My journey with Kotlin

Transcript

1. (My journey with)
   Kotlin
   Alexis Seigneurin
   

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2. My background
   • VB → Perl → C → C++ → Java → C# → Java → Scala
   • Statically typed languages (except Perl)
   • Almost exclusively using Scala for 3 years
   

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3. Why I looked at Kotlin
   • Now contributing to a Java project
   • … and it hurts!
   → I want to write useful code (not generate code)
   

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4. What hurts (me) in Java
   

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5. POJOs
   • Getters/setters
   • Can intercept assignation/retrieval
   • No one does that in practice → Boilerplate code
   • Convention for naming getters/setters
   • Not enforced / may not be respected
   • Getting/setting values = method calls :-(
   • Not using ‘=‘
   • Step-by-step debugging is unnecessarily complex
   • Have to write equals() / hashCode()
   • Not immutable
   

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6. POJOs on my project
   $ wc -l src/main/java/com/…/**/*.java | tail -n 1
   2059 total
   $ wc -l src/main/java/com/…/dto/*.java | tail -n 1
   485 total
   $ wc -l src/main/java/com/…/model/*.java | tail -n 1
   105 total
   • → 590 / 2059 lines = 29% boilerplate code
   • Code reviews…
   

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7. No type inference (or so few)
   NewTopic newTopic = new NewTopic(topicName,
   topicParameters.getNumPartitions(),
   topicParameters.getReplicationFactor());
   Duplicated type
   Method calls… just to get values
   Parsers no longer need
   semicolons (this is 2017)
   

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8. Streams API is verbose
   List userAclBindings = aclBindings.stream()
   .filter(aclBinding -> aclBinding.entry().principal().equals(filterUser))
   .collect(Collectors.toList());
   Boilerplate
   Boilerplate
   

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9. No operator overloading
   List userAclBindings = aclBindings.stream()
   .filter(aclBinding -> aclBinding.entry().principal().equals(filterUser))
   .collect(Collectors.toList());
   how about?
   == for object equality
   === for reference equality
   

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10. No string interpolation
    MessageFormat.format("{0}://{1}{2}:{3}", securityProtocol, instanceName, domain, port).toString()
    new RuntimeException("Tag [" + tagKey + "] not found")
    (No multiline strings either)
    

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11. One class per file
    • ClusterDto.java
    public class ClusterDto {
    private String clusterId;
    private NodeDto controller;
    private List nodes;
    …
    • NodeDto.java
    public class NodeDto {
    private int id;
    private String host;
    private int port;
    private String rack;
    …
    

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12. Define a “function”
    @FunctionalInterface
    public interface KafkaAdminFunction {
    T apply(AdminClient client) throws InterruptedException, ExecutionException;
    }
    Function = interface with one method!
    

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13. What are our options?
    

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14. Java 9?
    • Jigsaw, new GC, HTTP 2…
    • Syntax improvements (lol)
    • try-with-resource with a pre-defined variable
    • diamond operator on anonymous inner classes
    • interface with private methods
    

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15. Java 10?
    • Value types
    • Reified generics
    • …
    

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16. Scala
    • Extremely powerful
    • Can be overwhelming
    • Not acceptable in my team
    • Scala - Java interop isn’t the best
    • Requires a stop-the-world project migration
    

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17. Non-JVM language?
    • Language change + ecosystem change
    → Too disruptive
    

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18. Other JVM language?
    • Clojure → Too functional
    • JRuby, JPython… → Clunky
    • Groovy → Too dynamic
    • …
    • Kotlin → YES!
    

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19. Kotlin
    

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20. Kotlin
    • JVM language - Initiated by JetBrains
    • Intent:
    • make the code concise + readable
    • make the code safer than pure Java
    • Made for 100% interoperability with Java
    • Migrate one class at a time
    • Statically typed
    • Compiles faster than Scala (troll)
    

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21. Maturity?
    • Tooling support
    • Build tools: Maven, Gradle
    • IDE: mature support in IntelliJ IDEA (who cares about Eclipse)
    • Official programming language on Android
    • Extended support for Kotlin in Spring 5
    • (To write more idiomatic code)
    

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22. Interoperability
    • Migrate one class at a time
    • IntelliJ can convert a class / a package at once
    • Perfect interoperability
    • Call Java from Kotlin (seamless)
    • Call Kotlin from Java (seamless)
    

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23. Java code
    Kotlin code
    

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24. Basic syntax
    • Defining a class + methods
    class KafkaDescribeClient(val adminClientFactory: KafkaAdminClientFactory) {
    fun describeCluster(region: String, cluster: String): ClusterDto {
    …
    }
    }
    Function
    name: Type
    Constructor with 1 parameter
    

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25. Basic syntax
    • Defining an immutable variable
    val region = "us-east-1"
    val cluster: String = "engine"
    val adminClient = KafkaDescribeClient(adminClientFactory)
    • Defining a variable (you don’t really need that)
    var something = 12
    something = 42
    No `new` keyword
    

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26. Basic syntax
    • No `static` methods
    • Define a function in a file (in a package, no class)
    fun nodeToDto(node: Node): NodeDto {
    return NodeDto(node.id(), node.host(), node.port(), node.rack())
    }
    • Import the functions of the class
    import com.….dto.*
    • Call the function
    nodeToDto(controller)
    

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27. Basic syntax
    • Reference equality
    ===
    !==
    • Structural equality
    == → equals()
    Note that the `==` operator in Kotlin code is translated into a call to [equals]
    when objects on both sides of the operator are not null.
    • … meaning it’s safe, and `o == null` still works
    

