What is the equivalent of Java static methods in Kotlin

Question

There is no static keyword in Kotlin   What is the best way to represent a static Java method in Kotlin

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Kotlin has no any static keyword  You used that for java    class AppHelper           public static int getAge                 return 30                    and For Kotlin  class AppHelper           companion object               fun getAge     Int   30                   Call for Java  AppHelper getAge      Call for Kotlin  AppHelper Companion getAge      I think its working perfectly

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A  Old Java Way     Declare a companion object to enclose a static method   variable  class Foo  companion object       fun foo     println  Foo       val bar   bar             Use     Foo foo             Outputs Foo     println Foo bar     Outputs bar        B  New Kotlin way   Declare directly on file without class on a  kt file   fun foo     println  Foo   val bar   bar   Use  the methods variables with their names   After importing them   Use    foo             Outputs Foo           println bar     Outputs bar

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Use object to represent val var method to make static  You can use object instead of singleton class also  You can use companion if you wanted to make static inside of a class  object Abc       fun sum a  Int  b  Int   Int   a   b         If you need to call it from Java   int z   Abc INSTANCE sum x y     In Kotlin  ignore INSTANCE

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All static member and function should be inside companion block     companion object        JvmStatic     fun main args  Array lt String gt                fun staticMethod

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You can achieve the static functionality in Kotlin by Companion Objects   Adding companion to the object declaration allows for adding the static functionality to an object even though the actual static concept does not exist in Kotlin   A companion object can access all members of the class too  including the private constructors   A companion object is initialized when the class is instantiated   A companion object cannot be declared outside the class   class MyClass       companion object            val staticField    This is an example of static field Object Decleration           fun getStaticFunction    String               return  This is example of static function for Object Decleration                        Members of the companion object can be called by using simply the class name as the qualifier   Output   MyClass staticField    This is an example of static field Object Decleration  MyClass getStaticFunction        This is an example of static function for Object Decleration

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except Michael Anderson s answer  i have coding with other two way in my project     First   you can white all variable to one class  created a kotlin file named Const  object Const       const val FIRST NAME 1    just      const val LAST NAME 1    YuMu      You can use it in kotlin and java code   Log d  stackoverflow   Const FIRST NAME 1    Second   You can use Kotlin s extension function created a kotlin file named Ext  below code is the all code in Ext file  package pro just yumu         Created by lpf on 2020-03-18       const val FIRST NAME    just  const val LAST NAME    YuMu    You can use it in kotlin code   Log d  stackoverflow   FIRST NAME    You can use it in java code   Log d  stackoverflow   ExtKt FIRST NAME

