How to replace the builder pattern in Kotlin

· Stephan Schröder

Kotlin was developed as a “better Java”, with the interoperability with it being a primary concern. Not totally coincidentally that makes moving from Java to Kotlin very straightforward. Most features of Java and its immediate ecosystem including the likes of Lombok have direct or not-so-direct counterparts. So while rewriting a Java project to Kotlin, should you e.g. encounter a Lombok @Data annotation you can simply use a data class on Kotlin’s side.

But what about Lombok’s @Builder annotation? There seems to be no in-build builder facility in Kotlin. Do we have to implement a Builder for each class manually?

TLDR: No, we don’t. Use optional parameters (=parameters with default arguments) in your Kotlin class and use named parameters when invoking it.

What is the problem Lombok @Builder is solving?

Let’s assume we have a DTO containing many properties including username and password. The class has one constructor to initialize all its properties. Invoking this constructor in Java>=10 will look something like this:

var a = new A(null, null, "a_schmidt", "1234", null, null);

The issues here include:

  • a lot of visual noise (having to write null for all the parameters you don’t have a value for)
  • are you sure that username and password are on the right position without jumping to the constructor definition? The compiler only guarantees that the types are matching, but how do you know that you didn’t mix up the order of - in this case - username and password?

How Lombok @Builder helps

adding a @Builder annotation to A will allow you to write code like this

var a = A.builder()

As you can see, we no longer have to provide the parameters we don’t have values for, and it’s immediate apparent that you didn’t confuse the order of parameters. It’s definitely progress.

Side note: If you’re very concerned about securing that you don’t mix up usernames and passwords, you can use wrapper types. After all your organisation might give out usernames like “1234” and allow passwords like “a_schmidt”. With Lombok’s help, this would look like

class Username {
    private String value;

class Password {
    private String value;

class A {
    private Role role;
    private String x1;
    private Username username;
    private Password password;
    private String x2;
    private String x3;

Now even invoking the constructor without a Builder mixed up would fail (at least for username and password), since

var a = new A(null, null, new Password("1234"), new Username("a_schmidt"), null, null);

is a compiletime error.

Why this isn’t a problem in Kotlin to begin with

You don’t need to employ the Builder in Kotlin because Kotlin provides optional parameters, named parameters and nullability also plays a certain part. So given a class declaration like this:

data class A(
    val role: Role = Role.User, // provide a sensible default if such a default should exist 
    val x1: String? = null,     // or use a nullable type and initialise it with null by default
    val username: String,
    val password: String,
    val x2: String? = null,
    val x3: String? = null,

you can invoke the constructor like this

val a = A(
    username = "a_schmidt",
    password = "1234",

As you can see, this looks very close to what a Builder in Java gives you, but comes out of the box in Kotlin. Yes, you do have to think a bit more when writing the constructor in order to determine which parameters are optional and which ones have to be provided every time. This gives you additional security though (not even thinking about nullability here). In Java you can write code like

var a = A.builder()

and create an instance that is bound to trigger a NullPointer exception (or fail a check) at runtime. In Kotlin the equivalent code

val a = A(
    password = "1234",

won’t even compile. Most likely you will notice it immediately because your IDE will point it out to you.

Misconception: I heard there was a Lombok plugin for Kotlin. So can’t I simply use the @Builder on a Kotlin class anyway? Yes and No. Yes, there is a Lombok plugin for Kotlin with support for the most common annotations. Yes, since Kotlin 1.8 there is also support for the @Builder annotation. No, you can’t use it on your Kotlin class. The plugin only allows Kotlin code to understand Java code annotated with Lombok’s most common annotations, but it won’t let you annotate Kotlin classes with it. In more words: It simply helps your Kotlin compiler to understand that a @Builder annotation on a java class means that it’s ok to call a builder() function on that class even though there’s no function declaration to be found in the source code. So the plugin is really useful while converting a Java project to Kotlin, but you can (and should) safely remove it, once the conversion is done.

Side note: wrapper types don’t cause a runtime overhead in Kotlin. Using wrapper types to increase type safety were mentioned in the Java section, so let’s see how this topic is handled in Kotlin. Obviously you can also write wrapper types in Kotlin, since in Java wrapper types were normal classes and Kotlin got those as well. Not so obvious is that Kotlin provided a better/more runtime efficient way! The general tradeoff with normal wrapper classes is that they cause an additional pointer indirection overhead. For this reason some people avoid wrapper types even though the gained compiletime safety would probably be worth it. Kotlin provides specialised classes called inline classes, that are (most often) compiled away. So you are left with all the safety and (mostly) none of the runtime overhead. (The most common case, where the overhead can’t be compiled away, is when you create a generic collection of an inline class. But simply passing an inline class around or storing it in a property is completely runtime performance penalty free.)

An inline class can only wrap a single property and looks like this:

value class Username(private val value: String)

value class Password(private val value: String)

Extra: What about mapstruct?

TLDR: Mapstruct is a Java code generator to simplify the generation of mapper classes (a class containing a map-function converting one Java bean to another). When you use it or implement your mapper classes in Java by hand, this section boils down to the advice to use extension functions to implement mapping-functionality in Kotlin.

With what to replace or how to use mapstruct is the second most common question I heard when refactoring a Java project to Kotlin. Mapstruct itself will tell you that you can use it in Kotlin projects. The problem is that it knows nothing about Kotlin nullability or optional parameter, so all the properties of your Kotlin class would have to be nullable and without default value. This is obviously not idiomatic for Kotlin, so don’t use mapstruct in Kotlin.

To the best of my knowledge, there’s no Kotlin equivalent compiler plugin either to take over mapper generation in Kotlin land. But I suspect that the reason for this is that writing mappers manually in Kotlin is less cumbersome than in Java.

Not only do you have optional and named parameters, I normally don’t even see a point in writing a mapper class. I write an extension function instead:

fun A.toB(): B = B(
    username = this.username,
    password = this.password,
    isImportant = this.role == User.Admin,

The reason why I do this as an extension function - and not as a normal function in A - is that mapping doesn’t belong to the core responsibilities of A. At its declaration site, A doesn’t even need to know that B exists. But it’s still nice to convert an instance of A to B on the fly by writing a.toB(). Of course, this way it’s no longer possible to mock the mapping part.