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- Herausgeber: Packt Publishing
- Kategorie: Fachliteratur
- Sprache: Englisch
Quickly learn the fundamentals of the Kotlin language and see it in action on the web. Easy to follow and covering the full set of programming features, this book will get you fluent in Kotlin for Android.
Das E-Book können Sie in Legimi-Apps oder einer beliebigen App lesen, die das folgende Format unterstützen:
Seitenzahl: 542
Veröffentlichungsjahr: 2017
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Table of Contents
Programming Kotlin
Programming Kotlin
Copyright © 2017 Packt Publishing
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Every effort has been made in the preparation of this book to ensure the accuracy of the information presented. However, the information contained in this book is sold without warranty, either express or implied. Neither the authors, nor Packt Publishing, and its dealers and distributors will be held liable for any damages caused or alleged to be caused directly or indirectly by this book.
Packt Publishing has endeavored to provide trademark information about all of the companies and products mentioned in this book by the appropriate use of capitals. However, Packt Publishing cannot guarantee the accuracy of this information.
First published: January 2017
Production reference: 1130117
Published by Packt Publishing Ltd.
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ISBN 978-1-78712-636-7
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Credits
Authors
Stephen Samuel
Stefan Bocutiu
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Reviewers
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About the Authors
Stephen Samuel is an accomplished developer with over 17 years of experience. He has worked with Java throughout his career, and in the past 5 years has focused on Scala. He has a passion for concurrency and Big Data technologies. Having spent the last few years in investment banking, he is currently working with Kotlin on a major Big Data ingestment project. Stephen is also active in the open source community, being the author of several high profile Scala and Kotlin libraries.
I would like to thank my wife for being patient with all the days and nights spent on the computer, developing. I would also like to thank Stefan for kindly agreeing to co-author this book with me.
Stefan Bocutiu is a Big Data consultant with over 13 years of experience in software development. He enjoys coding in Scala, C#, and has a passion for stream processing technologies. With the team at DataMountaineer--a consultancy company offering delivery of solutions for streaming/ fast data platforms - he focuses on providing scalable, unified, real-time data pipelines allowing reactive decision making, analytics, and Hadoop integration.
Stefan is passionate about motorsports, and while his racing skills are not good enough to allow him to compete, he tries to attend as many MotoGP races as possible. When he is not coding, he can be found at the climbing wall or at the gym. Occasionally, hiking and scrambling trips are on his calendar, and during the winter season, skiing trips are a must for him.
About the Reviewers
Antonios Chalkiopoulos is a distributed systems specialist, engineering Big Data systems in the past 5 years on Media, Betting, Retail, Investment Banks, and FinTech companies in London. He is the author of Programming MapReduce with Scalding, one of the first books presenting how Scala can be used for Big Data solutions, and an open source contributor to a number of projects.
He is the founder of LANDOOP, a company that specializes in Fast Data and Big Data solutions and provides numerous tools and capabilities around Apache Kafka and real-time streaming systems.
Alexander Hanschke is a co-founder and CTO at techdev Solutions GmbH, a software company based in Berlin. He graduated from University of Mannheim and has worked in the financial sector, building Java enterprise applications for 8 years.
At his company, these days Alex is working on web applications written in Java and Kotlin. He frequently talks at user groups and conferences and is writing about Kotlin-related topics.
You can find him on Twitter at @alexhanschke.
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Preface
Kotlin is typically associated with Android development, and most discussion about it revolves gravitates around that. But the language has much more to offer and is ideal for modern server side developers. While any Android developer will find useful snippets in this book, the book is targeting Java and Scala developers primarily. The book will start with a introduction to Kotlin and explain how you set up your environment before moving on to the basic concepts. Once the basics are out of the way, the focus will shift towards more advanced concepts, and don't be surprised if you see a few bytecode listings. Once you have completed the book you should have all the knowledge required to start using Kotlin for your next project.