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Top-level   companion object for static properties Top-level When the properties are somewhat related to a class  define them as top-level properties just before the class declaration  const val MAX ATTEMPTS   3 private const val DEFAULT NAME    quot Guest quot  private const val MIN AGE   16  data class User val id  String  val name  String   DEFAULT NAME   This is similar to the static properties in Java  When the properties are completely independent of any class  you can define them as top-level in a separate file without the class  companion object When the properties are closely related to a class and will be used only in that class  define them inside a companion object  data class User val id  String  val name  String   DEFAULT NAME        companion object           const val DEFAULT NAME    quot Guest quot          const val MIN AGE   16           Top-level   companion object for static methods Top-level Similar to the properties above  when the functions are somewhat related to a class  define them just above the class  fun getAllUsers        fun getProfileFor userId  String       data class User val id  String  val name  String   Usage  val userList   getAllUsers    companion object When the functions are closely related to a class  define them inside a companion object  data class User val id  String  val name  String         companion object            fun getAll                fun profileFor userId  String               Usage  val userProfile   User profileFor  quot 34 quot    This is similar to the static methods in Java  The top-level functions are often more idiomatic to Kotlin  A better reason to define functions inside the companion object is when you are extending a companion object with an interface  An example of this is shown in the singleton section   Nested class for static class When the classes with related functionalities belong together  they can be grouped together by nesting  class User val id  String  val name  String        class UserAccess   UserDao           override fun add user  User              override fun remove id  String               This is equivalent to the static nested classes in Java  The UserAccess class here implements an interface UserDao  Usage  fun main         val john   User  quot 34 quot    quot John quot       val userAccess   User UserAccess       userAccess add john      Singleton object for static INSTANCE Top-level When you just want a single object of a class  you no longer need to create a static INSTANCE inside a class like in Java  Simply use a top-level object declaration  object UserAccess   UserDao       override fun add user  User          override fun remove id  String         Notice also how easy it is to extend an interface or a class in a singleton  The code above  under the hood  produces the following static INSTANCE singleton pattern in Java  simplified   public final class UserAccess implements UserDao      public static final UserAccess INSTANCE      public void add User user          public void remove String id          private UserAccess           static   INSTANCE   new UserAccess        companion object When the singleton is closely related to a class use companion object  data class User val id  String  val name  String        companion object   UserDao           override fun add user  User              override fun remove id  String               This way you get more elegant naming  User add john   Also  you make the intent clear that this singleton is only used as a utility for the User class  You can also use the object without the companion keyword inside the class  if you want multiple singletons or groups of functions properties   companion object for static factory Factory functions in Koltin are created using the companion object  Factory functions are useful when you want to provide multiple ways to create an object where the object construction process is complex or when multiple constructors are not expressive enough  For example  the newInstance   factory function in the following snippet creates a user by generating the id automatically  class User private constructor val id  Long  val name  String        companion object           private var currentId   0L          fun newInstance name  String    User currentId    name           This is equivalent to the static factory methods in Java  The constructor is kept private but the companion object has access to the constructor  In the code above  consistency of the next id generation is guaranteed because a companion object is a singleton  only one object will keep track of the id  there won t be any duplicate ids  Also notice that companion objects can have properties  currentId in this case  to represent state  Usage  val john   User newInstance  quot John quot      JvmStatic for Java interoperability The static concept of Java doesn t exist in Kotlin  A companion object is an instance of a real class called Companion  So  when you call the Kotlin code from Java  an object of the Companion class is first instantiated behind the scenes  You need to call the function using the Companion object in Java  Profile userProfile   User Companion profileFor  quot 34 quot     For the idiomatic Java naming and less verbosity  use  JvmStatic annotation on that function or property  companion object        JvmStatic     fun profileFor userId  String   Profile        The  JvmStatic annotation creates a separate pure static copy of the getProfileFor   function  Now you can use it from Java with regular syntax  Profile userProfile   User profileFor  quot 34 quot      That s it  Hope the examples are useful for your projects

User · Answer

You place the function in the  companion object    So the java code like this   class Foo     public static int a     return 1        will become  class Foo     companion object        fun a     Int   1         You can then use it from inside Kotlin code as   Foo a      But from within Java code  you would need to call it as  Foo Companion a       Which also works from within Kotlin    If you don t like having to specify the Companion bit you can either add a  JvmStatic annotation or name your companion class   From the docs      Companion Objects      An object declaration inside a class can be marked with the companion    keyword   class MyClass      companion object Factory          fun create    MyClass   MyClass                Members of the companion object can be called by using simply the class    name as the qualifier   val instance   MyClass create                  However  on the JVM you can have members of companion objects generated    as real static methods and fields  if you use the  JvmStatic   annotation   See the Java interoperability section for more details    Adding the  JvmStatic annotation looks like this  class Foo     companion object        JvmStatic     fun a     Int   1          and then it will exist as a real Java static function  accessible from both Java and Kotlin as Foo a      If it is just disliked for the Companion name  then you can also  provide an explicit name for the companion object looks like this   class Foo     companion object Blah       fun a     Int   1          which will let you call it from Kotlin in the same way  but from java like Foo Blah a    which will also work in Kotlin

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The exact conversion of the java static method to kotlin equivalent would be like this  e g  Here the util class has one static method which would be equivalent in both java and kotlin  The use of  JvmStatic is important   Java code        class Util           public static String capitalize String text            return text toUpperCase                Kotlin code       class Util           companion object                JvmStatic             fun capitalize text String   String                   return text toUpperCase

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You can use Companion Objects - kotlinlang  Which it can be shown by first creating that Interface  interface I lt T gt         Then we have to make a function inside of that interface   fun SomeFunc    T   Then after  We need a class   class SomeClass      inside that class we need a companion Object inside that class   companion object   I lt SomeClass gt       inside that Companion Object we need that old SomeFunc function  But we need to over ride it   override fun SomeFunc    SomeClass   SomeClass     Finally below all of that work  We need something to power that Static function  We need a variable   var e I lt SomeClass gt    SomeClass