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28. Strings
    • String comparison: use ‘==’
    if(port == "9092") {
    • String interpolation
    private fun buildUri(securityProtocol: String, instanceName: String, port: String):
    String {
    return "${securityProtocol}://${instanceName}${domain}:${port}"
    }
    • Multi-line strings
    System.err.println("""
    ========== Data checking ==========
    Purpose: checks data quality of messages in a topic.
    - Kafka brokers = ${config.kafkaBrokers}
    - Kafka group ID = ${config.applicationId}
    ...
    """)
    

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29. Data classes
    • Data class = (immutable) POJO
    data class AclParameters(
    val operation: AclOperation,
    val user: String,
    val hostname: String?,
    val groupId: String?)
    • Create an instance (no `new`)
    val aclParams = AclParameters(AclOperation.READ, "u1", "h1", “g1")
    • equals() / hashCode() / toString() → generated
    • From Java = POJO with a constructor + getters
    • For Jackson: there’s a module
    

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30. Data classes
    • Immutable → copy if you need to update
    val aclParamsCopy = aclParams.copy(groupId = groupName)
    • Destructuring
    val (operation, user, hostname, groupId) = aclParams
    • Partially destructuring
    val (operation, _, hostname, _) = aclParams
    

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31. Expressions
    • Evaluating an expression and assigning a variable
    val res = if (operation == CREATE) {
    createAcl(aclParameters)
    } else if (operation == ALTER) {
    alterAcl(aclParameters)
    } else if (operation == DELETE) {
    deleteAcl(aclParameters)
    } else {
    throw RuntimeException("Operation not supported: ${operation}")
    }
    

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32. Streams
    • Remember this piece of Java code?
    List userAclBindings = aclBindings.stream()
    .filter(aclBinding -> aclBinding.entry().principal().equals(filterUser))
    .collect(Collectors.toList());
    • The same in Kotlin
    val userAclBindings = aclBindings.filter { it.entry().principal() == filterUser }
    Type inference No stream(), no Collector Implicit argument Replaces equals()
    

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33. Streams
    • Java
    private List buildKafkaUris(List instances, String securityProtocol, String port) {
    return instances.stream()
    .map(i -> getTagOrFail(i, TAG_KEY_NAME))
    .map(name -> buildUri(securityProtocol, name, port))
    .collect(Collectors.toList());
    }
    • Kotlin
    private fun buildKafkaUris(instances: List, securityProtocol: String, port: String): List {
    return instances
    .map { getTag(it, TAG_KEY_NAME) }
    .map { buildUri(securityProtocol, it, port) }
    }
    Null safety through types
    (see further)
    

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34. Null safety
    • Kotlin tries to eliminate NullPointerExceptions
    • Suffix the type with ‘?’ to declare a nullable var/param
    var s: String? = null
    • Need to test for nulls before being able to use the value
    return s.length
    Error:(37, 17) Kotlin: Only safe (?.) or non-null asserted (!!.) calls are
    allowed on a nullable receiver of type String?
    if (s !=null) {
    return s.length
    }
    

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35. Null safety
    • Can’t return a potentially null value if not declared as nullable
    private fun getTag(instance: Instance, tagKey: String): String {
    val tag = instance.tags
    .filter { it.key.equals(tagKey, ignoreCase = true) }
    .map { it.value }
    .firstOrNull()
    return tag
    }
    Error:(40, 16) Kotlin: Type mismatch: inferred type is String? but String was expected
    • Safe:
    private fun getTag(instance: Instance, tagKey: String): String {
    val tag = instance.tags
    .filter { it.key.equals(tagKey, ignoreCase = true) }
    .map { it.value }
    .firstOrNull()
    if (tag == null) {
    throw RuntimeException("Tag [${tagKey}] not found")
    }
    return tag
    }
    • (Or just call first() in this case)
    tag is of type String?
    tag is of type String
    

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36. Null safety
    • Elvis operator
    val hostname = aclParameters.hostname ?: “*”
    • Safe calls
    val length: Int? = s?.length
    Return value is a nullable type
    

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37. Functions
    • First class support for functions
    protected fun withAdminClient(func: (AdminClient) -> R): R {
    val adminClient = adminClientFactory.createAdminClient()
    return adminClient.use { func(adminClient) }
    }
    A function with one param
    of type AdminClient
    Call it… as a function
    (Bye bye functional interfaces)
    

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38. Type alias
    • Can alias any type
    typealias Clusters = List
    • Example
    typealias KakfaAdminFunc = (AdminClient) -> R
    protected fun withAdminClient(func: KakfaAdminFunc): R {
    …
    

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39. Extension functions
    • Extend existing classes with new methods
    • Resolved statically / locally
    • Example
    fun AmazonEC2.findInstances(request: DescribeInstancesRequest): ArrayList {
    val allInstances = ArrayList()
    var token: String?
    do {
    val result = this.describeInstances(request)
    val instances = result.reservations.flatMap { it.instances }
    allInstances.addAll(instances)
    token = result.nextToken
    request.nextToken = token
    } while (token != null)
    return allInstances
    }
    ec2.findInstances(request)
    Class to extend
    Method
    

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40. Wrapping up
    

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41. Conclusion
    • Easy to use & intuitive
    • Greater expressivity than Java
    • Simpler than Scala
    • Very positive experience so far
    

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42. Try it out!
    @aseigneurin
    

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