constants-in-kotlin.md

Where Should I Keep My Constants in Kotlin?

There has recently been a discussion online about the best approach for storing global constants, or public static final fields in Javaspeak. Here, we will discuss multiple methods for creating global constants in kotlin.

Companion objects

There's no static keyword in Kotlin. If you want static access to certain fields or methods of your class, you should put them under a companion object. A naive way to declare a constant would look like this:

class Constants {  
  companion object {
    val FOO = "foo"
  }
}

The field will be available globally and accessible through

var myval = Constants.FOO

const vals

A simple way to improve our example is to mark FOO as const:

class Constants {  
  companion object {
    const val FOO = "foo"
  }
}

But this will only with primitive types and Strings. This will not work for custom classes such as:

class Constants {  
  companion object {
    // won't compile
    const val FOO = Foo()
  }
}

A workaround is to use the @JvmField annotation on the val:

class Constants {  
  companion object {
    @JvmField val FOO = Foo()
  }
}

Dropping the class and the object

If all we need is just a set of constants - we can safely drop both the class and the object and use top-level vals:

const val FOO = "foo"  

For example, overall constants will be something like:

@file:JvmName("Constants")

package com.example.package.app

const val DB_NAME = "myofflinedb"
const val BASE_URL = "http://myexample.baseurl.com/"
const val DUMMY_VALUE = 14

Your file name can be anything but you can access the values by:

var mydb = Constants.DB_NAME
var myUrl = Constants.BASE_URL