Hi Guys! is there a more clean-kotlin-way to writ...

# getting-started

[arturro]

Hi Guys!

is there a more clean-kotlin-way to write this code:

if (v1 == null && v2 == null) {
            return 0
        }
        if (v1 != null && v2 == null) {
            return 1
        }
        if (v1 == null && v2 != null) {
            return -1
        }

[diesieben07]

arturro: Is that in a Comparator?

[arturro]

yes

[arturro]

It looks like this:

override fun compare(o1: T, o2: T): Int {
        val property = o1.javaClass.kotlin.declaredMemberProperties.first { it -> it.name == propertyName }

        val v1 = property.get(o1) as? Comparable<Any>
        val v2 = property.get(o2) as? Comparable<Any>

        if (v1 == null && v2 == null) {
            return 0
        }
        if (v1 != null && v2 == null) {
            return 1
        }
        if (v1 == null && v2 != null) {
            return -1
        }
        return v1!!.compareTo(v2!!)
    }

[diesieben07]

You could just do val c = compareBy(MyClass::property)

[arturro]

I dont know the property

[arturro]

I know it only at runtime, as a string name

[diesieben07]

Then you look it up using reflection and then pass it to compareBy.

[diesieben07]

And then you can use nullsLast resp. nullsFirst to allow nulls.

[arturro]

I am not sure how to do this.

Now I have:

fun <T : Any> List<T>.sortByProperty(propertyName: String, asc: Boolean = true) : List<T> {
    val field = ???
    return this.sortedBy { field }
}

But it does'n compile.

[diesieben07]

T would have to be reified so you can get it's class. Then you can use T::class to look up the property

[diesieben07]

propertyName should really not be a string though..

[cedric]

Do not use reflection in a comparator

[arturro]

I have to 🙂

[cedric]

Then at least don't do so for every call of compare

[arturro]

Now I have something like this:

inline fun <reified T : Any> List<T>.sortByPropertyName(propertyName: String) : List<T> {
    val prop = T::class.declaredMemberProperties.first { it.name == propertyName }
    return this.sortedBy { prop }
}

But it doesn't compile, the error is:

Error:(11, 17) Kotlin: Type parameter bound for R in inline fun <T, R : Comparable<R>> Iterable<T>.sortedBy(crossinline selector: (T) -> R?): List<T>
 is not satisfied: inferred type KProperty1<T, *> is not a subtype of Comparable<KProperty1<T, *>>

[arturro]

Any suggestion how to fix it are welcome 🙂

[cedric]

@artur prop itself does not implement Comparable, you need code in there that can be mapped by the compiler to a Comparable

[cedric]

e.g. return this.sortedBy { -1 } (obviously not what you want but you get the idea)

[artur]

Thks Cédric!