What this book covers
Chapter 1, Getting Started with Kotlin, covers how to install Kotlin, the Jetbrains Intellij IDEA, and the Gradle build system. Once the setup of the tool chain is complete, the chapter shows how to write your first Kotlin program.
Chapter 2, Kotlin Basics, dives head first into the basics of Kotlin, including the basic types, basic syntax, and program control flow structures such as if statements, for loops, and while loops. The chapter concludes with Kotlin-specific additions such as when expressions and type inference.
Chapter 3, Object-Oriented Code in Kotlin, focuses on the object-orientated aspects of the language. It introduces classes, interfaces, objects and the relationship between them, subtypes, and polymorphism.
Chapter 4, Functions in Kotlin, shows that functions, also known as procedures or methods, are the basic building blocks of any language. This chapter covers the syntax for functions, including the Kotlin enhancements such as named parameters, default parameters, and function literals.
Chapter 5, Higher Order Functions and Functional Programming, focuses on the functional programming side of Kotlin, including closures--also known as lambdas--and function references. It further covers functional programming techniques such as partial application, function composition, and error accumulation.
Chapter 6, Properties, explains that properties work hand in hand with object-orientated programming to expose values on a class or object. This chapter covers how properties work, how the user can best make use of them, and also how they are represented in the bytecode.
Chapter 7, Null Safety, Reflection, and Annotations, explains that null safety is one of the main features that Kotlin provides, and the first part of this chapter covers in depth the whys and hows of null safety in Kotlin. The second part of the chapter introduces reflection--run time introspection of code--and how it can be used for meta programming with annotations.
Chapter 8, Generics, explains that generics, or parameterized types, are a key component of any advanced type system, and the type system in Kotlin is substantially more advanced than that available in Java. This chapter covers variance, the type system including the Nothing type, and algebraic data types.
Chapter 9, Data Classes, shows that immutability and boiler-plate free domain classes are a current hot topic, due to the way they facilitate more robust code and simplify concurrent programming. Kotlin has many features focused on this area, which it calls data classes.
Chapter 10, Collections, explains that collections are one of the most commonly used aspects of any standard library, and Java collections are no different. This chapter describes the enhancements that Kotlin has made to the JDK collections, including functional operations such as map, fold, and filter.
Chapter 11, Testing in Kotlin, explains that one of the gateways into any new language is using it as a language for writing test code. This chapter shows how the exciting test framework KotlinTest can be used to write expressive, human-readable tests, with much more power than the standard jUnit tests allow.
Chapter 12, Microservices in Kotlin, shows that microservices have come to dominate server-side architecture in recent years, and Kotlin is an excellent choice for writing such services. This chapter introduces the Lagom microservice framework and shows how it can be used to great effect with Kotlin.
Chapter 13, Concurrency, explains that as multi-core aware programs are becoming more and more important in server-side platforms, This chapter is focused on a solid introduction to concurrent programming techniques that are vital in modern development, including threads, concurrency primitives, and futures.
What you need for this book
This book requires a computer running MacOS, Linux, or Windows, capable of running the latest versions of Java. It is recommended that the machine has enough memory to run a recent version of Jetbrains' Intellij IDEA.
Who this book is for
This book is aimed those who have little or no Kotlin experience and wish to learn the language quickly. The focus of the book is on server-side development in Kotlin and would be best suited to a developer who is currently a server-side developer or who wishes to learn. No prior knowledge of functional or object-orientated programming is required, but knowledge of some other programming language is recommended.
Some chapters contain brief sections comparing Java implementations to their Kotlin cousins, but these pages can be skipped by those who have no prior Java knowledge.
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Chapter 1. Getting Started with Kotlin
It is time to write code. In this chapter, we will go over and write the typical entry code for every language: the famous Hello World! In order to do that, we will need to set up the initial environment required to develop software with Kotlin. We will provide a few examples using the compiler from the command line, and then we will move towards the typical way of programming using the IDEs and build tools available.