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Write them directly to files   In Java  ugly    package xxx  class XxxUtils     public static final Yyy xxx Xxx xxx    return xxx xxx          In Kotlin    file JvmName  XxxUtils   package xxx fun xxx xxx  Xxx   Yyy   xxx xxx     Those two pieces of codes are equaled after compilation  even the compiled file name  the file JvmName is used to control the compiled file name  which should be put just before the package name declaration

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object objectName       fun funName

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Docs recommends to solve most of the needs for static functions with package-level functions  They are simply declared outside a class in a source code file  The package of a file can be specified at the beginning of a file with the package keyword   Declaration  package foo  fun bar          Usage  import foo bar   Alternatively  import foo     You can now call the function with   bar     or if you do not use the import keyword   foo bar     If you do not specify the package the function will be accessible from the root   If you only have experience with java  this might seem a little strange  The reason is that kotlin is not a strictly object-oriented language  You could say it supports methods outside of classes   Edit   They have edited the documentation to no longer include the sentence about recommending package level functions  This is the original that was referred to above

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A lot of people mention companion objects  which is correct   But  just so you know  you can also use any sort of object  using the object keyword  not class  i e     object StringUtils       fun toUpper s  String    String             Use it just like any static method in java   StringUtils toUpper  foobar     That sort of pattern is kind of useless in Kotlin though  one of its strengths is that it gets rid of the need for classes filled with static methods   It is more appropriate to utilize global  extension and or local functions instead  depending on your use case   Where I work we often define global extension functions in a separate  flat file with the naming convention   className Extensions kt i e   FooExtensions kt   But more typically we write functions where they are needed inside their operating class or object

User · Answer

This also worked for me  object Bell        JvmStatic     fun ring           from Kotlin  Bell ring     from Java  Bell ring

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There is no static keyword in kotlin   kotlin docs recommends to use package-level functions if u want to follow DRY      Create a file with  kt extension and put your method in it   package p     fun m          fun body         after compilation m will have a signature of public static final void  and  import p m

