Convert java class to kotlin

This free online converter lets you convert code from Java to Kotlin in a click of a button. To use this converter, take the following steps —

Type or paste your Java code in the input box.
Click the convert button.
The resulting Kotlin code from the conversion will be displayed in the output box.

Key differences between Java and Kotlin

Characteristic	Java	Kotlin
Syntax	Java has a verbose syntax with a lot of boilerplate code, whereas Kotlin has a concise and expressive syntax that reduces the amount of code needed to accomplish the same task.	Kotlin has a concise and expressive syntax that reduces the amount of code needed to accomplish the same task compared to Java.
Paradigm	Java is an object-oriented programming language that supports imperative and declarative programming paradigms, whereas Kotlin is a statically-typed programming language that supports both object-oriented and functional programming paradigms.	Kotlin is a statically-typed programming language that supports both object-oriented and functional programming paradigms.
Typing	Java is a statically-typed language, which means that the type of a variable is determined at compile-time, whereas Kotlin is also a statically-typed language but supports type inference, which means that the type of a variable can be inferred by the compiler.	Kotlin is a statically-typed language that supports type inference, which means that the type of a variable can be inferred by the compiler.
Performance	Java has a reputation for being fast and efficient due to its Just-In-Time (JIT) compiler, which compiles bytecode into machine code at runtime. Kotlin also has good performance, but it is slightly slower than Java due to its additional features.	Kotlin has good performance, but it is slightly slower than Java due to its additional features.
Libraries and frameworks	Java has a vast collection of libraries and frameworks, including Spring, Hibernate, and Struts, which are widely used in enterprise applications. Kotlin can also use these libraries and frameworks, but it also has its own set of libraries and frameworks, such as Ktor and Anko.	Kotlin can use Java’s vast collection of libraries and frameworks, but it also has its own set of libraries and frameworks, such as Ktor and Anko.
Community and support	Java has a large and active community with a wealth of resources and support available, including documentation, forums, and tutorials. Kotlin’s community is growing rapidly, but it is still smaller than Java’s.	Kotlin’s community is growing rapidly, but it is still smaller than Java’s. However, there are still a wealth of resources and support available, including documentation, forums, and tutorials.
Learning curve	Java has a steep learning curve due to its verbose syntax and complex concepts, such as object-oriented programming and memory management. Kotlin has a much gentler learning curve due to its concise syntax and ease of use.	Kotlin has a much gentler learning curve than Java due to its concise syntax and ease of use.

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Convert Java to Kotlin

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

Add new functionality to existing classes with Kotlin extension functions.
Use Kotlin bean definition DSL.
Better configure your application using lateinit.
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 is built to be interoperable with Java. This means that Kotlin applications can have both Kotlin and Java source files.

Sometimes we might want to convert Java code to Kotlin. In this tutorial, we’ll see various ways to do that.

2. Advantages of Kotlin Over Java

Let’s quickly understand a few benefits of Kotlin over Java, as that might also guide our decision on whether we want to convert Java code to Kotlin:

Unlike Java, Kotlin doesn’t allow null values by default. Hence, Kotlin provides better code stability.
To extend methods in Java, we need to inherit the class and override them. However, Kotlin makes it easier for us with extension functions that don’t compel us to inherit from a class.
Java compels us to check the type of variable and cast it accordingly. Kotlin, on the other hand, comes with a smart casting feature.

3. Different Ways to Convert Java Code to Kotlin

We can convert our Java code to Kotlin without explicitly rewriting the Kotlin code equivalent.

Now, let’s see various approaches.

3.1. Using IntelliJ IDE Software or Android Studio Copy/Paste Method

We can use both Android Studio and IntelliJ IDE to convert our Java code to Kotlin code.

First, we must create or have an existing Kotlin project. We open this project on either IDE and within a .kt file, we paste some Java code. Then, the IDE prompts us with a dialog asking if we wish to convert the Java code to Kotlin.

