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Hello
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Hello
class
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Hello
class
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Hello
class
class List
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Any
The root of the hierarchy
public open class Any {
public open operator fun equals(other: Any?): Boolean
public open fun hashCode(): Int
public open fun toString(): String
}
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Any
The root of the hierarchy
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No content
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No content
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val maybeString: String? = null
val definitelyString = maybeString ?: "replacement"
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No content
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No content
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No content
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No content
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No content
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No content
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No content
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No content
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No content
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Unit
public object Unit {
override fun toString() = "kotlin.Unit"
}
The type with only one value
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val noResult: Task< >
Unit
Unit
The type with only one value
fun () {
// Empty!
}
empty() {
// Empty!
}
Task< > noResult;
Unit void
Void
:
empty
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Unit
Unit
The type with only one value
void
Void
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Nothing
A value that never exists
public class Nothing private constructor()
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Nothing
A value that never exists
fun loopy(): Nothing {
while (true) {
println("Loop!")
}
}
fun exceptional(): Nothing {
throw IllegalStateException()
}
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Nothing
A value that never exists
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Nothing
The bottom type
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Nothing
The bottom type
fun processData(data: List) {
// Use data
}
fun main(args: Array) {
val data: Nothing = loopy()
processData(data)
}
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Nothing
Elvis’ best friend
fun calculate(someParam: Int?) {
val x = someParam ?: exceptional()
val y = x * 2
println(y)
}
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Nothing
Elvis’ best friend
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No content
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No content
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No content
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exceptional()
Nothing
Elvis’ best friend
fun calculate(someParam: Int?) {
val x = someParam ?:
val y = x * 2
println(y)
}
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Nothing
Elvis’ best friend
throw IllegalArgumentException("!")
fun calculate(someParam: Int?) {
val x = someParam ?:
val y = x * 2
println(y)
}
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* 2
someParam ?: return
fun calculate(someParam: Int?) {
val y =
println(y)
}
Nothing
Elvis’ best friend
x
val x =
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someParam ?: return
* 2
fun calculate(someParam: Int?) {
val y =
println(y)
}
someParam
Nothing
Elvis’ best friend
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Nothing?
Nothing.
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Nothing?
Nothing.
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Nothing?
Nothing.
var x = null
x = "foo"
Type mismatch.
Required: Nothing?
Found: String
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Nothing
Edge cases
fun oddity() {
throw return return throw return
}
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Resources
• Illustration style credit
http://natpryce.com/articles/000818.html
• More about Nothing
http://oneeyedmen.com/nothing-can-save-us.html
https://blog.kotlin-academy.com/kotlins-nothing-its-
usefulness-in-generics-5076a6a457f7