Kotlin object vs companion-object vs package scoped methods

Question

I have written this methods in Kotlin and analysed the bytecode:

Situation 1

class A {
    object b {
        fun doSomething() {}
    }
}

Situation 2

class A {
    companion object b {
        fun doSomething() {}
    }
}

Situation 3

fun doSomething() {}

Bytecode Result

• Situation 1: class Test$asb, public final doSomething()I
• Situation 2: class Test$Companion, public final doSomething()I
• Situation 3: class TestKt, public final static doSomething()I

My questions are:

• I have an enum class, and I want to return an enum instace given an enum variable, for instance, findById (enum(id, color)). How would I do it? Companion Object? object?

• It seems the only way to have a real static method is in package level, without class declaration. But that becomes a little bit too global. Is there any way to access it via: ClassName.staticMethod, staticMethod being really static.

• Provide meaningfull examples of package declaration methods, companion object and object.

Context. I have been coding in Kotlin and I find it amazing. But sometimes I need to make a decision: for example, a heavy immutable property which in java I would declare as static final, but in Kotlin I find it hard to "find an equivalent".

Answer 1

I would suggest to develop voddan answer:

enum class Color {

    RED,
    BLUE,
    GREEN;


    companion object Utils {
        fun findById(color: Color): Color {
            return color;
        }
    }
}

And to test

@Test
fun testColor() {
    println(Color.Utils.findById(Color.valueOf("RED")));
}