1. A Friend In Need Is a Friend Indeed
Kotlin / Java Interop
Andrey Breslav
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2. Kotlin
Modern Language for Industry
• Smart compiler • Static typing
➡ Less boilerplate • Readability
• Flexible abstractions • Tool support
➡ Powerful libraries • Interoperability
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3. Stuff
• Apache 2 • IDE
• Target ➡ IntelliJ
➡ JVM (1.6) ➡ Web Demo
➡ JavaScript
*
• Build
➡ Ant
➡ Maven
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* Planned
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4. Outline
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5. Outline
• Enhancing Existing APIs
➡ Extensions
➡ Splitting Collections Interfaces
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6. Outline
• Enhancing Existing APIs
➡ Extensions
➡ Splitting Collections Interfaces
• Fighting for Safety
➡ Nullable types
➡ Interaction with generics
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7. Outline
• Enhancing Existing APIs
➡ Extensions
➡ Splitting Collections Interfaces
• Fighting for Safety
➡ Nullable types
➡ Interaction with generics
• Statics
➡ Packages
➡ Class objects
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8. Making Existing
Things Nicer
Extensions
Variance Tricks
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9. Extensions
• Demo
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10. Collections & Variance
Java:
Kotlin:
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11. Declaration-Site Variance
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12. Translation
Ø
java.util.List
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13. Translation: Inheritance
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14. Translation: Variance
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15. Calling Java From Kotlin
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16. Summary
• Use JDK collections in Kotlin
• Extend them with useful operations
• Make them variant for Kotlin
• At the expense of some unsoundness
around the edges
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17. Fighting NPEs
Nullable Types
Annotating Java Code
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18. Nullable Types
• Demo
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19. Nullable Types Summary
• null is a legitimate value, not a sin
• No overhead (as opposed to Option)
• Proper subtyping:
➡ List<String> is a List<String?>
• At the expense of:
file.getParent()!!.getName()!!
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20. Annotations to The Rescue
String String?
JSR 305: Annotations for Software Defect Detection
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21. Generics?!
@NotNull
doesn't
help
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22. More Use-Cases of @Kotlin
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23. External Annotations
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24. It Is Not So Bad
• IDE aids annotating with quick fixes
• Some annotations can be inferred
automatically
• Alternative: have a special kind of types
for Java things
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25. Statics
Packages
Class Objects
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26. Package-Level Code
file1.kt
foo/namespace.class
file2.kt
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27. Package-Level Code
file1.kt
foo/namespace.class
Compiled From: file1.kt
file2.kt
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28. Package-Level Code
file1.kt
foo/namespace.class
foo/namespace$file1.class
from:
file1.kt
foo/namespace$file2.class
file2.kt from:
file2.kt
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29. Package-Level Code
file1.kt
foo/namespace.class
foo/namespace$file1.class
from:
Initialization Order?! file1.kt
foo/namespace$file2.class
file2.kt from:
file2.kt
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30. Kotlin Classes
Have No Statics
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31. Class Object Translation
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32. Class Object Translation
Only fields!
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33. Resources
• Docs: http://kotlin.jetbrains.org
• Demo: http://kotlin-demo.jetbrains.com
• Code: http://github.com/jetbrains/kotlin
• Twitter:
➡ @project_kotlin
➡ @abreslav
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