Kotlin: How to access field from another class?

Question

package example

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

Class:

package example

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

Error:

'APPLE_SIZE_KEY' has private access in 'example.Apple'

But official documentation describes that if we do not specify any visibility modifier, public is used by default.

Why is above error coming?

more on static variable https://stackoverflow.com/questions/43857824/kotlin-static-methods-and-variables/43857895 — Bharatesh, Jan 05 '18 at 08:49

score 29 · Accepted Answer · answered Jan 04 '18 at 18:35

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"
    }
}

It may help to mention `@JvmStatic` which would enable normal static access (`Apple.APPLE_SIZE_KEY`) from Java code, as opposed to `Apple.Companion.INSTANCE.APPLY_SIZE_KEY`. — Salem, Jan 04 '18 at 22:13

score 9 · Answer 2 · answered Jan 04 '18 at 18:34

9

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)
}

answered Jan 04 '18 at 18:34

zsmb13

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`const val` could be used since the type is `String`, which avoids generating an unnecessary getter. And of course the type can be inferred by the compiler, so: `const val APPLE_SIZE_KEY = "APPLE_SIZE_KEY"` – Michael Jan 04 '18 at 20:34

Kotlin: How to access field from another class?

2 Answers2