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Kotlin
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Is Kotlin easy to learn for a Java developer?
Easy to learn
For anyone with existing developer experience, understanding and learning Kotlin will be almost effortless. Kotlin's syntax and design are simple to comprehend and yet very powerful to use. This is a key reason why Kotlin has surpassed Java as being the go-to language for Android app development.How long does it take to learn Kotlin for a Java developer?
The estimated duration of this course is two weeks, and the ideal skill level is intermediate. It means that your learning experience would be best if you have at least some knowledge of programming in general and familiarity with the basic OOP (object-oriented programming) concepts.Can I learn Kotlin instead of Java?
Kotlin language is a popular choice for android development. Kotlin has a range of features that make it more accessible and ideal for android development. However, Kotlin cannot replace Java, which is an old programming language used for several projects other than android development.- Can I learn Kotlin without Java? You can start with Kotlin without having any knowledge of Java. You can find many resources on the internet to learn Kotlin. You can comparatively find fewer examples of codes in Kotlin than in Java.
Kotlin
#kotlinTable of Contents
About1
Chapter 1: Getting started with Kotlin2
Remarks2
Compiling Kotlin2
Versions2
Examples3
Hello World3
Hello World using an Object Declaration3
Hello World using a Companion Object4
Main methods using varargs5
Compile and Run Kotlin Code in Command Line5
Reading input from Command Line5
Chapter 2: Annotations7
Examples7
Declaring an annotation7
Meta-annotations7
Chapter 3: Arrays9
Examples9
Generic Arrays9
Arrays of Primitives9
Extensions10
Iterate Array10
Create an array10
Create an array using a closure10
Create an uninitialized array11
Chapter 4: Basic Lambdas12
Syntax12
Remarks12
Examples13
Lambda as parameter to filter function13
Lambda passed as a variable13
Lambda for benchmarking a function call13
Chapter 5: Basics of Kotlin14
Introduction14
Remarks14
Examples14
Basic examples14
Chapter 6: Class Delegation16
Introduction16
Examples16
Delegate a method to another class16
Chapter 7: Class Inheritance17
Introduction17
Syntax17
Parameters17
Examples17
Basics: the 'open' keyword17
Inheriting fields from a class18
Defining the base class:18
Defining the derived class:18
Using the subclass:18
Inheriting methods from a class18
Defining the base class:18
Defining the derived class:18
The Ninja has access to all of the methods in Person18Overriding properties and methods19
Overriding properties (both read-only and mutable):19Overriding methods:19
Chapter 8: Collections20
Introduction20
Syntax20
Examples20
Using list20
Using map20
Using set20
Chapter 9: Conditional Statements21
Remarks21
Examples21
Standard if-statement21
If-statement as an expression21
When-statement instead of if-else-if chains22
When-statement argument matching22
When-statement as expression23
When-statement with enums23
Chapter 10: Configuring Kotlin build25
Examples25
Gradle configuration25
Targeting JVM25
Targeting Android25
Targeting JS25
Using Android Studio26
Install the plugin26
Configure a project26
Converting Java26
Migrating from Gradle using Groovy script to Kotlin script27Chapter 11: coroutines29
Introduction29
Examples29
Simple coroutine which delay's 1 second but not blocks29Chapter 12: Delegated properties30
Introduction30
Examples30
Lazy initialization30
Observable properties30
Map-backed properties30
Custom delegation30
Delegate Can be used as a layer to reduce boilerplate31Chapter 13: DSL Building33
Introduction33
Examples33
Infix approach to build DSL33
Overriding invoke method to build DSL33
Using operators with lambdas33
Using extensions with lambdas34
Chapter 14: Enum35
Remarks35
Examples35
Initialization35
Functions and Properties in enums35
Simple enum36
Mutability36
Chapter 15: Exceptions37
Examples37
Catching exception with try-catch-finally37
Chapter 16: Extension Methods38
Syntax38
Remarks38
Examples38
Top-Level Extensions38
Potential Pitfall: Extensions are Resolved Statically38 Sample extending long to render a human readable string39Sample extending Java 7+ Path class39
Using extension functions to improve readability39 Sample extending Java 8 Temporal classes to render an ISO formatted string40 Extension functions to Companion Objects (appearance of Static functions)40Lazy extension property workaround41
