Kotlin Tutorial for Java Developers

Ryan Thelin - Apr 12 '21 - - Dev Community

Kotlin is a rising programming language that aims to address the flaws of Java and be more seamlessly tied with the adaptable needs of the modern world. While once a niche language, Kotlin is quickly growing and has many companies switching from Java to Kotlin.

In fact, Google announced in 2017 that it would officially support Kotlin, rather than Java, as the main language for Android development.

Now, more and more companies are looking for Kotlin developers. Today, we'll help you transition to this new language by highlighting the differences and syntax of each fundamental Kotlin concept.

Here’s what we’ll cover today:

Transition to Kotlin in half the time

Get right to what you need without wasting time on programming fundamentals you already know.

Kotlin Crash Course for Programmers

Kotlin vs Java

Kotlin is an open-source, statically typed programming language developed by Jetbrains, which is famous for creating the IntelliJ IDEA IDE. The goal of Kotlin is to be an easier-to-use, more concise language for any type of project.

Kotlin is mainly used for Android app development or front-end development with frameworks like React.

Kotlin is unique because it is completely interoperable with Java code, meaning you can use existing Java libraries or frameworks. Kotlin code can transpile to JavaScript or Java bytecode for use on the Java Virtual Machine (JVM) or Android development.

Modern companies are particularly interested in Kotlin’s versatility because developers can use the same language for any project the company needs.

As agile development becomes more prevalent, companies like Google, Amazon, Netflix, Uber have started hiring Kotlin developers. In fact, it was ranked 4th fastest growing language in 2020 in Github's yearly report, with and many more reporting implementations of Kotlin developers.

Static Typing:

Like Java, variables can only be assigned values that match their type. This allows you to easily catch errors found by the compiler before execution.


var m : Int = 12

m = 10          // ok

m = "twelve" // error!

m = 10.0        // error!
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Non-local jumps:

Kotlin allows you to exit a function anywhere.

fun main() {

    intArrayOf(4, 5, 6).forEach lambda@ {
        if (it == 5) return@lambda
        println(it)
    }

    println()

    loop@ for (i in 0 .. 3) {

        for (j in 0 .. 3) {

            if (i + j == 4) continue@loop

            if (i + j == 5) break@loop

            println(i + j)
        }
    }
}
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Collection filtering:

Allows you to search collections for any data that matches the passed criteria.


val languageArray = arrayOf("Serbian", "Swahili", "Japanese", "German", "Spanish")

val selectedLang = languageArray

.filter { name -> name.startsWith("s", ignoreCase = true) }

//or: .filter { it.startsWith("s", ignoreCase = true) }

.sortedBy { name -> name.length }

.first()
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Extension functions:

Extension functions allow you to extend existing components without writing methods inside them, leading to cleaner code.


class Person {
  var firstName = ...
  var lastName = ...
}

// Maybe the author of the Person class forgot to include this

fun Person.setName(firstName: String, lastName: String) {
  this.firstName = firstName
  this.lastName = lastName
}
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Higher-order functions:

Kotlin functions are treated as first-class, meaning they can be passed or returned from other functions.


people.filter { person -> person.headCount == 1 }
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Lazy loading:

Decreases loading time by only loading resources it'll immediately need.

Key Differences between Kotlin and Java

Kotlin

  • Allows cross-platform transpiling from Kotlin to Android, JVM, and more.

  • Built from the ground up for both functional programming and object-oriented programming.

  • Designed for conciseness.

  • Built-in smart casting and implicit casting.

  • Cannot assign null values to ensure null safety

  • Out-of-the-box support for modern features like coroutines, extension functions, and data classes.

Java:

  • Not cross-platform, cannot transpile.

  • Built for object oriented programming with extremely limited functional support.

  • Designed for specificity.

  • Strict explicit typing rules.

  • Allows nulls but throws nullPointerException, which must be caught with exception handling.

  • Limited support for modern features through complex workarounds.

Keep the learning going.

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Kotlin Crash Course for Programmers

Hello World with Kotlin

Let's take a look at a Hello World program in both Java and Kotlin to see the differences.

You can write Kotlin in any Java IDE using the Kotlin plugin or even directly on the command line.

//java
class HelloWorld {

    public static void main(String[] args) {
        System.out.println("Hello, World!"); 
    }
}
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//kotlin
fun main() {

    println("Hello, World!")

}
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Clearly, Kotlin is more concise, only using two lines of code. This is because Kotlin allows functions to exist outside of classes, meaning we don't have to declare a HelloWorld class.

  • On line 1 of the Kotlin code, we define the main method. Like Java, every program needs a main function. Kotlin version 1.3+ removed inputs and outputs from main to make a simpler and less visually cluttered syntax.

  • On line 2, we print the string Hello, World! using the built-in println() function. This works the same way as Java's print function but does not need to be pulled from an external System class.

  • At the end of line 2, notice there is no semicolon to mark the line's end. Kotlin detects the end of statements by line and does not require semicolons.