Suppose, we wish to paste this piece of Java code. A simple function to add two numbers and return their sum:

public class TestClass < public static void main(String[] args) < System.out.println("Sum demo " + sum(2, 4)); System.out.println("Product demo " + product(2, 4)); >public static int sum(int x, int y) < int a = x; int b = y; return a + b; >public static int product(int x, int y) < int a = x; int b = y; return a * b; >>

Now, pasting this code into our .kt file on IntelliJ displays a prompt:

Now, we click on Yes to allow IntelliJ to perform the code conversion.

By the end of the conversion, the Java code will be replaced by the Kotlin code in the same file as seen in the image below:

It’s important to note that this method is very much convenient for converting snippets of Java code at a time.

However, having to paste Java code into the IDE for automatic conversion to Kotlin means a slower process when having to convert a large amount of Java code. Also, this method may not work well for Kotlin projects with existing Java source code files.

3.2. Using IntelliJ IDE Software or Android Studio Menu Option Method

Another way to perform the conversion using IntelliJ or Android Studio IDE software is to create a .java file in our Kotlin project.

Let’s create two Java classes, Summation, and Product, in our project:

To convert these Java classes to Kotlin, we can navigate to the Code -> Convert Java File to Kotlin File menu entry:

Once the conversion process finishes, we get a .kt file with the same name as the Java class. The .java files will then be removed from the project folder:

Besides using the navigation menu, we also have keyboard shortcuts for Mac and Windows to achieve the same result: Shift + Alt + Cmd + k (in Mac) and Shift + Alt + Ctrl + k (in Windows).

This method is very convenient for converting all Java source files to Kotlin in a mixed Java/Kotlin project. It performs the conversion quickly and project-wide.

3.3. Using JavaInUse Online

JavaInUse is a great online tool we can also use to convert Java code to Kotlin. All we need to do is write or paste the Java code we intend to convert into the left (editable) text box, and then click the Submit button to perform the conversion:

Using this online tool, however, has some downsides. Firstly, we can only convert a chunk of Java code at a time. This means that we can’t convert all Java code to Kotlin by uploading the project folder. We must perform the conversion in bits or per Java class.

Secondly, there are security risks in uploading our Java code since this is a third-party site.

Lastly, we recommend this tool for simple Java code conversions. It may not work if we try to convert Java code with many methods, method calls, and complex constructs.

4. Conclusion

This article shows two major ways to convert Java code to Kotlin. Firstly, we can use Android Studio or IntelliJ IDE software, and secondly via an online tool.

We also discussed the limitations of both approaches. The first methods work only with IntelliJ and Android Studio IDE software, while the online tool can be used with any editor.

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Mixing Java and Kotlin in one project – tutorial

Kotlin provides the first-class interoperability with Java, and modern IDEs make it even better. In this tutorial, you’ll learn how to use both Kotlin and Java sources in the same project in IntelliJ IDEA. To learn how to start a new Kotlin project in IntelliJ IDEA, see Getting started with IntelliJ IDEA.

Adding Java source code to an existing Kotlin project

Adding Java classes to a Kotlin project is pretty straightforward. All you need to do is create a new Java file. Select a directory or a package inside your project and go to File | New | Java Class or use the Alt + Insert/Cmd + N shortcut.

If you already have the Java classes, you can just copy them to the project directories.

You can now consume the Java class from Kotlin or vice versa without any further actions.

For example, adding the following Java class:

lets you call it from Kotlin like any other type in Kotlin.

Adding Kotlin source code to an existing Java project

Adding a Kotlin file to an existing Java project is pretty much the same.

If this is the first time you’re adding a Kotlin file to this project, IntelliJ IDEA will automatically add the required Kotlin runtime.

You can also open the Kotlin runtime configuration manually from Tools | Kotlin | Configure Kotlin in Project.

Converting an existing Java file to Kotlin with J2K

The Kotlin plugin also bundles a Java to Kotlin converter (J2K) that automatically converts Java files to Kotlin. To use J2K on a file, click Convert Java File to Kotlin File in its context menu or in the Code menu of IntelliJ IDEA.

While the converter is not fool-proof, it does a pretty decent job of converting most boilerplate code from Java to Kotlin. Some manual tweaking however is sometimes required.

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