KOTLIN COROUTINES FOR ANDROID DEVELOPMENT

 

OBJECT ORIENTED PROGRAMMING WITH KOTLIN II

Hello and welcome back to our Kotlin series. In this post we now want to focus on Object oriented programming with KOTLIN. In this second part of Object-Oriented Programming with Kotlin, we cover Kotlin inner and nested classes, Kotlin Data classes, Kotlin sealed classes, Kotlin objects and companion objects and finally we briefly look at Kotlin extension functions.

Kotlin inner and nested classes

Another fun fact about Kotlin is a file can have more than class in it. We can also nest classes in other classes. You can also nest interfaces. You can nest an interface in a class, a class in an interface and interfaces in interfaces.

Nested classes marked as "inner” are called inner class. Inner class cannot be declared inside interfaces or non-inner nested classes. The advantage of marking a nested class inner is that it is able to access members of outer class even if it is marked private.

Kotlin Data classes

Data classes are created solely to hold data. We mark the class as data to create a data class.

To ensure consistency and meaningful behavior of the generated code, data classes have to fulfill the following requirements: 

·       The primary constructor needs to have at least one parameter;

·       All primary constructor parameters need to be marked as val or var;

·       Data classes cannot be abstract, open, sealed or inner and Data classes may only implement interfaces.

For Data class, the compiler automatically generates:

·       A Copy () function, equals () and hashCode() pair, and a toString() form of the primary.

·       Constructor componentN() functions.

Kotlin Sealed classes

To create a sealed class, we use sealed modifier. All subclasses of a sealed class must be declared in the same file where the sealed class is declared. A sealed class is abstract and therefore you cannot instantiate objects from it. Constructors of sealed classes are private by default.

Sealed class restricts the possibility of creating subclasses and when you handle all subclasses of a sealed class in an when expression, it's not necessary to use else branch.

Kotlin Objects and Companion Objects

In Kotlin we can create singletons using the object declaration feature. The object keyword is used. A singleton is a class that allows one instance of itself to be created and gives access to that created instance. It is used when a user wants to restrict instantiation of a class to only one object.

An object declaration can contain properties, methods and so on. They are not allowed to have constructors. Similar to objects of a normal class, you can call methods and access properties by using the (.) notation.

In Kotlin, you can also call methods by using the class name. For that, you need to create a companion object by marking object declaration with companion keyword.

If we need a function that can be called without having a class instance but needs access to the internals of a class, we use companion object declaration inside that class. Companion Objects help access the members of the class using the class name only without explicitly creating the instance of the class.

Kotlin Extension Functions.

In Koltin, you can also use extension function to extend a class with new functionality. Basically, an extension function is a member function of a class that is defined outside the class.

Thanks for reading, till next time keep safe and happy coding.

So, you're a little weird? Work it! A little different? Own it! Better to be a nerd than one of the herds!     -Mandy Hale