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Welcome to kotlin Series



Kotlin OOP series:

1. OOP Basic
2. Constructor
3. Inheritance
4. Method Overriding

This is the fourth post in this Kotlin OOP series. In this series, we are going to cover a very important topic that is method overriding. This uses in the android app development is huge. First, we create two classes and we use one class method for other class. But we already know it how we can do that. we can do this? In the third part of this series. But in this tutorial, we are going to do some advance. We modify first class method in the second class.

Let's start.
Create a class name Math3 and also create 2 two class. Like previous Math class.

See the code-

class Math3 {

    fun add(a:Int, b:Int):Int{
        return a+b
    }

    fun sub(a:Int, b:Int):Int{
        return a-b
    }
}

Now create a second class name Math4 and it also has two methods.(like previous second math class). But take an extra variable and we use this variable in the override methods.
See the code-

class Math4(val extra:Int) {

    fun mul(a:Int, b:Int):Int{
        return a*b
    }

    fun div(a:Int, b:Int):Int{return a/b}
}

two class creation complete. Now we do our main job. Before inheriting one class to another make class as open and methods also.

See the code-

open class Math3 {

    open fun add(a:Int, b:Int):Int{
        return a+b
    }

    open fun sub(a:Int, b:Int):Int{
        return a-b
    }
}

Now inherit the first class, to second class.

class Math4(val extra:Int) : Math3(){
     //some code
}

we override two methods from first math class (Math3.kt). in add and sub-method we just add our extra value.

See the code-
add method

override fun add(a:Int, b:Int):Int{
    return a+b+extra
}

submethod-

override fun sub(a:Int, b:Int):Int{
        return a-b+extra
}

That it we finish. Now we run this code in the Main method.

see full code of Math4.kt

class Math4(val extra:Int) : Math3(){

    override fun add(a:Int, b:Int):Int{
        return a+b+extra
    }

    override fun sub(a:Int, b:Int):Int{
        return a-b+extra
    }

    fun mul(a:Int, b:Int):Int{
        return a*b
    }

    fun div(a:Int, b:Int):Int{return a/b}
}

on the Main method, we first create an instance of the Math3 class and use Math3 methods.

fun main(args: Array<String>){

    val math = Math3()
    println("add: ${math.add(20,21)}")
    println("add: ${math.sub(22,21)}")
}

Output-

add: 41
Sub: 1

Now time for Math4. use 4 methods and pass value 10.

val math2 = Math4(10)
println("add: ${math2.add(20,21)}")
println("sub: ${math2.sub(22,21)}")

println("mul: ${math2.mul(20,21)}")
println("div: ${math2.div(22,21)}")

Output-

add: 51
sub: 11
mul: 420
div: 1

Now compare two those output. you see 10 different in the result of add and sub.
Main Method-

fun main(args: Array<String>){

    val math = Math3()
    println("add: ${math.add(20,21)}")
    println("Sub: ${math.sub(22,21)}")

    val math2 = Math4(10)
    println("add: ${math2.add(20,21)}")
    println("sub: ${math2.sub(22,21)}")

    println("mul: ${math2.mul(20,21)}")
    println("div: ${math2.div(22,21)}")

}

Output-

add: 41
Sub: 1
add: 51
sub: 11
mul: 420
div: 1

That's it.
Now we learn how we can modify one class methods in other class.

Thank you for reading this post

Happy coding.

Welcome to kotlin Series

Kotlin OOP series:

1. OOP Basic
2. Constructor
3. Inheritance
4. Method Overriding

This is the third post about Kotlin OOP. In this post, we are going to cover Kotlin Inheritance.
Let's start-
First, create a class named Math. This class contains two function name add and sub.

add method -> take two integers and return the addition result of that two integers.
sub method -> also take two integers and return the subtracted result of two integers.

Full math class code-

class Math {

  fun add(a:Int, b:Int):Int{
  return a+b
  }

  fun sub(a:Int, b:Int):Int{
  return a-b
  }
}

Now we create a new class and the class name is Math2. see the code-

class Math2 {

  fun mul(a:Int, b:Int):Int{
  return a*b
  }

  fun div(a:Int, b:Int):Int{return a/b}
}

Now we inherited Math class to Math2. But you get some error. Because by default all the class is final. So you can not modify the class. So that we need to modify the Math class and open keyword before class Math

open class Math {
//some code
}

Now the error will be solved. See the code-

class Math2 : Math(){

  fun mul(a:Int, b:Int):Int{
  return a*b
  }

  fun div(a:Int, b:Int):Int{return a/b}
}

Now in the main method- create a math instance and print use those the two method-

val math = Math()
println("add: ${math.add(20,21)}")
println("add: ${math.sub(22,21)}")

output-

add: 41
add: 1

so we see that we can an easily use add and sub-methods. Now we create the Math2 instance. Through this instance, we can use 4 methods. two class method and Math class method.
Let see-

val math2 = Math2()
println("add: ${math2.add(20,21)}")
println("sub: ${math2.sub(22,21)}")

println("mul: ${math2.mul(20,21)}")
println("div: ${math2.div(22,21)}")

see we use both Math and Math2 class methods.
Output-

add: 41
sub: 1
mul: 420
div: 1

Full main method-

fun main(args: Array<String>){

  val math = Math()
  println("add: ${math.add(20,21)}")
  println("add: ${math.sub(22,21)}")

  val math2 = Math2()
  println("add: ${math2.add(20,21)}")
  println("sub: ${math2.sub(22,21)}")

  println("mul: ${math2.mul(20,21)}")
  println("div: ${math2.div(22,21)}")

}

Output:

add: 41
add: 1
add: 41
sub: 1
mul: 420
div: 1

That's all. In this post, we learn about Kotlin inheritance. In Android app development we use this concept a lot.

