/learn-you-a-kotlin

Exercises for the tutorial "Learn You a Kotlin For All The Good It Will Do You"

Primary LanguageJava

Exercise for the tutorial "Refactoring to Kotlin"

Aim of the session is to introduce Kotlin by converting Java code.

First create a new branch. Check in after each change. This lets you easily show how auto-converting code to Kotlin affects how its API looks when used from Java

Suggested progress

  • Part 0: Compiling Kotlin

    • Open project in IntelliJ, build on command line, run tests in IntelliJ
    • Tools/Kotlin/Configure Kotlin in Project
    • Look at the changes to build.gradle
    • Build and test
    • Tools/Kotlin/Configure Kotlin Plugin Updates - use New Java To Kotlin Converter

  • Part 1: Class syntax and data classes

    • Presenter
      • note that this an immutable value class with a public final field
      • convert to Kotlin, accept change other files.
      • try to run tests - see compilation failure, talk about converter
      • talk about properties v fields, fix JsonFormat.java (Alt-Enter)
      • talk through the bits of the Kotlin class - ctor, property, methods
      • run tests
      • remove equals, hashcode, to string - show tests fail
      • make a data class - show tests now pass
      • remove the unneeded class body
      • check in
    • Session
      • note that this is an immutable value class with public fields, one of which is nullable, and it defensively copies the presenters
      • also that we have 2 constructors - one a convenience vararg
      • convert to Kotlin
      • Note subtitle is a String? - talk about nullability
      • Note primary v secondary constructor, observe primary ctor invocation
      • Note we can have a free property - presenters, initialised in class body
      • Talk about init block, but then remove it
      • show tests pass, remove equals etc - show tests fail
      • convert to a data class
      • can't have a val outside ctor, remove it
      • No need to wrap List in unmodifiableList: discuss List/MutableList split, show List defn
      • Observe spread operator in constructor, remove it and replace with presenters.toList() - discuss asList()
      • Remove empty ctor body
      • Convert withXxx methods to single expression - note lack of new
      • Convert withXxx methods to invoke .copy (do via add argument names and talk about argument names)
      • run the tests, check diffs, talk about diffs, checkin
    • SessionTests
      • convert to Kotlin
      • run tests
      • talk about "internal"
      • talk about "var" vs "val"
      • talk about lack of a "new" keyword -- classes look like, and can be used as, functions
      • talk about listOf vs Arrays.asList -- Kotlin stdlib has lots of useful collection methods
      • talk about lack of return type when Unit
      • Now all of our Session clients are Kotlin we can inline the 'copy's, except for withPresenters, which we can make vararg
      • Now the copy invocations don't need testing
      • Move withPresenters methods out of class into extension ... much nicer in Kotlin, yeah?
      • Explain extension functions in more detail ... syntactic sugar for static methods
      • Move withPresenters into SessionTests where it is used to illustrate convenience extensions
      • Talk about top-level functions
      • Rename test to illustrate convenience extension methods and talk about names
      • run the tests, check diffs, talk about diffs, checkin
    • Slots
      • Convert Slots. It's all Kotlin!!! That was easy!
      • run the tests, check diffs, talk about diffs, checkin

  • Part 2: Null and nullability

    • Look at Sessions - a bunch of static convenience methods to manage a collection
    • Look at SessionsTests - already Kotlin
    • Talk about companion object, static etc
    • nulls test
      • show TypeCastException when we change the title
      • change cast to !! and show KotlinNullPointerException when we change the title
      • Show infers second reference cannot be null because of flow typing
      • Show the type of notNullSession given as or !!
    • Convert Sessions to Kotlin
    • Run tests
    • subtitleOf
      • Compare with Java - talk about ?.
    • subtitleOrPrompt
      • Compare with Java - talk about ?:
    • Move Session static methods to top level scope - talk about static scope
    • Make into extension functions
    • Note use of extension functions on nullable types
    • Remove boilerplate
    • Convert subtitleOrPrompt to property (Alt-Enter)
    • Talk about properties v functions
    • Convert findWithTitle to Kotlin (remove .stream()) - note lambda syntax and destructuring
    • Remove the destructuring as unhelpful (even risky -- explain risks)
    • Talk about the difference between iterables and sequences
    • Use predicate form of firstOfNull
    • Run tests
    • Talk about API design by adding extension methods to existing types instead of defining new types
    • typealias List to Sessions
    • run the tests, check diffs, talk about diffs, checkin

  • Part 3: modules and functions

    • Look at JsonFormatTests
      • note that we want to marshall session to and from JSON
    • Look at JsonFormat
      • we're groping towards a Java DSL for JSON, using Json
    • Look at Json
      • Try annotating props param of obj method with @Nullable so comments about nullability are not necessary -- you cannot!
      • Note use of Map.Entry - used as a pair. But Kotlin has a pair.
      • Import Map.Entry, replace Entry< with Pair<, fix issues,
      • Run tests, checkin
      • Convert to Kotlin, applying changes to affected code
        • You'll get compiler errors - ignore them for now
        • Look at the changes. JsonFormat and JsonFormatTests are full of INSTANCE!
          • Explain Kotlin objects -- they are singletons!!! :scream-emoji:
          • Revert.
        • We could annotate all methods in Json with @JvmStatic. Or we could convert the dependent classes first. Let's do the latter.
    • JsonFormatTests
      • Convert to Kotlin AND RERUN THE TESTS
      • They fail, because JUnit needs approval to be a field. Annotate with @JvmField
      • Also mention the @Throws annotations, and then remove them
      • Run and checkin
    • JsonFormat
      • Convert to Kotlin - IJ doesn't do a very good job in the face of Java lambdas sometimes
      • Try the pedagogical object: java.util.function.Function<Presenter, JsonNode> fix
      • Fix compilation errors by removing explicit Function<...> SAM notation
      • Explain it variable in lambdas
      • Don't convert lambdas to references - do move them outside parameter list
      • That collect turns out to be collect<List<Presenter>, Any>(Collectors.toList<Presenter>() as Collector<in Presenter, Any, List<Presenter>>?
      • Run tests
      • Remove @Throws: it's not called from Java any more (we'll talk about type safe error handling later if we have time)
      • Convert streams code to Kotlin map/flatMap/etc. (Remember that JsonNode is iterable, so has map, etc. defined for it)
      • move functions to module scope
      • convert to extension methods on domain types and JsonNode
      • Test and checkin
    • Back to Json
      • Convert to Kotlin
      • To make it compile:
        • Use Kotlin's function type syntax instead of java.util.Function<T,U>
        • remove some explicit type params that are not needed
        • use nullable types to indicate that array and iterable elements can be null
      • move functions to module scope
      • remove streams
      • use infix to
      • observe object
      • replace props.forEach with filterNotNull().toMap()
      • in array use apply to initialise result ...
      • ... but then replace it with ArrayNode(nodes, elements.toList())
      • Convert functions to extension methods where applicable
      • We can get rid of Iterable.array(fn) now
      • Convert prop(name,value) to name of value (infix function)
        • Discuss gradual introduction of mini-DSLs, rather than up-front DSL design which often ends up inflexible
    • Back to JsonFormat
      • convert the JSON as text into multiline strings
      • make extension properties from nonBlank functions
      • use isNullOrBlank
      • use let in Session.toJson

Themes

  • pragmatic language
  • Java interop
  • tooling
  • much less classy than Java
  • extension functions for fun and profit

There is a lot we still haven't covered

  • delegation
  • sealed classes
  • when expressions
  • sequences
  • inline functions
  • reified types in functions
  • coroutines
  • error handling
  • ...