Pitfalls when mixing Kotlin & Java

I've had my fair share of android projects being converted to Kotlin from Java and I thought I'd share the lessons I've learned doing so. There are a few subtle pitfalls that are very easy to fall into.
Here I share my experience with mixing and what to look out for.

Transcript

1. 
 Pitfalls when mixing Kotlin & Java Bob Dahlberg Hedy Tech Hedy
2. Bob Dahlberg Hedy Tech
 Challenge Accepted GDG Stockholm Android Stockholm Kotlin Meetup Who’s Bob?
 - Building software
 - CrossFit enthusiast
 - Father of one
 - Arranging meetups
 - Loves a good challenge!
3. Background Android Been an Android developer since Android Cupcake (2009). Only Java 6
4. Background Android Scala Learned Scala and tried to build Android with it..
5. Background Android Scala Learned Scala and tried to build Android with it.. It’s plausible.
6. Background Android Scala Learned Scala and tried to build Android with it.. It’s plausible. But you rather die a little than trying to mix it with Java.
7. Background Android Scala Kotlin Found Kotlin M12 in 2015 and it blended almost seamlessly with Java. 

8. Background Android Scala Kotlin Found Kotlin M12 in 2015 and it blended almost seamlessly with Java. A few years later even Google understood it and made it an official language on Android. 

9. Background Android Scala Kotlin Found Kotlin M12 in 2015 and it blended almost seamlessly with Java. A few years later even Google understood it and made it an official language on Android. But there are a few things to keep in mind when mixing.
10. The goal should be to move completely to Kotlin.
11. All the way Blending Although blending Java and Kotlin works great you can’t write idiomatic Java nor Kotlin code when blending. You should choose.

12. All the way Blending Although blending Java and Kotlin works great you can’t write idiomatic Java nor Kotlin code when blending. You should choose.
 You have to choose!
13. All the way Blending Choose Java - If you’re fairly new to Kotlin and wicked at Java. - Don’t know how to write idiomatic Kotlin code. - Can’t invest in learning in the short run or want to slowly adapt to it.
14. All the way Blending Choose Java Choose Kotlin - Already know how to write idiomatic Kotlin code. - Can invest in learning. - If it doesn’t mean to re-write the entire app upfront.
15. All the way Blending
 Choose Java Choose Kotlin Be pragmatic The idioms should be a goal to strive for. 
 Not enforced in every change. Don’t mix if you feel you need to rewrite the entire app before releasing.
16. All the way Blending
 Choose Java Choose Kotlin Be pragmatic Permanent mixing If you have a great separation of concern and code, a modular architecture, you can have idiomatic Java and idiomatic Kotlin mixed permanently. But it’s a rare case.
17. Pace yourself
18. Pace yourself Auto convert Don’t use it, please!

19. Pace yourself Auto convert Don’t use it, please!
 Only one file at the time!
20. Pace yourself Auto convert Explanation mark bonanza
21. Pace yourself Auto convert Explanation mark bonanza Kotlin Friendly If you’ve written your Java code so that it can be easily interpreted, the converted Kotlin code is better. 
 

22. Pace yourself Auto convert Explanation mark bonanza Kotlin Friendly If you’ve written your Java code so that it can be easily interpreted, the converted Kotlin code is better. final
 

23. Pace yourself Auto convert Explanation mark bonanza Kotlin Friendly If you’ve written your Java code so that it can be easily interpreted, the converted Kotlin code is better. final
 @NotNull

24. Pace yourself Auto convert Explanation mark bonanza Kotlin Friendly If you’ve written your Java code so that it can be easily interpreted, the converted Kotlin code is better. final
 @NotNull
 @Nullable
25. Pace yourself Auto convert Explanation mark bonanza Kotlin Friendly Refactor the converted code
26. Pace yourself Auto convert Explanation mark bonanza Kotlin Friendly Refactor the converted code
27. Pace yourself Auto convert Explanation mark bonanza Kotlin Friendly Refactor the converted code
28. Where to start?
29. Path of least resistance Hit it were it don’t hurt Proof of Concept
 Prove that your project works with Kotlin and that you and the team understand how to mix it.
30. Path of least resistance Hit it were it don’t hurt Isolated Tests
31. Path of least resistance Hit it were it don’t hurt Isolated Small POJOs
32. Path of least resistance Hit it were it don’t hurt Isolated Small Independent Utilities & Helpers
33. Preparations
34. Decide upfront How should you handle…
35. Decide upfront How should you handle… static java code
 