Extensions for easier reference View from code41
Extensions41
Usage42
Chapter 17: Functions43
Syntax43
Parameters43
Examples43
Functions Taking Other Functions43
Lambda Functions44
Function References44
Basic Functions46
Shorthand Functions46
Inline Functions46
Operator functions47
Chapter 18: Generics48
Introduction48
Syntax48
Parameters48
Remarks48
Implied Upper Bound is Nullable48
Examples49
Declaration-site variance49
Use-site variance49
Chapter 19: Idioms51
Examples51
Creating DTOs (POJOs/POCOs)51
Filtering a list51
Delegate to a class without providing it in the public constructor51Serializable and serialVersionUid in Kotlin52
Fluent methods in Kotlin52
Use let or also to simplify working with nullable objects53 Use apply to initialize objects or to achieve method chaining53Chapter 20: Interfaces55
Remarks55
Examples55
Basic Interface55
Interface with default implementations55
Properties55
Multiple implementations56
Properties in Interfaces56
Conflicts when Implementing Multiple Interfaces with Default Implementations57 super keyword57Chapter 21: Java 8 Stream Equivalents59
Introduction59
Remarks59
About laziness59
Why are there no Types?!?59
Reusing Streams60
See also:60
Examples61
Accumulate names in a List61
Convert elements to strings and concatenate them, separated by commas61Compute sum of salaries of employee61
Group employees by department61
Compute sum of salaries by department61
Partition students into passing and failing62
Names of male members62
Group names of members in roster by gender62
Filter a list to another list62
Finding shortest string a list62
Different Kinds of Streams #2 - lazily using first item if exists63 Different Kinds of Streams #3 - iterate a range of Integers63 Different Kinds of Streams #4 - iterate an array, map the values, calculate the average63 Different Kinds of Streams #5 - lazily iterate a list of strings, map the values, convert 63 Different Kinds of Streams #6 - lazily iterate a stream of Ints, map the values, print res64 Different Kinds of Streams #7 - lazily iterate Doubles, map to Int, map to String, print e64 Counting items in a list after filter is applied64 How streams work - filter, upper case, then sort a list64 Different Kinds of Streams #1 - eager using first item if it exists65 Collect example #5 - find people of legal age, output formatted string65 Collect example #6 - group people by age, print age and names together66 Collect example #7a - Map names, join together with delimiter67 Collect example #7b - Collect with SummarizingInt67Chapter 22: JUnit69
Examples69
Rules69
Chapter 23: Kotlin Android Extensions70
Introduction70
Examples70
Configuration70
Using Views70
Product flavors71
Painfull listener for getting notice, when the view is completely drawn now is so simple a72Chapter 24: Kotlin Caveats73
Examples73
Calling a toString() on a nullable type73
Chapter 25: Kotlin for Java Developers74
Introduction74
Examples74
Declaring Variables74
Quick Facts74
Equality & Identity75
IF, TRY and others are expressions, not statements75Chapter 26: logging in kotlin76
Remarks76
Examples76
kotlin.logging76Chapter 27: Loops in Kotlin77
Remarks77
Examples77
Repeat an action x times77
Looping over iterables77
While Loops78
Break and continue78
Iterating over a Map in kotlin78
Recursion79
Functional constructs for iteration79
Chapter 28: Null Safety80
Examples80
Nullable and Non-Nullable types80
Safe call operator80
Idiom: calling multiple methods on the same, null-checked object80Smart casts81
Eliminate nulls from an Iterable and array81
Null Coalescing / Elvis Operator81
Assertion82
Elvis Operator (?:)82
Chapter 29: Ranges83
Introduction83
Examples83
Integral Type Ranges83
downTo() function83 step() function83 until function83Chapter 30: RecyclerView in Kotlin84
Introduction84
Examples84
Main class and Adapter84
Chapter 31: Reflection86
Introduction86
Remarks86
Examples86
Referencing a class86
Referencing a function86
Inter-operating with Java reflection86
Getting values of all properties of a class87
Setting values of all properties of a class87
Chapter 32: Regex90
Examples90
Idioms for Regex Matching in When Expression90
Using immutable locals:90
Using anonymous temporaries:90
Using the visitor pattern:90
Introduction to regular expressions in Kotlin91
The RegEx class91
Null safety with regular expressions91