Kotlin is a JVM language, and so the compiler will emit Java bytecode. Because of this, of course, Kotlin code can call Java code, and vice versa! Therefore, you need to have the Java JDK installed on your machine. To be able to write code for Android, where the most recent supported Java version is 6, the compiler needs to translate your code to bytecode that is compatible at least with Java 6. For this book, however, all the code examples will be run with Java JDK 8. If you are new to the JVM world, you can get the latest version from http://www.oracle.com/technetwork/java/javase/downloads/index.html.
In this chapter you will learn how to:
Using the command line to compile and run Kotlin code
To write and execute code written in Kotlin, you will need its runtime and the compiler. At the time of writing, version 1.1 milestone 4 is available (the stable release is 1.0.6). Every runtime release comes with its own compiler version. To get your hands on it, navigate to https://github.com/JetBrains/kotlin/releases/tag/v1.1-M04, scroll to the bottom of the page, and download and unpack the ZIP archive kotlin-compiler-1.1-M04.zip to a known location on your machine. The output folder will contain a subfolder bin with all the scripts required to compile and run Kotlin on Windows, Linux, or OS X. Now you need to make sure the bin folder location is part of your system PATH in order to call the kotlinc without having to specify the full path.
If your machine runs Linux or OS X, there is an even easier way to install the compiler by using sdkman. All you need to do is execute the following commands in a terminal:
$ curl -s https://get.sdkman.io | bash$ bash$ sdk install kotlin 1.1-M04Alternatively, if you are using OS X and you have homebrew installed, you could run these commands to achieve the same thing:
$ brew update
$ brew install [email protected]Now that all of this is done, we can finally write our first Kotlin code. The application we will be writing does nothing else but display the text Hello World! on the console. Start by creating a new file named HelloWorld.kt and type the following:
fun main(args: Array<String>) { println("Hello, World!") }From the command line, invoke the compiler to produce the JAR assembly (include-runtime is a flag for the compiler to produce a self-contained and runnable JAR by including the Kotlin runtime into the resulting assembly):
kotlinc HelloWorld.kt -include-runtime -d HelloWorld.jarNow you are ready to run your program by typing the following on your command line; it is assumed your JAVA_HOME is set and added to the system path:
$ java -jar HelloWorld.jarThe code is pretty straight forward. It defines the entry point function for your program, and in the first and only line of code, it prints the text to the console.
If you have been working with the Java or Scala languages, you might raise an eyebrow because you noticed the lack of the typical class that would normally define the standard static main program entry point. How does it work then? Let's have a look at what actually happens. First, let's just compile the preceding code by running the following command. This will create a HelloWorld.class in the same folder:
$ kotlinc HelloWorld.ktNow that we have the bytecode generated, let's look at it by using the javap tool available with the JDK (please note that the file name contains a suffix Kt):
$ javap -c HelloWorldKt.classOnce the execution completes, you should see the following printed on your terminal:
Compiled from "HelloWorld.kt"public final class HelloWorldKt { public static final void main(java.lang.String[]); Code: 0: aload_0 1: ldc #9 // String args 3: invokestatic #15 // Method kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull:(Ljava/lang/Ob ject;Ljava/lang/String;)V 6: ldc #17 // String Hello, World! 8: astore_1 9: nop 10: getstatic #23 // Field java/lang/System.out:Ljava/io/PrintStream; 13: aload_1 14: invokevirtual #29 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V 17: return}You don't have to be an expert in bytecode to understand what the compiler has actually done for us. As you can see on the snippet, a class has been generated for us, and it contains the program entry point with the instructions to print Hello World! to the console.
I would not expect you to work with the command line compiler on a daily basis; rather, you should use the tools at hand to delegate this, as we will see shortly.
Kotlin runtime
When we compiled Hello World! and produced the JAR, we instructed the compiler to bundle in the Kotlin runtime. Why is the runtime needed? Take a closer look at the bytecode generated, if you haven't already. To be more specific, look at line 3. It invokes a method to validate that the args variable is not null; thus, if you compile the code without asking for the runtime to be bundled in and try to run it, you will get an exception.
