Constants in Kotlin -- what s a recommended way to create them

Question

How is it recommended to create constants in Kotlin  And what s the naming convention  I ve not found that in the documentation   companion object         1     val MY CONST    something         2     const val MY CONST    something         3     val myConst    something      Or

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If you put your const val valName   valValue before the class name  this way it will creates a  public static final YourClass Kt that will have the public static final values   Kotlin   const val MY CONST0   0 const val MY CONST1   1 data class MyClass var some  String    Java decompiled   public final class MyClassKt       public static final int MY CONST0   0      public static final int MY CONST1   1       rest of MyClass java

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For primitives and Strings       The empty String     const val EMPTY STRING        For other cases       The empty array of Strings      JvmField val EMPTY STRING ARRAY   arrayOfNulls lt String gt  0    Example         Copyright 2018 Vorlonsoft LLC       Licensed under The MIT License  MIT       package com vorlonsoft android rate  import com vorlonsoft android rate Constants Utils Companion UTILITY CLASS MESSAGE         Constants Class - the constants class of the AndroidRate library         constructor Constants is a utility class and it can t be instantiated      since       1 1 8     version     1 2 1     author      Alexander Savin     internal class Constants private constructor             Constants Class initializer block         init           throw UnsupportedOperationException  Constants UTILITY CLASS MESSAGE                         Constants Date Class - the date constants class of the AndroidRate library                 constructor Constants Date is a utility class and it can t be instantiated          since       1 1 8         version     1 2 1         author      Alexander Savin             internal class Date private constructor                 Constants Date Class initializer block             init               throw UnsupportedOperationException  Constants Date UTILITY CLASS MESSAGE                          The singleton contains date constants             companion object                   The time unit representing one year in days                 const val YEAR IN DAYS   365 toShort                                   Constants Utils Class - the utils constants class of the AndroidRate library                 constructor Constants Utils is a utility class and it can t be instantiated          since       1 1 8         version     1 2 1         author      Alexander Savin             internal class Utils private constructor                 Constants Utils Class initializer block             init               throw UnsupportedOperationException  Constants Utils UTILITY CLASS MESSAGE                          The singleton contains utils constants             companion object                   The empty String                 const val EMPTY STRING                      The empty array of Strings                  JvmField val EMPTY STRING ARRAY   arrayOfNulls lt String gt  0                  The part 2 of a utility class unsupported operation exception message                 const val UTILITY CLASS MESSAGE     is a utility class and it can t be instantiated

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First of all  the naming convention in Kotlin for constants is the same than in java  e g   MY CONST IN UPPERCASE    How should I create it    1  As a top level value  recommended   You just have to put your const outside your class declaration   Two possibilities   Declare your const in your class file  your const have a clear relation with your class   private const val CONST USED BY MY CLASS   1  class MyClass           I can use my const in my class body      Create a dedicated constants kt file where to store those global const  Here you want to use your const widely across your project     package com project constants const val URL PATH    https      Then you just have to import it where you need it    import com project constants  MyClass       private fun foo             val url   URL PATH         System out print url     https              2  Declare it in a companion object  or an object declaration   This is much less cleaner because under the hood  when bytecode is generated  a useless object is created    MyClass       companion object           private const val URL PATH    https             const val PUBLIC URL PATH    https   public     Accessible in other project files via MyClass PUBLIC URL PATH           Even worse if you declare it as a val instead of a const  compiler will generate a useless object   a useless function     MyClass       companion object           val URL PATH    https               Note    In kotlin  const can just hold primitive types  If you want to pass a function to it  you need add the  JvmField annotation  At compile time  it will be transform as a public static final variable  But it s slower than with a primitive type  Try to avoid it    JvmField val foo   Foo

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Kotlin static and constant value  amp  method declare  object MyConstant     JvmField      for access in java code  val PI  Double   3 14   JvmStatic    JvmStatic annotation for access in java code fun sumValue v1  Int  v2  Int   Int       return v1   v2        Access value anywhere  val value   MyConstant PI val value   MyConstant sumValue 10 5

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Something that isn t mentioned in any of the answers is the overhead of using companion objects  As you can read here  companion objects are in fact objects and creating them consumes resources  In addition  you may need to go through more than one getter function every time you use your constant  If all that you need is a few primitive constants you ll probably just be better off using val to get a better performance and avoid the companion object   TL DR  of the article   Using companion object actually turns this code  class MyClass        companion object           private val TAG    TAG             fun helloWorld             println TAG            Into this code   public final class MyClass       private static final String TAG    TAG       public static final Companion companion   new Companion            synthetic     public static final String access getTAG cp             return TAG             public static final class Companion           private final String getTAG                 return MyClass access getTAG cp                          synthetic         public static final String access getTAG p Companion c                return c getTAG                         public final void helloWorld             System out println Companion access getTAG p companion              So try to avoid them

