........................................................................................................................... ....... 46 Section 22.2: Declaring an annotation ...................................................................................................................... 46 Chapter 23: Type aliases ............................................................................................................. ............................ 47 Section 23.1: Function type ........................................................................................................................... .............. 47 Section 23.2: Generic type

........................................................................................................................... ............... 47 Chapter 24: Type-Safe Builders ............................................................................................................. ............ 48 Section 24.1: Type-safe tree structure builder .......................................................................................................... 48 Chapter 25: Delegated properties ............................................................................................................. ........ 49 Section 25.1: Observable properties .......................................................................................................................... 49 Section 25.2: Custom delegation ........................................................................................................................... .... 49 Section 25.3: Lazy initialization ........................................................................................................................... ....... 49 Section 25.4: Map-backed properties ....................................................................................................................... 49 Section 25.5: Delegate Can be used as a layer to reduce boilerplate .................................................................. 49 Chapter 26: Reflection ............................................................................................................. ................................ 51 Section 26.1: Referencing a class ........................................................................................................................... .... 51

Section 26.2: Inter-operating with Java reflection .................................................................................................. 51 Section 26.3: Referencing a function ......................................................................................................................... 51 Section 26.4: Getting values of all properties of a class ......................................................................................... 51 Section 26.5: Setting values of all properties of a class .......................................................................................... 52 Chapter 27: Extension Methods ............................................................................................................. .............. 54 Section 27.1: Potential Pitfall: Extensions are Resolved Statically .......................................................................... 54 Section 27.2: Top-Level Extensions ........................................................................................................................... 54 Section 27.3: Lazy extension property workaround ................................................................................................ 54 Section 27.4: Sample extending Java 7+ Path class ............................................................................................... 55 Section 27.5: Sample extending long to render a human readable string ........................................................... 55 Section 27.6: Sample extending Java 8 Temporal classes to render an ISO formatted string .......................... 55 Section 27.7: Using extension functions to improve readability ............................................................................. 55 Section 27.8: Extension functions to Companion Objects (appearance of Static functions) .............................. 56 Section 27.9: Extensions for easier reference View from code .............................................................................. 57 Chapter 28: DSL Building .............................................................................................................

........................... 58 Section 28.1: Infix approach to build DSL .................................................................................................................. 58 Section 28.2: Using operators with lambdas ............................................................................................................ 58 Section 28.3: Overriding invoke method to build DSL ............................................................................................. 58 Section 28.4: Using extensions with lambdas ........................................................................................................... 58 Chapter 29: Idioms ............................................................................................................. ........................................ 60 Section 29.1: Serializable and serialVersionUid in Kotlin ......................................................................................... 60 Section 29.2: Delegate to a class without providing it in the public constructor .................................................. 60 Section 29.3: Use let or also to simplify working with nullable objects ................................................................. 61 Section 29.4: Use apply to initialize objects or to achieve method chaining ........................................................ 61 Section 29.5: Fluent methods in Kotlin ...................................................................................................................... 61 Section 29.6: Filtering a list ........................................................................................................................... .............. 62 Section 29.7: Creating DTOs (POJOs/POCOs) ........................................................................................................ 62 Chapter 30: RecyclerView in Kotlin ............................................................................................................. ...... 63

Section 30.1: Main class and Adapter ........................................................................................................................ 63 Chapter 31: logging in kotlin ............................................................................................................. ..................... 65 Section 31.1: kotlin.logging ........................................................................................................................... ............... 65 Chapter 32: Exceptions ............................................................................................................. ................................ 66 Section 32.1: Catching exception with try-catch-finally ........................................................................................... 66 Chapter 33: JUnit ............................................................................................................. ........................................... 67 Section 33.1: Rules ........................................................................................................................... ............................. 67 Chapter 34: Kotlin Android Extensions ............................................................................................................ 68 Section 34.1: Using Views ........................................................................................................................... ................. 68 Section 34.2: Configuration ...........................................................................................................................