$ kotlinc HelloWorld.kt -d HelloWorld.jar$ java -jar HelloWorld.jarException in thread "main" java.lang.NoClassDefFoundError: kotlin/jvm/internal/Intrinsicsat HelloWorldKt.main(HelloWorld.kt)Caused by: java.lang.ClassNotFoundException: kotlin.jvm.internal.IntrinsicsThe runtime footprint is very small; with ~800 K one can't argue otherwise. Kotlin comes with its own standard class library (Kotlin runtime), which is different from the Java library. As a result, you need to merge it into the resulting JAR, or provide it in the classpath:
$ java -cp $KOTLIN_HOME/lib/kotlin-runtime.jar:HelloWorld.jar HelloWorldKtIf you develop a library for the exclusive use of other Kotlin libraries or applications, then you don't have to include the runtime. Alternatively there is a shorter path. This is done via a flag passed to the Kotlin compiler:
$kotlinc -include-runtime HelloWorld.kt -d HelloWorldThe REPL
These days, most languages provide an interactive shell, and Kotlin is no exception. If you want to quickly write some code that you won't use again, then the REPL is a good tool to have. Some prefer to quickly test their methods, but you should always write unit tests rather than using the REPL to validate that the output is correct.
You can start the REPL by adding dependencies to the classpath in order to make them available within the instance. To give an example, we will use the Joda library to deal with the date and time. First, we need to download the JAR. In a terminal window, use the following commands:
$ wget https://github.com/JodaOrg/joda-time/releases/download/v2.9.4/joda-time-2.9.4-dist.tar.gz$ tar xvf joda-time-2.9.4-dist.tar.gzNow you are ready to start the REPL, attach the Joda library to its running instance, and import and use the classes it provides:
$ kotlinc-jvm -cp joda-time-2.9.4/joda-time-2.9.4.jarWelcome to Kotlin version 1.1-M04 (JRE 1.8.0_66-internal-b17)Type :help for help, :quit for quit>>> import org.joda.time.DateTime>>> DateTime.now()2016-08-25T22:53:41.017+01:00Kotlin with Maven
If you still prefer to stick with good old Maven, there is no problem. There is a plugin for it to support Kotlin as well. If you don't have Maven on your machine, you can follow the instructions at https://maven.apache.org/install.html to get it installed on your local machine.
Just as we did with Gradle, let's use the built-in templates to generate the project folder and file structure. From the terminal, within a brand new folder, you will have to run the following command:
$ mvn archetype:generate -DgroupId=com.programming.kotlin - DartifactId=chapter01 -DarchetypeArtifactId=maven-archetype- quickstart -DinteractiveMode=falseThis will generate the pom.xml file and the src folder for Maven. But before we add the file containing the kotlin code, we need to enable the plugin. Just as before, start by deleting App.java and AppTest.java from src/main/java/com/programming/kotlin and test/main/java/com/programming/kotlin/, and create the src/kotlin folder (the subdirectory structure matches the namespace name):
$ mkdir -p src/main/kotlin/com/programming/kotlin/chapter01$ mkdir -p src/test/kotlin/com/programming/kotlin/chapter01In an editor of your choice, open up the generated pom.xml file and add the following:
<pluginRepositories> <pluginRepository> <snapshots> <enabled>true</enabled> </snapshots> <id>bintray-kotlin-kotlin-dev</id> <name>bintray</name> <url>http://dl.bintray.com/kotlin/kotlin-dev</url> </pluginRepository> </pluginRepositories> <repositories> <repository> <snapshots> <enabled>true</enabled> </snapshots> <id>bintray-kotlin-kotlin-dev</id> <name>bintray</name> <url>http://dl.bintray.com/kotlin/kotlin-dev</url> </repository> </repositories> <properties> <kotlin.version>1.1-M04</kotlin.version> <kotlin.test.version>1.3.3</kotlin.test.version> </properties> <build> <sourceDirectory>${project.basedir}/src/main/kotlin</sourceDirecto ry> <testSourceDirectory>${project.basedir}/src/test/kotlin</testSourc eDirectory> <plugins> <plugin> <artifactId>kotlin-maven-plugin</artifactId> <groupId>org.jetbrains.kotlin</groupId> <version>${kotlin.version}</version> <executions> <execution> <id>compile</id> <phase>process-sources</phase> <goals> <goal>compile</goal> </goals> </execution> <execution> <id>test-compile</id> <phase>process-test-sources</phase> <goals> <goal>test-compile</goal> </goals> </execution> </executions> </plugin> </plugins> </build>All we have done so far is to enable the Kotlin plugin and make it run in the process-stages phase to allow the mixing of Java code as well. There are cases when you might have part of the source code written in good old Java. I am sure you also noticed the addition of source directory tags, allowing for the kotlin files to be included in the build.
