Author: Swarnim Dixit

After the Android team’s announcement of first class support for Kotlin in the Google I/O keynote in 17^th May 2017, Kotlin came to limelight and Android developers started thinking of using Kotlin as the programming language instead of Java (ADT).

Kotlin is a statically-typed programming language developed by Jet Brains that runs on the Java virtual machine (JVM) and also can be compiled to JavaScript source code or uses the LLVM (Low Level Virtual Machine) compiler infrastructure. It is designed to interoperate with Java code and is dependent on Java Class Library, such as the collections framework. Kotlin is a fully supported programming language on Android. Kotlin was written by the same folks who created Android Studio, so its background comes from the mobile industry.

According to jetBrains, following are the reasons to use Kotlin for Android development.

• Concise – Drastically reduces the amount of boilerplate code. More concise than Java. The code looks clean and easy to understand.
• Interoperability – Use any existing library on the JVM, as there’s 100% interoperability. Target either the JVM or JavaScript.
• Safe – Prevents more kinds of errors than Java and avoid entire classes of errors like null pointer exception.
• Simpler – Way simpler than Scala! For a Java developer, getting started with Kotlin is very easy. The automated Java to Kotlin converter included in the Kotlin plugin helps with the first steps.
• Performance – Compiles as fast as Java.
• Small footprints – Kotlin has a very compact runtime library, which can be further reduced through the use of ProGuard. In a real application, the Kotlin runtime adds only a few hundred methods and less than 100K to the size of the .apk file. This means Kotlin adds just a little to .apk file size.

Installation:

The Kotlin plugin is bundled with Android Studio starting from version 3.0. If you use an earlier version, you’ll need to install the Kotlin plugin.

1. Go to File | Settings | Plugins | Install JetBrains plugin…
2. Then search for and install Kotlin.
3. You’ll need to restart the IDE after this completes.
4. After restarting, Android studio will prompt to configure the Kotlin plugin, select the latest version and click OK.
5. It will again prompt to sync the gradle, click on sync.

Create a Project or convert existing project:

1. Create a project normally.
2. If you have existing code and want to convert in Kotlin then press ctrl+shift+A to find action.
3. Write convert Java file to Kotlin file and press enter or press the shortcut ctrl+alt+shift+K (this will convert only the current file).

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4. Or start writing code with Kotlin to call methods written in Java or vice-versa as it is 100% interoperable with Java.

Everything is now ready to build the application and run it on an emulator or device. You can make a release or build an APK of the application and sign it similarly. Kotlin compiler produces byte-code, thus there really is no difference in terms of look and feel of Kotlin applications versus those written in Java.

Nice features of Kotlin:

1. Completely Null safe:

Kotlin completely removes null references by making all datatypes non-nullable (by default) i.e. the compiler won’t let you to use a non-initialized, non-nullable variable. To declare the type as nullable, add a question mark after the type. The compiler will enforce a null-check before accessing the variable. This practice drastically reduces bugs.

You can check if it’s nullable variable in two ways.
(a).The first is a traditional if statement:

if (nullVar != null)
 nullVar.fooboo();

(b).The second is with NULL safe call operator:

nullVar?.fooboo()

For example:

println(nonNullVar.substring(0, 3)) //prints first 3 characters
 println(nullVar?.substring(0, 3)) //prints first 3 characters if the string is not null, prints null otherwise

In some cases programmer may need to test the code if it is null pointer exception or not ,or if he knows the variable is nullable. Then he can use it as the compiler will not check for the null, and if it is null then throws null pointer exception.

println(nullVar!!.substring(0,3)) //prints first 3 characters if the string is not null, crashes with a NullPointerException otherwise.

2. Extension functions:

With an extension function, you can call a function from an object as if it were a part of its class. Kotlin lets you extend a class by adding additional functions to it without manipulating the original definition. Such functions are known as extension functions. The names of these functions should be preceded by the names of the classes they extend.

