Pitfalls when mixing Kotlin & Java

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  Pitfalls when mixing Kotlin & Java Bob Dahlberg Hedy Tech Hedy

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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!

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Background Android Been an Android developer since Android Cupcake (2009). Only Java 6

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Background Android Scala Learned Scala and tried to build Android with it..

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Background Android Scala Learned Scala and tried to build Android with it.. It’s plausible.

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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.

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Background Android Scala Kotlin Found Kotlin M12 in 2015 and it blended almost seamlessly with Java.  

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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.  

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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.

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The goal should be to move completely to Kotlin.

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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. 

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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!

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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.

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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.

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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.

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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.

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Pace yourself

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Pace yourself Auto convert Don’t use it, please! 

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Pace yourself Auto convert Don’t use it, please!  Only one file at the time!

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Pace yourself Auto convert Explanation mark bonanza

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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.    

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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   

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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 

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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

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Pace yourself Auto convert Explanation mark bonanza Kotlin Friendly Refactor the converted code

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Pace yourself Auto convert Explanation mark bonanza Kotlin Friendly Refactor the converted code

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Pace yourself Auto convert Explanation mark bonanza Kotlin Friendly Refactor the converted code

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Where to start?

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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.

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Path of least resistance Hit it were it don’t hurt Isolated Tests

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Path of least resistance Hit it were it don’t hurt Isolated Small POJOs

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Path of least resistance Hit it were it don’t hurt Isolated Small Independent Utilities & Helpers

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Preparations

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Decide upfront How should you handle…

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Decide upfront How should you handle… static java code               

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Decide upfront How should you handle… static java code  null             

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

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

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

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

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Decide upfront How should you handle… Differences Read up on the big differences and decide how you will tackle those challenges.

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When you need more

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Bits & Bytes Byte code In IntelliJ there’s a tool to see the byte code generated from your code.  

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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.

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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.

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Let’s see some code

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Code typealias

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Code typealias function parameters

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Code typealias function parameters

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Code typealias function parameters static-ish

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Code typealias function parameters static-ish

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Code typealias function parameters static-ish

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Code typealias function parameters static-ish

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Code typealias function parameters static-ish singelton

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Code typealias function parameters static-ish singelton extensions & default values

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Code typealias function parameters static-ish singelton extensions & default values

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Code typealias function parameters static-ish singelton extensions & default values data classes

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Code typealias function parameters static-ish singelton extensions & default values data classes null

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Code typealias function parameters static-ish singelton extensions & default values data classes null

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Code typealias function parameters static-ish singelton extensions & default values data classes null deconstructing

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Code typealias function parameters static-ish singelton extensions & default values data classes null deconstructing

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Code typealias function parameters static-ish singelton extensions & default values data classes null deconstructing

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Code typealias function parameters static-ish singelton extensions & default values data classes null deconstructing more conventions

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To sum up

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Take aways Differences Learn the differences - static  - visibility  - conventions  - null  - etc

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Take aways Differences Pragmatic Set a solid goal!  But be pragmatic about how to reach it.

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Take aways Differences Pragmatic Iterate As in all development, but maybe more important in this case. Convert  Refactor, Refactor, Refactor  Repeat

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Take aways Differences Pragmatic Iterate  Cheat Decompile the kotlin byte code to see what to expect from java. There’s no need to guess.

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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.

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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.

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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.

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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.

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Thank you!

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Bob Dahlberg [email protected] @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.