JDD 2017: Kotlin for Java developers (Tomasz Kleszczyński)

Kotlin for Java Developers
what every java developer should know about kotlin

Today I would like to share with you
● Why I care about Kotlin?
● 20 features I love
● 10 features you need to be aware of
● Ecosystem

Next JVM language?
● Statically typed programming language for multi platform applications
○ concise
○ safe
○ interoperable with Java
○ built together with tooling support
○ open source under Apache 2.0 license
● Version 1.0 released in 2015, current version 1.1.50 (September 2017)
● Google announced first-class support for Kotlin on Android
● Pivotal introduced Kotlin support in Spring Framework 5.0
● Easy to learn if you know Java

Extension functions and properties

Data classes
● equals() / hashCode() pair
● toString() “Account[email=test]”
● componentN() functions in their order
of declaration
● copy() function

Null safety
● Types defines nullability
○ Platform types
● Safe calls
● Elvis operator
● !!. operator
● Safe casts

Operator overloading
● +, -, *, /, %, ..
○ a + b -> a.plus(b)
○ a..b -> a.rangeTo(b)
● in, !in
○ a.contains(b)
● Indexed access []
○ a[i] -> a.get(i)
○ a[i] = b -> a.set(i, b)
● Invoke
○ a(i, j) -> a.invoke(i, j)
● a == b
○ a?.equals(b) ?: (b === null)
● a > b, a < b, a >= b, a <= b -> a.compareTo(b)

Destructuring objects
● underscore for unused
variables (1.1)
● destructuring in lambdas
(1.1)

Expressions and statements
● if and when are expressions, not statements
○ val length = if (a is String) a.length else -1
○ val action = when (test) {
in 0..5 -> OPEN
else -> CLOSE
}
● assignment is a statement, not an expression
○ if (a = b) does not compile
○ while ((line = bufferedReader.readLine()) != null) does not compile

Packages and source code structure
● packages
● import allows to import classes, functions, *
● type aliases
● multiple classes in one file
● arbitrary file names
● arbitrary directory structure
● visibility modifiers: private, protected, internal, public

Other languages have all these features
● Null safety
● No checked exceptions
● Extension functions
● Function types and lambdas
● Default and named parameters
● Properties
● Operator overloading
● Smart casts
● Data classes
● Immutable collections
● Enhanced switch-case
● String templates
● Ranges
● Infix notation
● Inline functions
● Coroutines (async/await)
● Great standard library
● Sealed classes
● Delegated and lazy properties
● Class delegation
● Singletons
● Nested functions
● Object decomposition
● Top-level functions
● Reified generics
● Raw strings
● 100% interoperable with Java 6
● And more...

Compile and run with Java code
● you can mix Java and Kotlin code in one project
● experiment with new language without breaking or rewriting the whole
application
● small memory footprint of the Kotlin standard library

What you need to know
● final by default and bytecode generation
● platform types and nullability
● no primitives, no implicit widening conversions for numbers
● standard library

Final by default and bytecode generation
● all classes, methods are final by default
○ tedious opening via ‘open’ keyword
○ interference with AOP (CGLIB), workarounds as compiler plugins ‘kotlin-spring’, ‘all-open’
● ‘override’ is a required keyword, not an annotation
● designing for inheritance
● kotlinc generates Java 6 or Java 8 bytecode
● on JVM lambdas does not use ‘invokedynamic’
● Kotlin Bytecode Generation and Runtime Performance

Platform types and nullability (!)
● any reference in Java may be null
● types of Java declarations are called platform types
● can be assigned to nullable or non-null type
● compiler, tools refers to them using as T! which means T ot T?
● nullability annotations (JSR-305, Android, Lombok, JetBrains, Eclipse)

Standard library
● kotlin-runtime and kotlin-stdlib -> kotlin-stdlib
○ < 1MB jar (JVM)
● Kotlin classes and extension functions to Java classes
○ kotlin
○ kotlin.collections
○ kotlin.comparisons
○ kotlin.concurrent
○ kotlin.io
○ kotlin.streams
○ kotlin.text
● kotlin.js

Platforms and tooling
● JVM
a. Java 6 and 8
● Android
● JavaScript (ES5.1)
a. compatible with module systems like AMD, CommonJS
● native (LLVM)
a. LLVM is used to compile Kotlin into native code
b. technology preview for iOS, linux, MAC, (windows in the work)
● IntelliJ IDEA (Java to Kotlin converter), Eclipse
● Gradle, Maven, Ant

Where to start?
● Try online https://try.kotlinlang.org/
● Kotlin is Awesome! https://kotlin.link/

What will be your next JVM
language?
https://en.wikipedia.org/wiki/List_of_JVM_languages

JDD 2017: Kotlin for Java developers (Tomasz Kleszczyński)

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