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In  object-oriented programming ,  inheritance  enables new objects to take on the properties of existing objects. We can have class that can inherit properties and methods from the parent class, when we inherit classes, we do not need to specify the method and properties again, in the new class (child class or sub class). In essence it allow us to avoid code duplication (important) and make our code more flexible and maintainable (also very important). By default, classes in kotlin are final i.e they cannot be inherited from. This makes sense because not all classes created will need to be inherited or opened Why do we Inherit classes? Let's say, we wanted to create a class for Student and Employee . What do student and employee have in common? they both need to eat they both sleep they both move they both need to bath they both have a name the both have hair (no matter how small) they both breath air they both have birthdays they both ... You g...

Class without a Constructor but with default variable (or properties or data) [box type="info" align="" class="" width=""]In class-based object-oriented programming, a constructor is a special type of subroutine called to create an object. It prepares the new object for use, often accepting arguments that the constructor uses to set required member variables.[/box] class Person { val name : String = "Peter" // Default Variable var age : Int = 30 // Default Variable } fun main (args: Array<String>){ val person = Person() // this line tell kotlin to instantiate a new object from 'Person' class person.age = 33 //accessing our 'Person' property person.name = "Peterr"// this won't compile and can't be re-assigned because 'val' keyword makes it immutable } Class without a Constructor but with properties (or variables or data) and Methods fun main (args: Array<S...

Simple Function. neither accepts nor return any variable or value fun helloWorld(){ println("Hello World") }  Function that takes a parameter but returns no value fun stringFunction (text : String){ for (c in text){ println("$c ") //KOTLIN -> K O T L I N } } fun intFunction(int : Integer){ println(int) } Functions that returns a value but takes no parameter fun getAuthorsName() : String { return "Olorunleke Opeyemi" } fun getTodaysDate() : Date { return Date() } Functions that accept a value and returns a value class Example{ fun main(args: Array<String>) { println(evaluateInteger(21, 23)) } fun evaluateInteger (a : Int, b: Int) : String { return if (a>b) { "$a is greater than $b" } else if (a<b){ "$b is greater than $a" } else {"they are equal"} } }  

Kotlin ArrayList Kotlin ArrayList is also very similar to Java Arraylist . ArrayList<T> is the resizable-array implementation of the  List  interface. The ArrayList class has only a few methods in addition to the methods available in the List interface . val myArrayList = arrayListOf("John", "Fred", "McKinsey", "Morgan", "Sarah") //an arraylist of my friends val newFriendList = arrayListOf("Musa", "Victor", "Timothy") myArrayList.add(0, newFriendList[0]) for (e in myArrayList){ print("$e -") } // Musa -John -Fred -McKinsey -Morgan -Sarah myArrayList.removeAt(0) // removes 'Musa' which is at postion 1 val allOfMyFriends = myArrayList + newFriendList for(element in allOfMyFriends){ print("$element -") } // John -Fred -McKinsey -Morgan -Sarah -Musa -Victor -Timothy println(allOfMyFriends.size) // 8 println(allOfMyFriends.isEmpty) // false val isRemoveFredSucces...

An interface is a collection of abstract methods. It is a reference type in Java/Kotlin. It is similar to class. A class implements an interface, thereby inheriting the abstract methods of the interface. public class HelloWorld { class Example implements InterfaceExample{ @Override public void doSomething() { // doSomething } @Override public int doAnotherThing(String s) { return 0; } @Override public boolean doEntirelyAnottherThing(Long l) { return false; } } interface InterfaceExample { // Collection of Abstract Methods void doSomething(); // a simple abstract method int doAnotherThing(String s); //accepts a String returns an Int boolean doEntirelyAnottherThing(Long l) // accepts a long value and returns a boolean } } Along with abstract methods, an interface may also contain constants, default methods, static methods, a...

1. Variables In Kotlin, we use " val"  to declare a constant or " var " keywords to declare a variable. You can specify a type such as String or Int after the variable name. In the example below, we declared a constant firstName of type String with the val keyword. val firstName: String = "Opeyemi" // this is similar to -> final String firstName = "Opeyemi" <- Java But you'll soon realize that in Kotlin , it's often possible to omit the type from the declaration and the compiler won't complain. val lastName = "Olorunleke" // will still compile In the code above, you'll observe that we did not explicitly state the type String. The code above will still work because the compiler has implicitly inferred the type using type inference. We'll come back to this! The difference between the val and var keywords is that the former is immutable ((its value cannot be changed or read-only), while the latter is mu...