For example, to add an extension function isTeenager to the Student class, you write the following code outside the class:

fun Student.isTeenager(): Boolean {
 // If age falls in the range
// 13-19 return true
 return age in 13..19 //range
}

In Kotlin, it is compiled to a static function that returns a boolean value. You’d call it as if it were a member function, like

var student = Student()
 student.isTeenager()

as if it is a member function of Student class

3. Data Classes:

We all know that most of our apps are data dependent, we often find ourselves creating model classes to hold data. In Java, this can be very tedious task, generating getter and setter functions for each field. With Kotlin, we can declare the class and all its properties in a single line. The compiler will generate all getter and setter functions, as well as the equality members, toString() and a copy() function.

For example, take this in Java:

public class Student{
 private String name;
 private int age;
 public User(String name, int age) {
 this.name = name;
 this.age = age;
 }
 public void setName(String name) {this.name = name;}
 public String getName() {return this.name;}
 public void setAge(int age) {this.age = age;}
 public int getAge() {return this.age;} 
}

To run the same function in Kotlin, all we need is:

data class Student(var name: String, var age: Int?)

You just have to add data keyword before class keyword and the compiler will take care of generating constructor and getter setters.

4. Singleton: Kotlin doesn’t support static member for class. So, how can we create singleton class in Kotlin?

A thread safe singleton design pattern

In Java if it is like:

public class SomeSingleton {
 private static SomeSingleton instance = null; private SomeSingleton(){
 }
 private synchronized static void createInstance() {
 if (instance == null) {
 instance = new SomeSingleton();
 }
 }
 public static SomeSingleton getInstance() {
 if (instance == null) createInstance();
 return instance;
 }
 }

In Kotlin, it is

object SomeSingleton

The object will be instantiated and its init blocks will be executed lazily upon first access, in a thread-safe way as constructors are private and in case programmer need to initialize something.

With an object declaration, you are getting a safe and efficient singleton class.

object SomeSingleton {
 init {
 println("init called")
 }
 }

5. More features that will save you from some more keystrokes:

(a). Type aliases: Assign an alias to any given type. This is most useful for long types with many generic parameters.

E.g.

typealias MofL = Map<String, List> fun useMap(map: MofL) { }

(b). Clicklistners:

Consider this click listener

In Java:

view.setOnClickListener(new View.OnClickListener() {
 @Override
 public void onClick(View v) {
 // perform click event
 }});

In Kotlin:

view.setOnClickListener { // perform click event }

(c). Semi colons: It is sufficient for Kotlin to sense the end of the statement through a line break. So if you forget to add semicolon at the end of the line, it’s okay!

(d). Constructors: Kotlin’s syntax to create a constructor is very compact. You don’t need to write a constructor separately to initialize member variables but you can add it to the class header only like in the example shown:

class Person(var name: String, var age: Int, var school: String?) {}

If you don’t need to write anything in class then you don’t even need curly braces.

You can add other constructors as well and are known as secondary constructors but you have to delegate it to the primary constructor using this keyword.

constructor(name: String, age: Int, school: String?, email: String) : this(name, age, college) {
 this.email = email
 }

Limitations:

Whenever a new programming language is launched, it takes some time to get into the market. When Kotlin launched, it wasn’t as mature and proper as it is now. Looking at the limitations, as it is new in town, you will not get proper tutorials and learning stuff. Programmers who already worked with Java can pick up Kotlin in a day just by going through the documentation. Kotlin has its own library that will be added on top of Java’s standard library, so file’s build size increases by 100kb and gradle build also takes little more time.

Conclusion:

So you can saw that Kotlin doesn’t have any ground breaking characteristics. While the goal was not to create something revolutionary, but to provide something that is usable and familiar to modern enterprise developers. Kotlin helps us in avoiding some common pitfalls which are common with Java like null references, which increases code efficiency and product quality and it is 100% interoperable with Java.