Raw strings in regex patterns92
find(input: CharSequence, startIndex: Int): MatchResult?92 findAll(input: CharSequence, startIndex: Int): Sequence92 matchEntire(input: CharSequence): MatchResult?93 matches(input: CharSequence): Boolean93 containsMatchIn(input: CharSequence): Boolean93 split(input: CharSequence, limit: Int): List93 replace(input: CharSequence, replacement: String): String94Chapter 33: Singleton objects95
Introduction95
Examples95
Use as repalcement of static methods/fields of java95Use as a singleton95
Chapter 34: Strings97
Examples97
Elements of String97
String Literals97
String Templates98
String Equality98
Chapter 35: Type aliases100
Introduction100
Syntax100
Remarks100
Examples100
Function type100
Generic type100
Chapter 36: Type-Safe Builders101
Remarks101
A typical structure of a type-safe builder101
Type-safe builders in Kotlin libraries101
Examples101
Type-safe tree structure builder101
Chapter 37: Vararg Parameters in Functions103
Syntax103
Examples103
Basics: Using the vararg keyword103
Spread Operator: Passing arrays into vararg functions103Chapter 38: Visibility Modifiers105
Introduction105
Syntax105
Examples105
Code Sample105
Credits106
About You can share this PDF with anyone you feel could benefit from it, downloaded the latest version from: kotlin It is an unofficial and free Kotlin ebook created for educational purposes. All the content is extracted from Stack Overflow Documentation, which is written by many hardworking individuals at Stack Overflow. It is neither affiliated with Stack Overflow nor official Kotlin. The content is released under Creative Commons BY-SA, and the list of contributors to each chapter are provided in the credits section at the end of this book. Images may be copyright of their respective owners unless otherwise specified. All trademarks and registered trademarks are the property of their respective company owners. Use the content presented in this book at your own risk; it is not guaranteed to be correct nor accurate, please send your feedback and corrections to info@zzzprojects.com https://riptutorial.com/1Chapter 1: Getting started with Kotlin
Remarks
Kotlin is a statically-typed object-oriented programming language developed by JetBrains primarily targeting the JVM. Kotlin is developed with the goals of being quick to compile, backwards- compatible, very type safe, and 100% interoperable with Java. Kotlin is also developed with the goal of providing many of the features wanted by Java developers. Kotlin's standard compiler allows it to be compiled both into Java bytecode for the JVM and into JavaScript.Compiling Kotlin
Kotlin has a standard IDE plugin for Eclipse and IntelliJ. Kotlin can also be compiled using Maven, using Ant, and using Gradle, or through the command line. It is worth noting in will output a java class file, in this case (Note the Kt appended to the class name). However if one was to run the class file using java will throw the following exception:In order to run the resulting class file using Java, one must include the Kotlin runt-time jar file to
the current class path.Versions
VersionRelease Date
1.0.02016-02-15
1.0.12016-03-16
1.0.22016-05-13
1.0.32016-06-30
1.0.42016-09-22
https://riptutorial.com/2VersionRelease Date
1.0.52016-11-08
1.0.62016-12-27
1.1.02017-03-01
1.1.12017-03-14
1.1.22017-04-25
1.1.32017-06-23
Examples
Hello World
All Kotlin programs start at the function. Here is an example of a simple Kotlin "Hello World" program: Place the above code into a file named (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 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: In this example, the main class to run would now be .See also:
Package level functions including annotation.•Annotation use-site targets•
Hello World using an Object Declaration
You can alternatively use an Object Declaration that contains the main function for a Kotlin program. https://riptutorial.com/3 The class name that you will run is the name of your object, in this case is . The advantage to this method over a top-level function is that the class name to run is more self- evident, and any other functions you add are scoped into the class . You then also have a singleton instance of to store state and do other work.See also:
Static Methods including the annotation•
Hello World using a Companion Object
Similar to using an Object Declaration, you can define the function of a Kotlin program using aCompanion Object of a class.