The only thing left to do now is to add the library dependencies for the Kotlin runtime as well as the unit tests. We are not going to touch upon the testing framework until later in the book. Replace the entire dependencies section with the following:
<dependencies> <dependency> <groupId>org.jetbrains.kotlin</groupId> <artifactId>kotlin-stdlib</artifactId> <version>${kotlin.version}</version> </dependency> <dependency> <groupId>io.kotlintest</groupId> <artifactId>kotlintest</artifactId> <version>${kotlin.test.version}</version> <scope>test</scope> </dependency> </dependencies>It is time now to add the Hello World! code; this step is similar to the one we took earlier when we discussed Gradle:
$ echo "" >> src/main/kotlin/com/programming/kotlin/chapter01/Program.kt$cat <<EOF >> src/main/kotlin/com/programming/kotlin/chapter01/Program.kt package com.programming.kotlin.chapter01 fun main(args: Array<String>) { println("Hello World!") }We are now in a position to compile and build the JAR file for the sample program:
$ mvn package$ mvn exec:java - Dexec.mainClass="com.programming.kotlin.chapter01.ProgramKt"The last instruction should end up printing the Hello World! text to the console. Of course we can run the program outside Maven by going back to executing Java, but we need to add the Kotlin runtime to the classpath:
$java -cp $KOTLIN_HOME/lib/kotlin-runtime.jar:target/chapter01-1.0- SNAPSHOT.jar "com.programming.kotlin.chapter01.ProgramKt"If you want to avoid the Classpath dependency setup when you run the application, there is an option to bundle all the dependencies in the resulted JAR and produce what is called a fat jar. For that, however, another plugin needs to be added:
<plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-shade-plugin</artifactId> <version>2.4.3</version> <executions> <execution> <phase>package</phase> <goals> <goal>shade</goal> </goals> <configuration> <transformers> <transformer implementation="org.apache.maven.plugins.shade.resource.ManifestRe sourceTransformer"> <mainClass>com.programming.kotlin.chapter01.ProgramKt</mainClass> </transformer> </transformers> </configuration> </execution> </executions> </plugin>We can execute the command to run our JAR without having to worry about setting the classpath since this has been taken care of by the plugin:
$ java -jar target/chapter01-1.0-SNAPSHOT.jarIntelliJ and Kotlin
Coding using Vim/nano is not everyone's first choice; working without the help of an IDE with its code completion, intelli-sense, shortcuts for adding files, or refactoring code can prove challenging the more complex the project is.
For a while now, in the JVM world, people's first choice when it comes to their integrated development environment has been IntelliJ. The tool is made by the same company that created Kotlin: JetBrains. Given the integration between the two of them, it would be my first choice of IDE to use, but, as we will see in the next section it is not the only option.
IntelliJ comes in two versions: Ultimate and Community (free). For the code we will be using in the course of this book, the free version is enough. If you don't have it already installed, you can download it from https://www.jetbrains.com/idea/download.
From version 15.0, IntelliJ comes bundled with Kotlin, but if you have an older version you can still get support for the language by installing the plugin. Just go to Settings|Plugins| Install IntelliJ plugins and type Kotlin in the search.
