Kotlin: private mutable property with only getter

Dmytro Titov :

I'm very new to Kotlin and trying to migrate one project from Java to it. I'm a bit confused with Kotlin's properties and getters/setters for them along with visibility scope.

What I want to achieve is:

public class Foo {

    private int number;

    public int getNumber() {
        return number;
    }

    public void increment() {
        number++;
    }

}

So basically I need to create a class with a private property number. This property should be accessible by the public getter, but it shouldn't have a setter. Also, this property should be internally mutable, so that its value can be changed using some method like increment().

As far as I understand, I can't declare it as val, because in that case, it would be completely immutable, even from within the instance of a class, so I can't increment it. Looks like it should be var. But then I don't see an option to make the property itself be private and the getter for this property to be public, because it seems that properties and their getters/setters should have the same visibility scope...

Roland :

If you use Intellij you could just paste your code into a Kotlin class and would have received the following:

class Foo {

  var number: Int = 0
    private set

  fun increment() {
    number++
  }
}

And this basically is also the answer to your problem...

private set tells that the setter is only privately visible. So you can access it within Foo, but not from outside. The 0 you set explicitly (in contrast to Java). Then: by default the property and functions are public.

Check also the Kotlin reference regarding properties (specifically getter/setter)