Kotlin Variables and Data Types

You can define Kotlin variables using either the var or val keywords.


var <variableName> = <value> //mutable

val <variableName> = <value> //read-only

val <variableName>: <dataType> = <value> // explicit type casting
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Variables in Kotlin are very similar to variables in Java except:

  • Kotlin offers immutable (read-only) data types for functional programming

  • Kotlin allows for smart typing, sometimes called implicit typing, where it automatically detects the data type of a variable without explicit tagging.

  • Kotlin does not allow you to assign a null value to variables or return values by default. With Java, you can assign null, but it will throw a nullPointerException if you try to access that value.

Let's see how these differences look in practice.

Read-only vs Mutable Variables:

Variables in Kotlin can be either read-only or mutable. The value of read-only variables cannot be changed after they've been initially assigned. Read-only variables are especially helpful in reducing side effects when using functional programming.

Mutable variables are like most variables in Java and can be changed after they've been initialized.

Read-only initialization:


val number = 17

println("number = $number")

number = 42  // Not allowed, throws an exception
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Mutable initialization:


var number = 17

println("number = $number")

number = 42  // var can be reassigned

println("number = $number")
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It's best practice to use read-only val variables whenever possible and only use mutable var when you specifically need it. This minimizes the overall complexity of your programs and makes it easier to understand their data flow.

Kotlin Data Types

Unlike Java, Kotlin does not have primitive data types. All the following types are objects at runtime but do transpile to the respective Java primitive types in Java bytecode.

Integer Types

Kotlin includes all standard integer types. Below examples of explicitly cast, read-only integers that are initialized to their greatest possible values.


val byte: Byte = 127

val short: Short = 32767

val int: Int = 2147483647

val long: Long = 9223372036854775807
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Floating-Point Numbers

Kotlin supports standard decimal and exponent notations. Float and Double behave the same, but Double can hold more numbers than Float. Kotlin smart casts any floating-point number to Double, meaning you have to include f at the end of the argument to set a variable as a Float.


val float: Float = 3.4028235e38f

val double: Double = 1.7976931348623157e308
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Text Types

Kotlin supports Java-like Char and String types to represent text. You denote single characters with single quotes,'c', and Strings with double quotes "this string".


val character: Char = '#'

val text: String = "Learning about Kotlin's data types"
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Unlike Java, Kotlin allows you to easily make multi-line strings using three sets of double quotes, """<multipleLines>""". Java only offers concat and line.separator for multi-line strings, which are less concise and require special implementations.

Boolean Type

Kotlin's Boolean type is identical to Java's in function.


val yes: Boolean = true

val no: Boolean = false
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You can use the as keyword for type conversion of one type to another.

Type inference

Type inference is a compiler feature that allows you to omit types in your code when the compiler can infer it for you. This is also sometimes called smart typing or implicit typing.

If a variable could be declared as two types of differing sizes, it will choose the default: Double for floating-point numbers and Int for integers.

To implicitly declare non-default types, you can add the type key at the end of the value, like L for type Long below:


val string = "Educative"

val int = 27

val long = 42L

val double = 2.71828

val float = 1.23f

val bool = true
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Kotlin's type inference system is very robust and allows implicit typing of literals, objects, and complex structures, like lambda expressions.

Kotlin Conditionals and Loops

Kotlin includes two conditional statements, if and when, and two loops, for and while.

All conditionals are defined using:


<conditional> (<desiredCondition>) <code>

<conditional> (<desiredCondition>) { <codeBlock>

}
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If statement

Kotlin if statement is just like Java's in that it contains a code block which executes if the listed condition becomes true. You can also add an else statement that executes if the if statement is skipped.


var max = a

if (a < b) max = b

// With else

var max: Int

if (a > b) {

    max = a

} else {

    max = b

}

// As expression

val max = if (a > b) a else b
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When Statement

Kotlin also has a when statement that works similarly to C's switch statement. The when statement creates multiple branches of checks, which each evaluated for a particular condition.


when (x) {

    1 -> print("x == 1") //branch 1

    2 -> print("x == 2") //branch 2

    else -> { // Note the block

        print("x is neither 1 nor 2")

    }

}
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when can be used either as an expression or as a statement. If used as an expression, the value of the first matching branch becomes the value of the overall expression. If it is used as a statement, the values of individual branches are ignored.

You can think of when as multiple if-else statements rolled into a more concise format.

For Loop

The Kotlin for loop works like a C for each loop rather than Java's for loop. It accepts a collection, like an array, and completes the same action on each element of the collection.

The syntax for a 1 for loop is:


for (item in collection) print(item)
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for (i in 1..3) {

    println(i)

}
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While Loop

The while loop executes the body of code repeatedly so long as the listed conditions remain met. Like Java, there are two types of while loop. The standard while loop checks for the condition before the code is executed, and the do-while loop checks after the code is executed.


while (x > 0) {

    x--

}

do {

    val y = retrieveData()

} while (y != null) // y is visible here!
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Kotlin Collections

Kotlin includes 3 basic collections: lists, sets, and arrays. Like Java, they each act as frameworks to store data in different ways. Collections are assigned to variables like any other value.

