Kotlin: How to access field from another class?

What you are trying to do is accessing a value of a class that has no instance. Here are three solutions:

package example

object Apple {
    val APPLE_SIZE_KEY: String = "APPLE_SIZE_KEY"
}

This way you do not need to instantiate anything because of the way objects work in Kotlin.

You could also just instantiate your class like this:

package example

class Store {
     fun buy() {
      val SIZE = Apple().APPLE_SIZE_KEY
    }
}

In this solution you also have an object of Apple, but Apple is still declared as a class.

The third option is a companion object, which behaves like static variables in Java.

package example

class Apple {
    companion object {
        val APPLE_SIZE_KEY: String = "APPLE_SIZE_KEY"
    }
}

If you want this to be a class level property instead of an instance level property, you can use a companion object:

class Apple {
    companion object {
        val APPLE_SIZE_KEY: String = "APPLE_SIZE_KEY"
    }
}

fun useAppleKey() {
    println(Apple.APPLE_SIZE_KEY)
}

What you currently have is an instance property, which you could use like this:

fun useInstanceProperty() {
    val apple = Apple()
    println(apple.APPLE_SIZE_KEY)
}