how to create a static method in kotlin

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How to create static methods and classes in Kotlin

One of the key differences between the object-oriented model in Kotlin and Java is the definition of static methods and classes. Generally speaking, a static class doesn’t have to be instantiated in order to be used. Similarly, to invoke a static method, we just have to know its name, not to instantiate an object of the class where such a method is defined.

In Java, classes may easily declare static fields or methods. However, the definition of a static class is a bit more complex. Kotlin, on the other hand, greatly simplifies the job with native constructs such as package-level functions, object s, and companion object s.

In this article, we are first going to look at the legacy Java way of declaring static classes and methods. Secondly, we’ll see how the same thing can be achieved, with much less effort, in Kotlin. Then, we’ll compare the benefits of Kotlin’s way with respect to code reusability.

Table of contents:

Static classes and methods in Java

Static classes in kotlin, package-level functions, companion objects, a comparison.

In object-oriented programming (OOP), static methods and fields are useful to model common values or operations that do not need to be tied to an instance of a class. For example, the Math class contains several static fields and methods for common constants (such as pi) and mathematical operations (such as max and min):

Static classes, on the other hand, are more tricky to define. In Java, only nested classes (that is, classes defined within another class) can be declared static. They do not need a reference to the outer class (the class they’re declared within). Hence, while we may not instantiate non-static nested classes without an instance of the outer class, static classes are independent.

Furthermore, the class loader loads the code of static classes when they are first used and not when the enclosing class gets loaded. This allows us to reduce the memory footprint of our application.

For example, we may want to use static classes to implement a thread-safe singleton class without paying the price of synchronizing the getInstance method:

In the example above, using a static class to hold a reference to the singleton instance gives us some nice properties:

• the class-loader only loads SingletonHolder the first time it’s accessed (via Singleton::getInstance() );
• when a Java class is loaded, it is guaranteed that all of its static properties are initialized. Hence, SingletonHolder::INSTANCE gets instantiated immediately before the first usage;
• SingletonHolder::INSTANCE can be declared final even though it’s lazily initialized;
• the class-loader itself is thread-safe, which makes the first two properties thread-safe

Besides performance reasons, static classes are often used to improve the readability and the maintainability of the code, as we can use them to move components closer to where they are used.

As Java, Kotlin allows us to define nested classes in our code. In Kotlin, however, a nested class is static by default. That is, by default nested classes in Kotlin do not hold a reference to the enclosing class:

In the example above, we defined a nested class Engine inside a class Car . As we can see, we can instantiate Car and Engine separately. In particular, we create an instance of Engine without providing a reference to an object of Car . The only clue that Engine is defined inside Car is its qualified name, Car.Engine . The example above prints Gasoline and SomeModel .

Static classes in Kotlin can access the properties of the companion object of the enclosing class. We’ll see more about companion object s below.

If we want to define a nonstatic nested class in Kotlin we have to declare it as inner :

Now that Engine is marked as inner , we have to create an instance of Car and use it to instantiate Engine . From within Engine , we can reference the outer object using this@Car . The example prints SomeModel - Gasoline .

Similar to Java, nested classes can be declared also in the scope of a method of the enclosing class. In this case, the new class would be a local type.

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The main benefit of Kotlin’s approach is that it limits the risks of memory leaks, by default. In Java, it is easier to overlook the fact that a given nested class holds a reference to the enclosing class. In Kotlin, on the other hand, such a reference does not exist by default.

Whether to use an inner class or a static one largely depends on the way we’re modeling our domain. Surely, static classes allow for greater code reusability, as we do not need to instantiate the enclosing class, while letting us define (possibly) dependent components close to one another.

Static methods in Kotlin

Kotlin greatly simplifies how we can define static methods or variables. In particular, it does so using (companion) object s and package-level functions.

Kotlin is not exclusively an object-oriented language because it also supports the functional programming paradigm: this is where package-level functions come from. As the name suggests, they are functions (or members) that do not belong to a given class but are instead defined within a package. Often, they are utility functions that are independent of any other class.

For example, we can use them to implement handy functions to initialize a class. Assume we have a class named Point to model a point in the Cartesian plane:

Then, in a different package, we might define the following functions:

We can then use the functions and values above just by importing them:

The main function above prints the strings Point(0, 0) and Point(5, 0) .

Note how we defined two package-level functions in the com.logrocket.blog.utils package, onXAxis and onYAxis . We also defined a package-level value, centerPoint . Both the functions and the value can be accessed without any references to any enclosing classes, as we’d have done in Java: we just have to import them.

Package-level functions and values are syntactic sugar for static fields and methods in Java. What the Kotlin compiler does is generate a Java class named after the Kotlin file, with static methods and fields in it. For example, the functions in com.logrocket.blog.utils.factory.kt will be compiled into a class named com.logrocket.blog.utils.FactoryKt (where the name of the class is built using the name of the file and Kt, in PascalCase):

If we wanted to change the name of the generated Java class, we could use the @JvmName annotation. For example, if we place the annotation @file:JvmName("PointFactory") at the beginning of factory.kt , the generated class will be named PointFactoryKt instead of FactoryKt . Such an annotation must appear before the package directive.