............. 68 Section 34.3: Painful listener for getting notice, when the view is completely drawn now is so simple and awesome with Kotlin's extension ....................................................................................................................... 69 Section 34.4: Product flavors ........................................................................................................................... .......... 69 Chapter 35: Kotlin for Java Developers ......................................................................................................... 71 Section 35.1: Declaring Variables ........................................................................................................................... .... 71 Section 35.2: Quick Facts ........................................................................................................................... ................. 71 Section 35.3: Equality & Identity ........................................................................................................................... ...... 71 Section 35.4: IF, TRY and others are expressions, not statements ......................................................................... 72 Chapter 36: Java 8 Stream Equivalents ......................................................................................................... 73 Section 36.1: Accumulate names in a List ................................................................................................................. 73 Section 36.2: Collect example #5 - find people of legal age, output formatted string ........................................ 73 Section 36.3: Collect example #6 - group people by age, print age and names together ................................. 73 Section 36.4: Dierent Kinds of Streams #7 - lazily iterate Doubles, map to

Int, map to String, print each ........................................................................................................................... ..................................................... 74 Section 36.5: Counting items in a list after filter is applied ..................................................................................... 75 Section 36.6: Convert elements to strings and concatenate them, separated by commas ............................... 75 Section 36.7: Compute sum of salaries of employee .............................................................................................. 75 Section 36.8: Group employees by department ...................................................................................................... 75 Section 36.9: Compute sum of salaries by department .......................................................................................... 75 Section 36.10: Partition students into passing and failing ....................................................................................... 75 Section 36.11: Names of male members ................................................................................................................... 76 Section 36.12: Group names of members in roster by gender ............................................................................... 76 Section 36.13: Filter a list to another list .................................................................................................................... 76 Section 36.14: Finding shortest string a list ............................................................................................................... 76 Section 36.15: Dierent Kinds of Streams #2 - lazily using first item if exists ....................................................... 76 Section 36.16: Dierent Kinds of Streams #3 - iterate a range of Integers .......................................................... 77 Section 36.17: Dierent Kinds of Streams #4 - iterate an array, map the values, calculate the average ........................................................................................................................... ..................................................... 77 Section 36.18: Dierent Kinds of Streams #5 - lazily iterate a list of strings, map the values, convert to Int, find max ........................................................................................................................... .................................... 77 Section 36.19: Dierent Kinds of Streams #6 - lazily iterate a stream of Ints, map the values, print results

........................................................................................................................... ..................................................... 77 Section 36.20: How streams work - filter, upper case, then sort a list ................................................................... 78 Section 36.21: Dierent Kinds of Streams #1 - eager using first item if it exists ................................................... 78 Section 36.22: Collect example #7a - Map names, join together with delimiter ................................................... 78 Section 36.23: Collect example #7b - Collect with SummarizingInt ....................................................................... 79 Chapter 37: Kotlin Caveats ............................................................................................................. ....................... 81 Section 37.1: Calling a toString() on a nullable type ................................................................................................ 81 Appendix A: Configuring Kotlin build ............................................................................................................. ... 82 Section A.1: Gradle configuration ........................................................................................................................... .... 82 Section A.2: Using Android Studio ........................................................................................................................... .. 83 Section A.3: Migrating from Gradle using Groovy script to Kotlin script ............................................................... 84 Credits ............................................................................................................. ................................................................. 86 You may also like ............................................................................................................. ........................................... 88

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Chapter 1: Getting started with Kotlin Version Release Date 1.0.0 2016-02-15 1.0.1 2016-03-16 1.0.2 2016-05-13 1.0.3 2016-06-30 1.0.4 2016-09-22 1.0.5 2016-11-08 1.0.6 2016-12-27 1.1.0 2017-03-01 1.1.1 2017-03-14 1.1.2 2017-04-25 1.1.3 2017-06-23 1.1.6 2017-11-13 1.2.2 2018-01-17 1.2.3 2018-03-01 1.2.4 2018-04-19

Section 1.1: Hello World All Kotlin programs start at the main function. Here is an example of a simple Kotlin "Hello World" program: package my.program fun main( args: Array< String>) { println( "Hello, world!" ) } Place the above code into a file named Main.kt (this filename is entirely arbitrary) When targeting the JVM, the function will be compiled as a static method in a class with a name derived from the filename. In the above example, the main class to run would be my.program .MainKt . To change the name of the class that contains top-level functions for a particular file, place the following annotation at the top of the file above the package statement: @file: JvmName( "MyApp" ) In this example, the main class to run would now be my.program .MyApp . See also: Package level functions including @JvmName annotation. Annotation use- site targets