 
 
 
 
 
 

36. Decide upfront How should you handle… static java code
 null
 
 
 
 
 
 

37. Decide upfront How should you handle… static java code
 null
 extensions
 
 
 
 
 

38. Decide upfront How should you handle… static java code
 null
 extensions
 visibility modifiers
 
 
 
 

39. Decide upfront How should you handle… static java code
 null
 extensions
 visibility modifiers
 POJOs
 
 
 

40. Decide upfront How should you handle… static java code
 null
 extensions
 visibility modifiers
 POJOs
 singeltons
 
 

41. Decide upfront How should you handle… Differences Read up on the big differences and decide how you will tackle those challenges.
42. When you need more
43. Bits & Bytes Byte code In IntelliJ there’s a tool to see the byte code generated from your code. 

44. Bits & Bytes Byte code
 Decompile In IntelliJ there’s a tool to see the byte code generated from your code. And if you decompile that you’ll see the equivalent java code.
45. Bits & Bytes Byte code
 Decompile Try In IntelliJ there’s a tool to see the byte code generated from your code. And if you decompile that you’ll see the equivalent java code. Try with a simple data class.
46. Let’s see some code
47. Code typealias
48. Code typealias function parameters
49. Code typealias function parameters
50. Code typealias function parameters static-ish
51. Code typealias function parameters static-ish
52. Code typealias function parameters static-ish
53. Code typealias function parameters static-ish
54. Code typealias function parameters static-ish singelton
55. Code typealias function parameters static-ish singelton extensions & default values
56. Code typealias function parameters static-ish singelton extensions & default values
57. Code typealias function parameters static-ish singelton extensions & default values data classes
58. Code typealias function parameters static-ish singelton extensions & default values data classes null
59. Code typealias function parameters static-ish singelton extensions & default values data classes null
60. Code typealias function parameters static-ish singelton extensions & default values data classes null deconstructing
61. Code typealias function parameters static-ish singelton extensions & default values data classes null deconstructing
62. Code typealias function parameters static-ish singelton extensions & default values data classes null deconstructing
63. Code typealias function parameters static-ish singelton extensions & default values data classes null deconstructing more conventions
64. To sum up
65. Take aways Differences Learn the differences - static
 - visibility
 - conventions
 - null
 - etc
66. Take aways Differences Pragmatic Set a solid goal!
 But be pragmatic about how to reach it.
67. Take aways Differences Pragmatic Iterate As in all development, but maybe more important in this case. Convert
 Refactor, Refactor, Refactor
 Repeat
68. Take aways Differences Pragmatic Iterate
 Cheat Decompile the kotlin byte code to see what to expect from java. There’s no need to guess.
69. Take aways Differences Pragmatic Iterate
 Cheat Help Kotlin Use final, @Nullable and @NotNull extensively in java. And set lint warnings to errors instead to avoid as many problems with null.
70. Take aways Differences Pragmatic Iterate
 Cheat Help Kotlin Organic Let the Kotlin code base grow and take over organically so that your team have time to adopt and learn. Isolated, small, independent.
71. Take aways Differences Pragmatic Iterate
 Cheat Help Kotlin Organic Conventions Use the strengths in Kotlin that theres a lot of conventions that is adopted for Java.
72. Take aways Differences Pragmatic Iterate
 Cheat Help Kotlin Organic Conventions Null And where kotlin has features to be better than Java. Be sure to use them smart.
73. Thank you!
74. Bob Dahlberg bob@hedy.tech @dahlberg.bob - medium.com bobdahlberg - speakersdeck.com When writing a lot of Kotlin code in a mixed project you’ll start to get a false sense of security around null.