CODEWITHSUNDEEP
kotlin
Luciano Jeong
Luciano Jeong

Reputation: 335

About type parameter of generic in Kotlin

I'm studying kotlin language and I can't distinguish these examples, especially nullable types in a 'where' clause of generic. Would you tell me the difference?

case 1

class Foo<T> where T: Comparable<T>
class Foo<T> where T: Comparable<T?>
class Foo<T> where T: Comparable<T>?
class Foo<T> where T: Comparable<T?>?

case 2

class Foo<T> where T: Comparable<T>? {
// If a class is declared like above, is a type 'T' already nullable?

// Then, 
fun bar(value: T?) { // Should I declare a member function like this to accept null or
// do something
}

fun bar(value: T) { // Should I declare like this instead?
}
}

Upvotes: 2

Views: 69

Answers (1)

Ricky Mo
Ricky Mo

Reputation: 7738

First, to distinguish T : Comparable<T> and T : Comparable<T?>, take a look at the following example. The difference is whether you can compare T with T?.

class Bar1(var bar : Int) : Comparable<Bar1>{

    override fun compareTo(other : Bar1) : Int {
        return bar - other.bar
    }
}

class Bar2(var bar : Int) : Comparable<Bar2?>{

    override fun compareTo(other : Bar2?) : Int {
        return bar - ( other?.bar ?: 0 )
    }
}

fun main(){
    println(Bar1(1) > Bar1(2))
    val bar2 : Bar2? = Bar2(2)
    println(Bar2(1) > bar2)
}

Output:

false

false

The difference is that 

val bar1 : Bar1? = Bar1(2)
println(Bar1(1) > bar1)

will not compile. bar1 must be unwrapped

Second, to distinguish class Foo<T> where T: Comparable<T>? and class Foo<T> where T: Comparable<T>?, it has nothing to do with comparable. Take a look at the following simplified example.

class Foo1<T>(val t : T) where T : Int{
    override fun toString() : String{
        return "$t"
    }
}

class Foo2<T>(val t : T) where T : Int?{
    override fun toString() : String{
        return "$t"
    }
}


fun main(){
    println(Foo1(5))
    val i : Int? = 5
    println(Foo2(i))
}

Output:

5

5

The difference is that println(Foo1(i)) will not compile. i must be unwrapped.

Upvotes: 3

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