What is the best way to compare two classes of the...

# getting-started

[Anthony Flores]

What is the best way to compare two classes of the same type, and copy all the properties of the second one that are different compared to an original?

For example, let's say I have 3 string variables stored in said class:

class SomeClass {
    var string1: String = "Default"
    var string2: String = "Default"
    var string3: String = "Default"
}

and lets say in the first class, string1 is "Text," string2, is "Other Text," and in the second class string1 is "String, " while string2 is still Default

I would want it so that the first string is "String," the second string is "Other Text," and string3 as "Default"

[Adam S]

when you say "first class" and "second class", you mean "first instance of SomeClass", and "second instance of SomeClass"? Or do you want to compare 3 different classes?

Why do you expect string2="Other Text"? Shouldn't it be "Default", because second.string1 != first.string1?

[Anthony Flores]

Yes, I'm referring them as all the same class, but with different properties

[Anthony Flores]

If nothing was changed in the first class, and in the second class the first variable was changed, then the output should produce the first variable as whatever the second class was and the rest as the values in the first class

[Michael de Kaste]

putting a "default" value on a string means it has a special meaning within your code. I think it might be better to just use String? instead, but regardless. I dont know what you want with the copying over, but you can do:

import SomeClass.Companion.DEFAULT

class SomeClass {
    companion object {
        const val DEFAULT = "Default"
    }
    var string1: String = DEFAULT
    var string2: String = DEFAULT
    var string3: String = DEFAULT
}

fun main() {
    val class1 = SomeClass().apply {
        string1 = "Text"
        string2 = "OtherText"
    }
    val class2 = SomeClass().apply {
        string1 = "String"
    }

    val result1 = class2.string1.takeUnless(DEFAULT::equals) ?: class1.string1
    val result2 = class2.string2.takeUnless(DEFAULT::equals) ?: class1.string2
    val result3 = class2.string3.takeUnless(DEFAULT::equals) ?: class1.string3

    println(result1)
    println(result2)
    println(result3)
}

this prints, in order:

String
OtherText
Default

if you instead have nullable string with null being the default, you can just do:

val result1 = class2.string1 ?: class1.string1
    val result2 = class2.string2 ?: class1.string2
    val result3 = class2.string3 ?: class1.string3

[Michael de Kaste]

it is ofcourse also possible to use reflection to automatically compare all fields, however, I don't think you can easily check for a default value this way. So If you pretend your defaults are "null" instead:

SomeClass::class.memberProperties.map {
    it.name to (it.get(class2) ?: it.get(class1))
}.forEach(::println)

prints

(string1, String)
(string2, OtherText)
(string3, null)

[Anthony Flores]

Alright, thanks!