The class name that you will run is the name of your class, in this case is . The advantage to this method over a top-level function is that the class name to run is more self- evident, and any other functions you add are scoped into the class . This is similar to the example, other than you are in control of instantiating any classes to do further work. A slight variation that instantiates the class to do the actual "hello": https://riptutorial.com/4See also:
Static Methods including the @JvmStatic annotation•Main methods using varargs
All of these main method styles can also be used with varargs:Compile and Run Kotlin Code in Command Line
As java provide two different commands to compile and run Java code. Same as Kotlin also provide you different commands. to compile java files. to run java files. Same as to compile kotlin files to run kotlin files.Reading input from Command Line
The arguments passed from the console can be received in the Kotlin program and it can be used as an input. You can pass N (1 2 3 and so on) numbers of arguments from the command prompt. A simple example of a command-line argument in Kotlin. https://riptutorial.com/5 Here, Enter two number from the command line to find the maximum number. Output :For !! Operator Please check Null Safety.
Note: Above example compile and run on Intellij.
Read Getting started with Kotlin online: https://riptutorial.com/kotlin/topic/490/getting-started-with-
kotlin https://riptutorial.com/6Chapter 2: Annotations
Examples
Declaring an annotation
Annotations are means of attaching metadata to code. To declare an annotation, put the annotation modifier in front of a class:Annotations can have meta-anotations:
Annotations, like other classes, can have constructors: But unlike other classes, is limited to the following types: types that correspond to Java primitive types (Int, Long etc.);• strings• classes ( Foo:: class)• enums• other annotations• arrays of the types listed above•Meta-annotations
When declaring an annotation, meta-info can be included using the following meta-annotations: : specifies the possible kinds of elements which can be annotated with the annotation (classes, functions, properties, expressions etc.)• specifies whether the annotation is stored in the compiled class files and whether it's visible through reflection at runtime (by default, both are true.)• allows using the same annotation on a single element multiple times.• specifies that the annotation is part of the public API and should be included in the class or method signature shown in the generated API documentation.•Example:
https://riptutorial.com/7 Read Annotations online: https://riptutorial.com/kotlin/topic/4074/annotations https://riptutorial.com/8Chapter 3: Arrays
Examples
Generic Arrays
Generic arrays in Kotlin are represented by .
To create an empty array, use factory function:
To create an array with given size and initial values, use the constructor:Arrays have and functions:
Arrays of Primitives
These types do not inherit from to avoid boxing, however, they have the same attributes and methods.Kotlin typeFactory functionJVM type
https://riptutorial.com/9Extensions
is defined for , , , , , and always returns : , , ... return an item of the array returns null if index is out of bounds, otherwise an item of the array returns a of all elements , creates and returns a new array with sorted elements of current , sort the array in-placeIterate Array
You can print the array elements using the loop same as the Java enhanced loop, but you need to change keyword from to .You can also change data type in for loop.
Create an array
Create an array using a closure
https://riptutorial.com/10Create an uninitialized array
The returned array will always have a nullable type. Arrays of non-nullable items can't be created uninitialized. Read Arrays online: https://riptutorial.com/kotlin/topic/5722/arrays https://riptutorial.com/11Chapter 4: Basic Lambdas
Syntax
Explicit parameters:•
{ parameterName: ParameterType, otherParameterName: OtherParameterType -> anExpression() }•Inferred parameters:•
val addition: (Int, Int) -> Int = { a, b -> a + b }•Single parameter shorthand•
val square: (Int) -> Int = { it*it }•Signature:•
() -> ResultType• (InputType) -> ResultType• (InputType1, InputType2) -> ResultType•Remarks
Input type parameters can be left out when they can be left out when they can be inferred from the context. For example say you have a function on a class that takes a function: You can use this function by passing a lambda, and since the parameters are already specified in the function signature there's no need to re-declare them in the lambda expression: This also applies when you are assigning a lambda to a variable: https://riptutorial.com/12 When the lambda takes one parameter, and the type can be inferred from the context, you can refer to the parameter by .Examples
Lambda as parameter to filter function
Lambda passed as a variable
Lambda for benchmarking a function call
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