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Even though this is a bit over 2 years old now  and had plenty of great answers  I am seeing some other ways of getting  static  Kotlin fields are missing  Here is an example guide for Kotlin-Java static interop      Scenario 1  Creating a static method in Kotlin for Java      Kotlin   file JvmName  KotlinClass     This provides a name for this file  so it s not defaulted as  KotlinClassKt  in Java package com frybits  class KotlinClass       companion object              This annotation tells Java classes to treat this method as if it was a static to  KotlinClass           JvmStatic         fun foo    Int   1            Without it  you would have to use  KotlinClass Companion bar    to use this method          fun bar    Int   2               Java  package com frybits   class JavaClass        void someFunction             println KotlinClass foo       Prints  1          println KotlinClass Companion bar       Prints  2   This is the only way to use  bar    in Java          println KotlinClass Companion foo       To show that  Companion  is still the holder of the function  foo                 Because I m way to lazy to keep typing  System out   but I still want this to be compilable      void println Object o            System out println o              Michael Anderson s answer provides more depth than this  and should definitely be referenced for this scenario     This next scenario handles creating static fields in Kotlin so that Java doesn t have to keep calling KotlinClass foo   for those cases where you don t want a static function      Scenario 2  Creating a static variable in Kotlin for Java      Kotlin   file JvmName  KotlinClass     This provides a name for this file  so it s not defaulted as  KotlinClassKt  in Java package com frybits  class KotlinClass        companion object              This annotation tells Kotlin to not generate the getter setter functions in Java  Instead  this variable should be accessed directly           Also  this is similar to   JvmStatic   in which it tells Java to treat this as a static variable to  KotlinClass            JvmField         var foo  Int   1            If you want something akin to  final static   and the value is a primitive or a String  you can use the keyword  const  instead           No annotation is needed to make this a field of  KotlinClass   If the declaration is a non-primitive non-String  use  JvmField instead         const val dog  Int   1            This will be treated as a member of the  Companion  object only  It generates the getter setters for it          var bar  Int   2            We can still use   JvmStatic  for  var  variables  but it generates getter setters as functions of KotlinClass           If we use  val  instead  it only generates a getter function          JvmStatic         var cat  Int   9               Java  package com frybits   class JavaClass        void someFunction               Example using  JvmField         println KotlinClass foo     Prints  1          KotlinClass foo   3             Example using  const val          println KotlinClass dog     Prints  1   Notice the lack of a getter function            Example of not using either  JvmField   JvmStatic  or  const val          println KotlinClass Companion getBar       Prints  2          KotlinClass Companion setBar 3     The setter for  bar             Example of using  JvmStatic instead of  JvmField         println KotlinClass getCat             KotlinClass setCat 0              void println Object o            System out println o                One of the great features about Kotlin is that you can create top level functions and variables  This makes it greate to create  classless  lists of constant fields and functions  which in turn can be used as static functions fields in Java      Scenario 3  Accessing top level fields and functions in Kotlin from Java      Kotlin    In this example  the file name is  KSample kt   If this annotation wasn t provided  all functions and fields would have to accessed   using the name  KSampleKt foo    to utilize them in Java  Make life easier for yourself  and name this something more simple  file JvmName  KotlinUtils    package com frybits    This can be called from Java as  KotlinUtils TAG   This is a final static variable const val TAG    You re it      Since this is a top level variable and not part of a companion object  there s no need to annotate this as  static  to access in Java    However  this can only be utilized using getter setter functions var foo   1    This lets us use direct access now  JvmField var bar   2    Since this is calculated at runtime  it can t be a constant  but it is still a final static variable  Can t use  const  here  val GENERATED VAL Long    123  toLong      Again  no need for  JvmStatic  since this is not part of a companion object fun doSomethingAwesome         println  Everything is awesome            Java  package com frybits   class JavaClass        void someFunction              println KotlinUtils TAG     Example of printing  TAG              Example of not using  JvmField          println KotlinUtils getFoo       Prints  1          KotlinUtils setFoo 3              Example using  JvmField         println KotlinUtils bar     Prints  2   Notice the lack of a getter function         KotlinUtils bar   3             Since this is a top level variable  no need for annotations to use this           But it looks awkward without the  JvmField         println KotlinUtils getGENERATED VAL                This is how accessing a top level function looks like         KotlinUtils doSomethingAwesome               void println Object o            System out println o                Another notable mention that can be used in Java as  static  fields are Kotlin object classes  These are zero parameter singleton classes that are instantiated lazily on first use  More information about them can be found here  https   kotlinlang org docs reference object-declarations html object-declarations  However  to access the singleton  a special INSTANCE object is created  which is just as cumbersome to deal with as Companion is  Here s how to use annotations to give it that clean static feel in Java      Scenario 4  Using object classes      Kotlin   file JvmName  KotlinClass      This provides a name for this file  so it s not defaulted as  KotlinClassKt  in Java package com frybits  object KotlinClass     No need for the  class  keyword here         Direct access to this variable     const val foo  Int   1        Tells Java this can be accessed directly from  KotlinClass       JvmStatic     var cat  Int   9        Just a function that returns the class name      JvmStatic     fun getCustomClassName    String   this  class java simpleName    boo          Getter Setter access to this variable  but isn t accessible directly from  KotlinClass      var bar  Int   2      fun someOtherFunction      What is  INSTANCE            Java  package com frybits   class JavaClass        void someFunction             println KotlinClass foo     Direct read of  foo  in  KotlinClass  singleton          println KotlinClass getCat       Getter of  cat          KotlinClass setCat 0     Setter of  cat           println KotlinClass getCustomClassName       Example of using a function of this  object  class          println KotlinClass INSTANCE getBar       This is what the singleton would look like without using annotations         KotlinClass INSTANCE setBar 23            println KotlinClass INSTANCE someOtherFunction       Accessing a function in the object class without using annotations            void println Object o            System out println o

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The kotlin documents provider three ways to do that  the first is define function in package without class   package com example  fun f     1   the second is use  JvmStatic annotation   package com example  class A   JvmStatic fun f     1     and the third is use companion object   package com example  clss A  companion object  fun f     1