Thanks for reading this post.

Happy coding.

Welcome to kotlin Series


Kotlin OOP series:

1. OOP Basic
2. Constructor
3. Inheritance
4. Method Overriding

This is the second post about Kotlin OOP Concept. In the previous post,  Kotlin OOP: Basic
we talk about a basic opp concept.
In this post, we are going to litter deeper.
Let's start-
From the previous post, we create an Apple class like previous.
let do this-

private class Apple(color:String,shape:String){

  var color:String? = null
  var shape:String? = null

  init {
  //println("Color: $color")
  //println("Shape: $shape")

  this.color = color
  this.shape = shape
  }

  fun GetColor():String?{
  return this.color
  }

  fun GetShape():String?{
  return this.shape
  }

}

In this class, we are working with the primary constructor. But If we have another option to working with the second constructor. Just like Java. How can I do this? Let's start- Let's make the primary constructor empty

private class Apple3(){
}

and declarer secondary constructor like below-

constructor(color:String,shape:String): this() {
}

Now declare class two class variable and assign it in the constructor.

var color:String? = null
var shape:String? = null

assign it-

//second constrictor
constructor(color:String,shape:String): this() {
  this.color = color
  this.shape = shape
}

Greater and setter method already there Check full class code-

private class Apple(){

  var color:String? = null
  var shape:String? = null

  //second constrictor
  constructor(color:String,shape:String): this() {
  this.color = color
  this.shape = shape
  }

  fun GetColor():String?{
  return this.color
  }

  fun GetShape():String?{
  return this.shape
  }

}

test this on the Main method-

fun main(args: Array<String>){
  val apple =Apple3("red","oval")

  println("Color: ${apple.GetColor()} Shape: ${apple.GetShape()}")
}

output-

Color: red Shape: oval

This is very similar to Java code. But if you are using this secondary constructor. you can not create a variable like the primary constructor.

This is a very smaller post. In this post we cover constructor. Some you need the secondary constructor and some time you need the primary constructor. If you come from Java then you should go through the secondary constructor. Then it looks link java. So you feel easy to learn Kotlin. Keep learning.

Thank you for reading this post.
Happy coding.

Welcome to kotlin Series

Kotlin OOP series:

1. OOP Basic
2. Constructor
3. Inheritance
4. Method Overriding

In this post, we are going to cover object-oriented programming concept with kotlin. In kotlin, all the item is an object. So we are going to create an object name apple. and it has two properties color and shape. Let's create an Apple object class-

private class Apple(color:String,shape:String){
}

now we are going to create two class variable and this variable can nullable. Those are color and shape

var color:String? = null
var shape:String? = null

Now initialize this value in init{}

init {
   this.color = color
   this.shape = shape
}

Not: it just like Java constructor. In java you assign your class variable in the constructor it works just like that. Now we are going to create two methods that return shape and color. Those methods are like Java getter method

fun GetColor():String?{
    return this.color
}

fun GetShape():String?{
    return this.shape
}

oh! we finished the apple class. you check my full apple class code.

private class Apple(color:String,shape:String){

    var color:String? = null
    var shape:String? = null

    init {
        this.color = color
        this.shape = shape
    }

    fun GetColor():String?{
        return this.color
    }

    fun GetShape():String?{
        return this.shape
    }

}

Now time to test this apple class.
Create a main Method-

fun main(args: Array<String>){
}

create a variable name apple and assign to apple class.

val apple =Apple("red","oval")

print apple color and shape-

println("Color: ${apple.GetColor()} Shape: ${apple.GetShape()}")

complete Main method-

fun main(args: Array<String>){
    val apple =Apple("red","oval")

    println("Color: ${apple.GetColor()} Shape: ${apple.GetShape()}")
}

Output:

Color: red Shape: oval

This code like too similar to Java. I try to make it similar to java. But we are learning new language Kotlin. we can make this class is very small through Kotlin concept. we make a new class Apple2. It also has same properties like color and shape.
Let's do that.
code-

class Apple2(var color: String, var shape: String)

our Apple class is complete in just one line. We don't need any greater or setter methods. No extra constructor need.
Let's test this apple class and check the output is same.

fun main(args: Array<String>){
    val apple =Apple2("red","oval")
    println("Color: ${apple.color} Shape: ${apple.shape}")
}

Output

Color: red Shape: oval

So we see that output is same. We don't need to write more code. Another big advantage is that you can define default value when you create the class variable.
See this-

private class Apple2(var color: String = "red",
                     var shape: String = "oval")

on the main method

val deafult = Apple2()
println("Color: ${deafult.color} Shape: ${deafult.shape}")

See we don't pass any value. Those values come from the default value when you assign it.
Output-

Color: red Shape: oval

we can also assign value through variable wise.
see the code-

val appleGreen = Apple2(color = "red",shape = "round")

This is very helpful when you have many variables in your constructor. So you don't check again and again is the value is assigned the correct one.
Check the main method-

val appleGreen = Apple2(color = "red",shape = "round")
println("Color: ${appleGreen.color} Shape: ${appleGreen.shape}")

Output

Color: red Shape: round

Full main Method code-

fun main(args: Array<String>){
    val apple =Apple2("red","oval")
    println("Color: ${apple.color} Shape: ${apple.shape}")

    val deafult = Apple2()
    println("Color: ${deafult.color} Shape: ${deafult.shape}")

    val appleGreen = Apple2(color = "red",shape = "round")
    println("Color: ${appleGreen.color} Shape: ${appleGreen.shape}")

}

and output is

Color: red Shape: oval
Color: red Shape: oval
Color: red Shape: round

That's it today. Thank you for reading this post.

Happy coding