Kotlin Inheritance With Example

What is Inheritance?

• Just like in real life, we inherit features and behaviors from our parents. Our parents inherit from their grandparents. We can apply this same logic in programming. 

• While designing the classes we can extract out common properties and behavior in a separate class and other classes that require these features will inherit from this class - this will create a parent-child relationship among the classes.

• In simple words, Inheritance is a way of organizing code in a parent-child relationship that helps in reducing code duplication. You define the functionality in a parent class that will be inherited by child classes. You don't need to define the same functionality in a child class - child classes will have it from its parent.

Inheritance in Kotlin

• Inheritance in Kotlin is no different than any other programming language that supports Object-Oriented Programming. 

• But in Kotlin, if you want to allow a class to be inheritable, you have to explicitly mark it as Open i.e. open for inheritance. Let's understand this with an example - 

open class Parent
{
    var parentProperty: String = "Parent"
    fun parentFn() = println("Parent function is called")
}

class Child: Parent()
{
    var childProperty: String = "Child"
    fun childFn() = println("Child function is called")
}
  

Explanation - 

• Here we have defined 2 classes - Parent and Child. A parent is marked as open i.e. open for inheritance. This class has one property and one method.

• Another class Child inherits from Parent class using a colon(:) followed by the name of Parent Class Constructor call i.e Parent()

• This is how we inherit a class in Kotlin. Simply put a colon followed by the name of the parent class constructor call. 

• Now child class objects have access to all the properties and methods defined in the parent class. They can treat these methods and properties as their own.

fun main() 
{
    val child = Child()
    println(child.parentProperty) // Parent
    child.parentFn() // Parent function is called
    child.childFn() // Child function is called
}
  

As you can see above, you can access parent's properties and methods from the Child class's object.

IS-A Relationship

• Inheritance is also known as defining the IS-A relationship. While defining your class structure, you can look for classes and arrange them in a hierarchy if they follow IS-A relationship. For e.g. 

• Car is a Vehicle, Truck is a Vehicle. Vehicle can be defined as Parent Class.
• Circle is a Shape, Square is a Shape. Shape can be defined as Parent Class.
• Full-Time Employee, Part-Time Employee. Employee can be defined as Parent Class.
• Savings Account, Current Account, Credit Card Account, Loan Account. Account can be defined as Parent Class.

• As you can see wherever we have an IS-A relationship, we can arrange those classes in a hierarchy and define the class structure accordingly.

• One more concept that is very common in Object-Oriented Programming is the HAS-A relationship. For instance, Car has Engine. Engine has Cylinder and Piston. This also defines a relationship which is known as Composition.

Some Facts - 

• Parent's Constructor is called before Child's constructor.

• A class can have only one parent i.e. it can inherit from one single class.

• You have to explicitly mark the class as Open if we want to make it inheritable.

That's it for this article. For more articles on Kotlin Tutorials For Beginner Series - refer to this.

Happy Learning !! Cheers From CheezyCode.

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