Lastly, if we want more utility functions to be compiled into the same generated Java class, or if we already have a file named pointfactory.kt , we can use the @JvmMultifileClass annotation. This way, the compiler will generate a Java façade class with the specified name and all the declarations from all the Kotlin files with the same JvmName .

By declaring an object in Kotlin we define a singleton, that is, a class with only one instance. Such an instance is created lazily, the first time it’s used, in a thread-safe manner.

For example, we could define the following object to group the functions and values we defined above:

Then, differently than before, we have to specify the name of the object to access its functions. In other words, an object defines a scope:

As there’s only one instance of each Kotlin object , the qualified name of the object is enough to access its members. This is similar to but slightly different than a Java class consisting of static methods or variables only. In the latter case, we could instantiate that Java class as many times as we wanted (assuming the constructor is not private ). In that case, the static variables would be the same for each different instance of the class. With Kotlin object s, on the other hand, we only have one instance.

In the example above, PointFactory is pretty tied to the Point class, as it contains several methods to instantiate a point. For cases like this, we can make it a companion object , making this tight coupling more explicit:

With companion object s, we can declare methods and tie them to a given class, rather than to its instances. As for “normal” object s, companion object s are singletons. Hence, we can reference the functions by specifying the name of the class:

Kotlin provides us with three different solutions to define static methods or fields.

Package-level functions and values are the most idiomatic way. Often there’s no need to scope utility methods inside a class. In such cases, package-level members are a fine choice allowing for greater reusability of the code. As a matter of fact, most of the standard library is implemented using them.

However, object s and companion object s do have some pros. For example, they allow for a better scoping of methods and fields. One of the main cons of package-level members is that they pollute the auto-completion suggestions available in most IDEs, making it more difficult to pick the right function. The scope of an object solves this issue.

Strictly speaking, in a pure object-oriented programming mindset, everything is better defined inside of a class. However, as we saw above, often we need methods that are different to place in an existing class. This can happen, for example, with utility methods that operate on a class but do not represent the behavior of that class.

In languages like Java, the normality is to define Utils or Helper classes full of the methods that are different to scope in a certain class. This gets easily out of control and leads to classes with different responsibilities and heterogeneous methods that are very difficult to read, maintain, and re-use.

Kotlin, on the hand, is not just an object-oriented language. It supports other programming paradigms, such as the functional one. Hence, it does not take the object orientation as strictly as Java, allowing us, for example, to define functions that are not tied to any class.

On the one hand, this improves the reusability and the maintainability of the code. Furthermore, we can use the package structure and the visibility keywords to choose which portion of our codebase can use a given function or object . Better still, with companion object s, we can define utility code as close as possible to the class it operates on. On the other hand, we ought to pay attention to the freedom and flexibility of Kotlin’s approach. For example, nothing prevents us from defining a mutable package-level variable, essentially a global one, which can be very harmful.

As is common with modern programming languages, we have a number of ways to model the same thing and achieve the same result. Hence, it is always a matter of experience and sensibility to figure out what the right construct is and to use it appropriately.

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How to write Static Method in Kotlin?

In this tutorial, you shall learn how to write a static method in a class in Kotlin, with examples.

Static Method in Kotlin

Unlike Java, we do not have a static keyword in Kotlin. But, that does not limit us to write methods in Kotlin that function as static methods in Java.

To achieve the functionality of a static method in Kotlin, use companion keyword modifier.

In the following example, we define a class MyClass with with a function greetings() , where the function is modified using companion keyword. We can call this method using Class name as MyClass.greetings() .

Kotlin Program

In this Kotlin Tutorial , we learned how to call a method using Class name, without instantiating an instance of the class type.

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How to create static variables and functions in Kotlin

The val keyword is used to create an immutable variable, just like the final keyword in java.

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Equivalent of Java Static Methods in Kotlin

Prerequisite : static Keyword in Java

Static Keyword in Java is a non-access modifier. It is useful for representing meta-data (usually data related to a class). The static keyword serves for efficient memory management as in spite of recurring usages the Java Virtual Machine only once allocates memory for the variable, method, class or block declared as static. Static members are not related to instances being created but are owned by the class itself. In Kotlin however, we don’t have such keywords. So this functionality has to be achieved using some other ways. The most popular methods of implementing functionality similar to that of the static keyword are listed below:

By the use of companion object

Object expressions or Object declarations in Kotlin are used to create instances of an anonymous class. Using companion identifiers along with such object expressions or declarations facilitates members of the companion object to be called just by using the class name as the qualifier. When the matching class is loaded (resolved), a companion object is created that has the same semantics as a Java static initializer.

In the case of Java, the method of companion object can be called but we need an additional ‘Companion’ field to be added before the function call after the class name. We can get rid of this too, by either giving our own name to the companion object or by using @JvmStatic annotation.

By using @JvmStatic annotation

Adding @JvmStatic annotation simply before the members to be declared as the equivalent of static ones in Java works well in Kotlin to provide the same functionality.

