Generate data class with KotlinPoet

Question

I want to generate simple data class with KotlinPoet:

data class User(val id: Int)

I do it like that:

val builder = KotlinFile.builder("", "Foo")
val classBuilder = TypeSpec.classBuilder("User").addModifiers(KModifier.DATA)
val ctor = FunSpec.constructorBuilder()
ctor.addParameter("id", Int::class)
classBuilder.primaryConstructor(ctor.build())
builder.addType(classBuilder.build())
builder.build().writeTo(System.out)

But what I get is this:

data class User(id: Int) {
}

How can I add val modifier to constructor parameter?

Answer 1

Here is an issue discussing this problem.

The conclusion is that the way to write this code is to create a property that matches the constructor parameter's name, and is initialized by it:

KotlinFile.builder("", "Foo")
        .addType(TypeSpec.classBuilder("User")
                .addModifiers(KModifier.DATA)
                .primaryConstructor(FunSpec.constructorBuilder()
                        .addParameter("id", Int::class)
                        .build())
                .addProperty(PropertySpec.builder("id", Int::class)
                        .initializer("id")
                        .build())
                .build()
        )
        .build()
        .writeTo(System.out)

However, this feature is not available as of the 0.2.0 release of KotlinPoet. It is already implemented, and there is a test for this in the repository that is passing, but you'll have to wait for the 0.3.0 release of KotlinPoet to be able to use this feature, which is supposed to come in the next few days.

This feature is available from 0.3.0, which is out now.

---

The previous code with an alternative formatting that matches your original style:

val builder = KotlinFile.builder("", "Foo")
val classBuilder = TypeSpec.classBuilder("User").addModifiers(KModifier.DATA)

val ctor = FunSpec.constructorBuilder()
ctor.addParameter("id", Int::class)
classBuilder.primaryConstructor(ctor.build())

val idProperty = PropertySpec.builder("id", Int::class).initializer("id").build()
classBuilder.addProperty(idProperty)

builder.addType(classBuilder.build())
builder.build().writeTo(System.out)