Kotlin 中 Java 静态方法的等价物是什么？

Question

There is no static keyword in Kotlin.

What is the best way to represent a static Java method in Kotlin?

Answer 1

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).

Answer 2

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.

Answer 3

A. Old Java Way :

1. Declare a companion object to enclose a static method / variable

   class Foo{ companion object { fun foo() = println("Foo") val bar ="bar" } }

2. Use :

    Foo.foo() // Outputs Foo println(Foo.bar) // Outputs bar

---

---

B. New Kotlin way

1. Declare directly on file without class on a .kt file.

    fun foo() = println("Foo") val bar ="bar"

2. Use the methods/variables with their names . ( After importing them )

   Use :

    foo() // Outputs Foo println(bar) // Outputs bar

---

Answer 4

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.

Answer 5

This also worked for me

object Bell {
    @JvmStatic
    fun ring() { }
}

from Kotlin

Bell.ring()

from Java

Bell.ring()

Answer 6

object objectName {
    fun funName() {

    }
}

Answer 7

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); } }

Answer 8

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
                      }
              }
    }

Answer 9

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!

Answer 10

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 = "Guest"
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 = "Guest"
        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("34")

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("34", "John")
    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("John")

---

@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("34");

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("34");

---

That's it! Hope the examples are useful for your projects.

Answer 11

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<String>) { 
    UtilClass.repeatIt5Times("Hello")
  }
}

or Using import

import Packagename.UtilClass.Companion.repeatIt5Times
class KotlinClass{
  fun main(args : Array<String>) { 
     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")
   }
}

Answer 12

Kotlin has no any static keyword. You can use below code for Java and Kotlin

 object AppHelper {
    @JvmStatic
    fun getAge() : Int = 30
}

Call for Java Class

AppHelper.getAge();

Call for Kotlin Class

AppHelper.getAge()

Its work perfectly for me. Thanks

Answer 13

Companion Objects is the alternate of java static keyword and you can make a class or a method as static by declaring them as Companion Objects.
You don't need to qualify companion objects with the class name either if you're calling them from within the same class.

For example:

class SomeClass() {

    val id: Int

    init {
       id = nextId++       
    }

    private companion object {
       var nextId = 1
    }
}

fun main(args: Array<String>) {
    repeat(2) { 
        println(SomeClass().id)
    }
} 

Answer 14

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.

Answer 15

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.

Answer 16

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!.

Answer 17

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

Answer 18

The exact conversion of the java static method to kotlin equivalent would be like this. eg 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()
            }
        }
    }

Answer 19

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);

Answer 20

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).

Answer 21

Let, you have a class Student . And you have one static method getUniversityName() & one static field called totalStudent .

You should declare companion object block inside your class.

companion object {
 // define static method & 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

Answer 22

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

Answer 23

The java code is like as below:

class Foo { public static int a() { return 1; } }

will become as below in kotlin:

class Foo { companion object { fun a() : Int = 1 } }

However, using @JvmStatic annotation on the JVM, we can have members of companion objects generated as real static methods and fields.

Answer 24

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)
                }
            }
}

Answer 25

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) ie,

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 ie, FooExtensions.kt. But more typically we write functions where they are needed inside their operating class or object.

Answer 26

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

☺

Answer 27

Simply use this approach

object Foo{
   fun foo() = println("Foo")
   val bar ="bar"  
}

Foo.INSTANCE.foo()

Answer 28

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.

Answer 29

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
}
}

Answer 30

All static member and function should be inside companion block

  companion object {
    @JvmStatic
    fun main(args: Array<String>) {
    }

    fun staticMethod() {
    }
  }

Answer 31

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<Car>()

        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)

Answer 32

You can use Companion Objects - kotlinlang

Which it can be shown by first creating that Interface

interface I<T> {

}

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<SomeClass> {}

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<SomeClass> = SomeClass()