By using a package-level function

An equivalent functionality as that of the static members and methods in Java can also be achieved by making that member, method, class, or block be a package-level member. This can be done by creating a .kt file and placing all such desired members into it without making any class inside. In this way, all these members will be compiled by the JVM as the static parts of the Kotlin class in which the package is being imported.

Kotlin File to achieve the functionality of static keyword:

Main Kotlin File:

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Kotlin static methods and variables

In Java, once a method is declared as "static", it can be used in different classes without creating an object. With static methods, we don't have to create the same boilerplate code for each and every class. Here we will take an example to demonstrate how Kotlin implements static methods.

Example – Static Method in Kotlin using Companion Object

In order to implement a static method in Kotlin, we will take the help of "companion objects".

Companion objects are the singleton objects whose properties and functions are tied to a class but not to the instance of that class. Hence, we can access them just like a static method of the class.

Note that only one companion class is allowed per class. More than one companion object per class will lead to a runtime error in Kotlin.

On execution, it will generate the following output −

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How can I create a static class in Kotlin? (static CLASS not method or variable)

Hello everyone,

I am looking for a way to create a static class in Kotlin. One example is when I want to use JavaFX, I normally need to create a static class that inherit the Application class, and sometimes I also want to create some inner classes that are static. Of course, I have a way to deal with this problem in the former case, but I want to know a general way to create Java static class in Kotlin because I am translating a lot of Java code to Kotlin which has a lot of static classes and did not work using the default translation in IDEA.

Thank you. I am looking forward to hearing from you.

As long as you mean static nested classes, in kotlin a nested class is static by default. To make it a non-static inner class it needs to have a inner flag.

You could use a companion object . If you want a singleton you could also use an object class .

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Mock Static Java Methods Using Mockk

Last updated: March 19, 2024

As a seasoned developer, you’re likely already familiar with Spring. But Kotlin can take your developer experience with Spring to the next level!

Join the >> "Advanced Kotlin Techniques for Spring Developers" webinar and learn how to:

Add new functionality to existing classes with Kotlin extension functions.

Use Kotlin bean definition DSL.

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Use sequences and default argument values to write more expressive code.

By the end of this talk, you’ll have a deeper understanding of the advanced Kotlin techniques that are available to you as a Spring developer, and be able to use them effectively in your projects.

1. Overview

Kotlin has great compatibility with Java, but one difference is that Kotlin doesn’t contain the static modifier , and when it comes to mocking, Java static methods aren’t convenient to access. The mocking library Mockk  has a feature that allows us to mock static Java methods easily.

In this tutorial, we’ll learn how to use this function and undo its effects after we’re done with it.

2. Using Mockk on a Java static Method

To set up our demonstration, let’s use RandomNumberGenerator as an example of a static Java class. We assume that this is whatever static method our external implementation gives us:

Let’s use it in a simple example to determine the result of a coin flip based on the class we have created:

Some unit tests won’t require any change or mocking to work. Validating that our coin flip never throws an exception is an example of that:

In some other cases, however, we might want to assert the mechanisms behind our flip, which is the static random() method. For example, we may need to validate specific rules based on the result of the method call.

In our sample function, the assertion we want depends on an assumption: Whenever the static method is called, we’ll replace it with a class that returns what we want . The test should have a format similar to:

To make sure we can control the returned value, let’s use  mockkStatic() to fine-tune the result:

3. Removing a Mockk After Using It

We need to ensure we don’t mess with the static namespace we used, as many other methods might use the same static method . In other words, we don’t want our mocked behavior to leak into other tests. So, let’s unmock the static class we mocked previously:

4. Removing All Mockks at Once After Tests

We can also unmock what we’ve mocked after any test finishes running by using unmockkAll() . That limits the unmockk calls to a single place and ensures we won’t leak any modification to the rest of our test suite.

To do that, let’s add a method to our unit test class and annotate it with @AfterEach :

5. Conclusion

In this tutorial, we’ve learned how to use Mockk’s mockkStatic to mock static Java methods. We also prepared our test suite to unmockk any mocks we might have set up, ensuring we don’t pollute the context for other tests.

As always, the implementation used in this article can be found on GitHub .

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Possibility to call a Java static method in Kotlin

Suppose we have a Java static method:

It is possible to call that method in Kotlin?

4 Answers 4

Yes, you can. Java code:

Kotlin code:

• 2 but from instance is not? –  qwert_ukg Commented Mar 15, 2018 at 5:53
• How could we call android.nfc.Tag.createMockTag from Kotlin? –  IgorGanapolsky Commented Aug 20, 2020 at 19:14

The answer from 0wl is generally correct.

I just wanted to add that some of the Java classes are mapped to special Kotlin classes. In this case you must fully qualify the Java class for this to work.

Yes . It's documented in the Java Interop

http://kotlinlang.org/docs/reference/java-interop.html

The docs show the following example

The only caveat I see is that they can't be passed around with an instance and accessed on instances of the class like you can in Java but this is usually considered bad practice anyway.

In Java it is possible to call static methods from child classes, in Kotlin this is not possible. If this is your case, call the method from the parent, instead.

Test.java :

Refer to this StackOverflow thread for more information.

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Hot Network Questions

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