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Talk about “Nothing”
In Kotlin
Kelvin Ko @ko_kelvin
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What is “Nothing”?
• From Kotlin Doc:
“ଘࡏ͠ͳ͍”
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What actually it is?
• A Class with private construcor
• Cannot be instaniated
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What is “Nothing”?
• “A value that never exist”, sounds easy to understand
• But
• What is the different between Null and Nothing?
“ଘࡏ͠ͳ͍”
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Kotlin type system
• Nothing is a “Bottom Type”
• SubType of any other Types (extends all other types)
• In opposite, Any is Super class of any other class
(World without Null) (World with Null)
Image Reference: https://medium.com/@m.sandovalcalvo/kotlin-type-system-unveiling-the-mystery-50613f0db893
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So, “Nothing” is
• Used as a Type, no instance
• Bottom Type (Sub Type of any other Type)
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Why we need this?
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Use cases
1. Indicate function never return
2. As a “empty” Type in Generic
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Example of Function that
never return
• listenIncomingConnection will run forever
(May not be a good way to implement like this, just simple example)
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Use case
• Adding Nothing, IDE will know the println is unreachable
This look correct,
but actually this line
will never run
(Video example is uploaded here: https://youtu.be/fjECMd1H-hI)
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Use case 2: Exception
• After throw Exception, program will end
• It is more accurate to use “Nothing” as return type to indicate it never return
Assume we would Load some data from Database,
and throw Exception if it return null
(Video example is uploaded here: https://youtu.be/HG210SN4_iI)
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• So IDE will warn you the code after TODO is unreachable
Stdlib’s TODO()
• Actually TODO() is implemented like this
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Why we need a class that is
Sub Type of any other classes?
Bottom Type
Image Reference: https://medium.com/@m.sandovalcalvo/kotlin-type-system-unveiling-the-mystery-50613f0db893
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2. Usage in Generic
• Example: A Linked List implementation
LinkedList Example Reference: https://www.codementor.io/@kotlin_academy/kotlin-s-nothing-its-usefulness-in-generics-j4wcu5taa
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Linked List implementation
• To represent Empty List, add a single instance object
• Won’t compile
• Kotlin Object cannot have type parameters.
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Linked List implementation
• How about remove Type parameter ?
• Won’t compile
• The reference to T is unresolved. We must specific a
type for T
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Linked List implementation
• Use “Nothing” as Type
• It work because Nothing is subtype of any other Type
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Inside Kotlin Stdlib
• emptyList() implementation:
•
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Conclusion
• Nothing is
• Subtype of any other type (Bottom Type)
• Useful in Generic
• Indicate function that Never return
• Infinite loop
• Throw exception
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I hope you learn Nothing today
Thank you for listening