Constructors

FunSpec is a slight misnomer; it can also be used for constructors:

val flux = FunSpec.constructorBuilder()
  .addParameter("greeting", String::class)
  .addStatement("this.%N = %N", "greeting", "greeting")
  .build()

val helloWorld = TypeSpec.classBuilder("HelloWorld")
  .addProperty("greeting", String::class, KModifier.PRIVATE)
  .addFunction(flux)
  .build()

Which generates this:

class HelloWorld {
  private val greeting: String

  constructor(greeting: String) {
    this.greeting = greeting
  }
}

For the most part, constructors work just like methods. When emitting code, KotlinPoet will place constructors before methods in the output file.

Often times you’ll need to generate the primary constructor for a class:

val helloWorld = TypeSpec.classBuilder("HelloWorld")
  .primaryConstructor(flux)
  .addProperty("greeting", String::class, KModifier.PRIVATE)
  .build()

This code, however, generates the following:

class HelloWorld(greeting: String) {
  private val greeting: String

  init {
    this.greeting = greeting
  }
}

By default, KotlinPoet won’t merge primary constructor parameters and properties, even if they share the same name. To achieve the effect, you have to tell KotlinPoet that the property is initialized via the constructor parameter:

val flux = FunSpec.constructorBuilder()
  .addParameter("greeting", String::class)
  .build()

val helloWorld = TypeSpec.classBuilder("HelloWorld")
  .primaryConstructor(flux)
  .addProperty(
    PropertySpec.builder("greeting", String::class)
      .initializer("greeting")
      .addModifiers(KModifier.PRIVATE)
      .build()
  )
  .build()

Now we’re getting the following output:

class HelloWorld(private val greeting: String)

Notice that KotlinPoet omits {} for classes with empty bodies.