CODEWITHSUNDEEP
androiddesign-patternskotlin
garbus
garbus

Reputation: 84

Kotlin - any substitute for companion object inside of another object (not class)?

I would like in my Kotlin Android app to have singleton object with some static definitions of it's inner states.

As I understand, object in Kotlin is for singleton, so I am trying this kind of approach:

object MySingleton
{
    public const val _DEF_DEFINITION_NO_ONE: Byte = 1;
    public const val _DEF_DEFINITION_NO_TWO: Byte = 2;
    (...)
}

This is fine, but the problem is, to being able use these definitions, now I must create Object's instance first.

Just wondering if I am able in Kotlin to create this kind of construction and have access to these definitions without creating MySingleton instance? Answer would be companion object working similar as static in other languages, but it's not allowed inside objects, only inside of classes.

Of course I can leave this as is or make these definitions global, but would like to know is it possible to do in a way I described? Or maybe should I design this in another yet way?

Upvotes: 3

Views: 2245

Answers (2)

Willi Mentzel
Willi Mentzel

Reputation: 29844

There are two ways to work with static data in Kotlin:

An object

object ASingleton {
    val CONSTANT_PROPERTY: Int = 1;
}

If you need a single instance class, which has only one state for each property, use a singleton. Note: There can only be one instance of this class and it is created for you by the compiler.

A class with a companion object

class ClassWithCompanionObject{

    val someProperty: Int = 0; // instance bound

    companion object {
        val CONSTANT_PROPERTY: Int = 1;
    }
}

If you need some static properties, and the rest should have a state which is bound to a certain instance, go with the class with companion object.

Usage:

println(ASingleton.CONSTANT_PROPERTY)
println(ClassWithCompanionObject.CONSTANT_PROPERTY)

Upvotes: 6

s1m0nw1
s1m0nw1

Reputation: 81899

As you said, MySingleton is an object and thus a singleton. There's no need to create an instance of it (not even possible). You simply access it's constants in a static way like this: MySingleton._DEF_DEFINITION_NO_ONE.

If you want to use the constants without prefixing the object name, just import them with the fully-qualified name and use it as follows:

import package.MySingleton._DEF_DEFINITION_NO_ONE
//...
println(_DEF_DEFINITION_NO_ONE)

Upvotes: 4

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