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local constants   const val NAME    name    Global constants   object MyConstants      val NAME    name      val ID     id      var EMAIL    email      access MyConstants NAME

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Values known at compile time can  and in my opinion should  be marked as constant   Naming conventions should follow Java ones and should be properly visible when used from Java code  it s somehow hard to achieve with companion objects  but anyway    The proper constant declarations are   const val MY CONST    something  const val MY INT   1

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class Myclass     companion object           const val MYCONSTANT   479     you have two choices you can use const keyword or use the  JvmField which makes it a java s static final constant     class Myclass         companion object               JvmField val MYCONSTANT   479         If you use the  JvmField annotation then after it compiles the constant gets put in for you the way you would call it in java   Just like you would call it in java the compiler will replace that for you when you call the companion constant in code    However  if you use the const keyword then the value of the constant gets inlined  By inline i mean the actual value is used after it compiles     so to summarize here is what the compiler will do for you      so for  JvmField   Foo var1   Constants FOO     and for const   Foo var1   479

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Like val  variables defined with the const keyword are immutable  The difference here is that const is used for variables that are known at compile-time   Declaring a variable const is much like using the static keyword in Java   Let s see how to declare a const variable in Kotlin   const val COMMUNITY NAME    wiki    And the analogous code written in Java would be   final static String COMMUNITY NAME    wiki     Adding to the answers above -      JvmField an be used to instruct the Kotlin compiler not to generate   getters setters for this property and expose it as a field      JvmField  val COMMUNITY NAME   Wiki    Static fields     Kotlin properties declared in a named object or a companion object   will have static backing fields either in that named object or in the   class containing the companion object    Usually these fields are private but they can be exposed in one of the following ways     JvmField annotation  lateinit modifier  const modifier    More details here - https   kotlinlang org docs reference java-to-kotlin-interop html instance-fields

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You don t need a class  an object or a companion object for declaring constants in Kotlin  You can just declare a file holding all the constants  for example Constants kt or you can also put them inside any existing Kotlin file  and directly declare the constants inside the file  The constants known at compile time must be marked with const   So  in this case  it should be   const val MY CONST    something   and then you can import the constant using   import package name MY CONST  You can refer to this link

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Avoid using companion objects  Behind the hood  getter and setter instance methods are created for the fields to be accessible  Calling instance methods is technically more expensive than calling static methods   public class DbConstants       companion object           val TABLE USER ATTRIBUTE EMPID     id          val TABLE USER ATTRIBUTE DATA    data          Instead define the constants in object   Recommended practice    object DbConstants           const val TABLE USER ATTRIBUTE EMPID     id          const val TABLE USER ATTRIBUTE DATA    data      and access them globally like this  DbConstants TABLE USER ATTRIBUTE EMPID

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In Kotlin  if you want to create the local constants which are supposed to be used with in the class then you can create it like below  val MY CONSTANT    Constants    And if you want to create a public constant in kotlin like public static final in java  you can create it as follow   companion object        const val MY CONSTANT    Constants

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There are a few ways you can define constants in Kotlin    Using companion object      companion object           const val ITEM1    item1          const val ITEM2    item2          you can use above companion object block inside any class and define all your fields inside this block itself  But there is a problem with this approach  the documentation says      even though the members of companion objects look like static members in other languages  at runtime those are still instance members of real objects  and can  for example  implement interfaces    When you create your constants using companion object  and see the decompiled bytecode  you ll something like below     ClassName Companion Companion   ClassName Companion   INSTANCE     NotNull   String ITEM1    item1      NotNull   String ITEM2    item2      public static final class Companion         NotNull      private static final String ITEM1    item1         NotNull      public static final String ITEM2    item2             FF  synthetic field      static final ClassName Companion   INSTANCE        private Companion                  static           ClassName Companion var0   new ClassName Companion              INSTANCE   var0               From here you can easily see what the documentation said  even though the members of companion objects look like static members in other languages  at runtime those are still instance members of real objects It s doing extra work than required   Now comes another way  where we don t need to use companion object like below   object ApiConstants         val ITEM1  String    item1       Again if you see the decompiled version of the byte code of above snippet  you ll find something like this   public final class ApiConstants        private static final String ITEM1    item1         public static final ApiConstants INSTANCE        public final String getITEM1                return ITEM1                private ApiConstants                   static            ApiConstants var0   new ApiConstants             INSTANCE   var0           CONNECT TIMEOUT    item1                   Now if you see the above decompiled code  it s creating get method for each variable  This get method is not required at all   To get rid of these get methods  you should use const before val like below   object ApiConstants        const val ITEM1  String    item1       Now if you see the decompiled code of above snippet  you ll find it easier to read as it does the least background conversion for your code   public final class ApiConstants       public static final String ITEM1    item1       public static final ApiConstants INSTANCE       private ApiConstants                 static           ApiConstants var0   new ApiConstants            INSTANCE   var0                  So this is the best way to create constants

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