We are going to use the IDE to create a Gradle project with Kotlin enabled, just as we did in the previous section. Once you have started IntelliJ, you will have to choose Create new project. You will get a dialog window from which you should select Gradle from the left-hand side section; check the Kotlin(Java) option from the right-hand side. As you can see here:
Selecting a project type
You should already have the system variable JAVA_HOME set up for the tool to pick it up automatically (see the Project SDK at the top of the screenshot). If this isn't the case, choose the New button and navigate to where your Java JDK is. Once you have selected it, you are ready to go to the next step by clicking on the Next button available on the bottom right-hand side of the screen.
The next window presented to you is asking you to provide the Group Id and Artifact Id. Let's go with com.programming.kotlin and chapter01 respectively. Once you have completed the fields, you can move to the next step of the process where you tick the Use auto-import flag as well as Create directories for empty directory roots automatically. Now carry on to the next step, where you are asked where you wish to store the project on your machine. Set the project location, expand More Settings, type chapter01 for the Module name, and hit the Finish button.
IntelliJ will go on and create the project, and you should have the outcome shown in the following screenshot:
Hello World! basic project
On the selected kotlin folder, right-click and choose the New | Package option, and type com.programming.kotlin.chapter01:
Setting up the package name
Below the kotlin folder, you should see a new one appear, matching what was typed earlier. Right click on that, choose New | Kotlin File/Class, and type Program.kt:
Creating Program.kt file
We are now ready to start typing our Hello World! Use the same code we created earlier in the chapter. You should notice the Kotlin brand icon on the left-hand side of the file editor. If you click on it, you will get the option to run the code, and if you look at the bottom of your IntelliJ window you should see the text Hello World! printed out:
Hello World! program
Well done! You have written your first Kotlin program. It was easy and quick to set up the project and code, and to run the program. If you prefer, you can have a Maven rather than a Gradle project. When you choose New | Project, you have to select Maven from the left-hand side and check Create from archetype while choosing org.jetbrains.kotlin:kotlin-archetype-jvm from the list presented:
Maven project
Eclipse and Kotlin
There might be some of you who still prefer Eclipse IDE to IntelliJ; don't worry, you can still develop Kotlin code without having to move away from it. At this point, I assume you already have the tool installed. From the menu, navigate to Help | Eclipse Marketplace, look for the Kotlin plugin, and install it (I am working with the latest distribution: Eclipse Neon).
Once you have installed the plugin and restarted the IDE, you are ready to create your first Kotlin project. From the menu, choose File | New | Project and you should see the following dialog:
New Kotlin project
Click the Next button to move to the next step and, once you have chosen the source code location, click the Finish button. This is not a Gradle or Maven project! You can choose one of the two, but then you will have to manually modify the build.gradle or pom.xml, as we did manually in the Kotlin with Gradle and Kotlin with Maven sections of this chapter. Similar to the IntelliJ project, click on the src folder, choose New package, and name it com.programming.kotlin.chapter01. To add our Program.kt, you will need to right-click on the newly created package, select New | Other, and select Kotlin | Kotlin File from the list. Once the file has been created, type the simple lines of code to print out the text to the console. You should have the following result in your Eclipse IDE:
Hello World! with Eclipse
Now you are ready to run the code. From the menu select Run | Run. You should be able to trigger the execution, and in the Console tab at the bottom of your IDE you should see the Hello World! text printed out.
Summary
This chapter has showed you how you can set up your development environment with the tools required to build and run Kotlin code. Now you are able to run and execute the examples created in the rest of the book, as well as experiment with your own Kotlin code.
In the next chapter you will delve into the basic constructs you will use daily when you code in Kotlin.
Comments
Comments in Kotlin will come as no surprise to most programmers as they are the same as Java, Javascript, and C, among other languages. Block comments and line comments are supported:
// line comment /* A block comment can span many lines */Imports
To enable classes, objects, interfaces, and functions to be used outside of the declared package we must import the required class, object, interface, or function:
import com.packt.myproject.Foo