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Use  JVMStatic Annotation  companion object           TODO  Rename and change types and number of parameters      JvmStatic     fun newInstance param1  String  param2  String                EditProfileFragment   apply                   arguments   Bundle   apply                       putString ARG PARAM1  param1                      putString ARG PARAM2  param2

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Simply you need to create a companion object and put the function in it    class UtilClass           companion object                JvmStatic             fun repeatIt5Times str  String   String   str repeat 5                      To invoke the method from a kotlin class   class KotlinClass    fun main args   Array lt String gt          UtilClass repeatIt5Times  Hello           or Using import  import Packagename UtilClass Companion repeatIt5Times class KotlinClass    fun main args   Array lt String gt           repeatIt5Times  Hello             To invoke the method from a java class    class JavaClass      public static void main String    args          UtilClass Companion repeatIt5Times  Hello               or by adding  JvmStatic annotation to the method  class JavaClass     public static void main String    args        UtilClass repeatIt5Times  Hello            or both by adding  JvmStatic annotation to the method and making static import in java   import static Packagename UtilClass repeatIt5Times class JavaClass     public static void main String    args        repeatIt5Times  Hello

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In Java  we can write in below way   class MyClass     public static int myMethod        return 1          In Kotlin  we can write in below way  class MyClass     companion object        fun myMethod     Int   1         a companion is used as static in Kotlin

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To make it short you could use  companion object  to get into Kotlin static world like      companion object       const val TAG    tHomeFragment      fun newInstance     HomeFragment       and to make a constant field use  const val  as in the code  but try to avoid the static classes as it is making difficulties in unit testing using Mockito

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For Java   public class Constants   public static final long MAX CLICK INTERVAL   1000     Equivalent Kotlin code   object  Constants   const val MAX CLICK INTERVAL  Long   1000    So for the equivalent of Java static methods is object class in Kotlin

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If you need a function or a property to be tied to a class rather than to instances of it  you can declare it inside a companion object   class Car val horsepowers  Int        companion object Factory           val cars   mutableListOf lt Car gt             fun makeCar horsepowers  Int   Car               val car   Car horsepowers              cars add car              return car                     The companion object is a singleton  and its members can be accessed directly via the name of the containing class  val car   Car makeCar 150  println Car Factory cars size

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You need to pass companion object for static method because kotlin don   t have static keyword - Members of the companion object can be called by using simply the class name as the qualifier   package xxx     class ClassName                 companion object                          fun helloWord str  String   String                               return stringValue

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There are 2 ways you can apply static in Kotlin  First make a companion object under class  For ex    class Test      companion object            fun isCheck a Int  Boolean               if a  0  true else false                        you can call this function as  Test Companion isCheck 2    Another way we can use is to make an object class  object Test         fun isCheck a Int  Boolean              if a  0  true else false              Happy Coding

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For Android using a string from a single activity to all the necessary activity  Just like static in java   public final static String TEA NAME    TEA NAME    Equivalent approach in Kotlin   class MainActivity   AppCompatActivity         companion object           const val TEA NAME    TEA NAME            override fun onCreate savedInstanceState  Bundle             super onCreate savedInstanceState          setContentView R layout activity main            Another activity where value is needed   val teaName   MainActivity TEA NAME

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I would like to add something to above answers   Yes  you can define functions in source code files outside class   But it is better if you define static functions inside class using Companion Object because you can add more static functions by leveraging the Kotlin Extensions   class MyClass       companion object              define static functions here             Adding new static function fun MyClass Companion newStaticFunction                    And you can call above defined function as you will call any function inside Companion Object

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Let  you have a class Student  And you have one static method getUniversityName    amp  one static field called totalStudent    You should declare companion object block inside your class    companion object       define static method  amp  field here      Then your class looks like      class Student var name  String  var city  String  var rollNumber  Double   0 0            use companion object structure     companion object               below method will work as static method         fun getUniversityName    String    MBSTU              below field will work as static field         var totalStudent   30           Then you can use those static method and fields like this way   println  University       Student getUniversityName        Total Student      Student totalStudent         Output         University   MBSTU  Total Student  30

[java] What is the equivalent of Java static methods in Kotlin?

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