You can create all collections in Kotlin using their unique constructor function. All constructors use the same naming convention, <collection>Of(<value1, value2, …>). For example, you can make a list of 2 integers with listOf(1, 2).

List

Lists are indexed linear data structure similar to Java arrays. The indices start at 0 and go to (list.size - 1). Unlike arrays, lists are dynamic and can resize to accommodate added or removed elements. Lists can contain any number of duplicate elements.


val numbers = listOf("one", "two", "three", "four")

println("Number of elements: ${numbers.size}")

println("Third element: ${numbers.get(2)}")

println("Fourth element: ${numbers[3]}")

println("Index of element \"two\" ${numbers.indexOf("two")}")
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Sets

A set is an unordered linear data structure that cannot contain duplicate elements. Sets do not have indices. Two sets are considered equal if both contain all the same unique elements.


val numbers = setOf(1, 2, 3, 4)

println("Number of elements: ${numbers.size}")

if (numbers.contains(1)) println("1 is in the set")

val numbersBackwards = setOf(4, 3, 2, 1)

println("The sets are equal: ${numbers == numbersBackwards}")
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The above sets are equal despite appearing in the opposite order in initialization.

Array

The array is not a native data type in Kotlin. It is represented by the Array class. These arrays behave the same as Java arrays: both contain mutable values but are fixed size.

You can create an array using the function arrayOf(<element1, element2, …>).

For example:


val priorities = arrayOf("HIGH", "MEDIUM", "LOW")

println(priorities[1])

priorities[1] = "NORMAL"

println(priorities[1])

println(priorities.size)
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Kotlin Functions

Functions in Kotlin require you to define the function's name, inputs, and outputs. All function definitions begin with the fun keyword and contain a block of code executed when the function is called. Unlike Java, you do not need to define the method as static, as Kotlin is less strictly tied to OOP.

Here's an example of Kotlin's function syntax:


fun <variableName>(<inputName>: <inputType>): <returnType>

fun fibonacci(index: Int): Long
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To mimic the behavior of Java's void return types, you can set the return type to Nothing.

One of Kotlin and Java's biggest differences is that functions can exist outside of classes in Kotlin. On the other hand, Java is rigidly bound to object-oriented programming and requires everything to be within class definitions.

Package-level functions (functions outside of any particular class) are used to hold tasks equally useful for any object in the program.
For example, a convert function that converts kg to lbs and lbs to kg would be a good package-level function because any class in the program might have to convert measurements.

Kotlin also allows for higher-order functions, meaning functions can be passed or returned like other values. This is invaluable for functional programming designs.

Extension Functions

Kotlin adapts the standard OOP formula with Extension Functions, which let you extend the functionality of existing classes without inheriting from them. Extension functions are useful when trying to use the functionality of third-party libraries you can't modify but can extend. Unlike inheritance, you can extend a class without full access to it.

For example, you could add additional behavior to an external API that would help closer integrate it with your solution.

To declare an extension function, we need to prefix its name with a receiver type, i.e. the type being extended. The following adds a swap function to MutableList –


fun MutableList < Int > .swap(index1: Int, index2: Int) {

val tmp = this[index1]

this[index1] = this[index2]

this[index2] = tmp

}
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The this keyword is used to reference values and methods from the extended type.

Functional Programming with Kotlin

The most significant difference between Kotlin and Java is that Kotlin is tailored for functional programming, not just OOP. Functional programming in Java is limited. While you can mimic many functional behaviors in Java, most developers agree that it's not applicable in the real world.

Kotlin is built from the ground up to be a fully-featured functional language, with higher-order functions, lambda expressions, operator overloading, lazy evaluation, operator overloading, and more.

// higher order functions

fun main( ) {

val numbers = arrayListOf(15, -5, 11, -39)

val nonNegativeNumbers = numbers.filter{
  it >= 0
 }

println(nonNegativeNumbers) //function as input parameter

}
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// lambda expression

val sum: (Int, Int) -> Int = { 
 x, y -> x + y
}
fun main( ) {
    println(sum(5,10))
}
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What to learn next with Kotlin

Congratulations on taking your first steps with Kotlin! This rising language is perfect for modern developers and is easy to pick up for existing Java developers.

You're now ready to move on to intermediate topics such as:

  • Nullable types

  • Conditions as expressions

  • Map collections

  • Shorthand function notation

  • Operator functions

To help you transition to Kotlin fast, Educative has made Kotlin Crash Course for Programmers. This course is tailored towards existing Java developers, meaning all lessons are hands-on and highlight what's different about the language. By the end, you'll know how to use all the basic features of Kotlin and will be ready to build your first Android application with Android Studio or another Kotlin program

Happy learning!

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