How do you initialize val fields with constructor params in Kotlin?

Question

In Java you can declare a private final member variable and then initialize it from your constructor, which is very useful and is a very common thing to do:

class MyClass {

  private final int widgetCount;

  public MyClass(int widgetCount) {
    this.widgetCount = widgetCount;
  }

In Kotlin how do you initialize final member variables (val types) with values passed in to a constructor?

Answer 1

It is as simple as the following:

class MyClass(private val widgetCount: Int)

This will generate a constructor accepting an Int as its single parameter, which is then assigned to the widgetCount property.

This will also generate no setter (because it is val) or getter (because it is private) for the widgetCount property.

Answer 2

class MyClass(private val widgetCount: Int)

That's it. If in Java you also have a trivial getter public int getWidgetCount() { return widgetCount; }, remove private.

See the documentation for more details (in particular, under "Kotlin has a concise syntax for declaring properties and initializing them from the primary constructor").

Answer 3

As well as defining the val in the constructor params, you can use an init block to do some general setup, in case you need to do some work before assigning a value:

class MyClass(widgetCount: Int) {

  private val widgetCount: Int
  
  init { 
     this.widgetCount = widgetCount * 100
  }
}

init blocks can use the parameters in the primary constructor, and any secondary constructors you have need to call the primary one at some point, so those init blocks will always be called and the primary params will always be there.

Best to read this whole thing really! Constructors