oops.md

Classes and Objects are the central concepts in any Object Oriented Programming(OOP) language.

Classes
A class is an entity that determines how an object will behave and what the object will contain.In other words, it is a blueprint or a set of instruction to build a specific type of object.

Create class use class keyword in kotlin

class Student {
}

To create an object for above class in kotlin so it has default constructor.
val Student = Student()
There was no need to use new keyword to create an object in kotlin.

Properties

class Student {
	// Properties or Member Variables
    var firstName: String
    var lastName: String
    val schollName: String = "GHSS"//initializing value & not changeable.
}

Using the var keyword is mutable or read-only use keyword val.

Constructors
Every class needs to have a constructor. If you don’t declare one yourself, the compiler generates a default constructor.
There are two types of constructors in Kotlin -
1.Primary Constructor
2.Secondary Constructor
1. Primary Constructor
The Primary constructor is part of the class header and declared after the class name

class Student constructor(firstName: String, lastName: String) {
}

You can omit the constructor keyword from the primary constructor

class Student(firstName: String, lastName: String) {
}

Initializer Block
The primary constructor cannot contain any code. So all the initialization logic is written inside an initializer block which is prefixed with the init keyword

class Student (firstName: String, lastName: String) {
  var firstName: String
  var lastName: String
  init{
  this.firstName=firstName
  thhis.lastName=lastName
  }
}

Note that, instead of initializing the properties in the init block, you can directly initialize them in the class body itself like this

class Student (firstName: String, lastName: String) { 
  var firstName: String = firstName 
  var lastName: String = =lastName   
} 

Default Values in the Constructor

class Student (firstName: String, lastName: String,schoolName: String="GHSS") {
  var firstName: String = firstName
  var lastName: String = =lastName
  init{
  println("Student data firstName = $firstName and lastName = $lastName" and school name $schoolName)
  }
}

So you could omit the school name to create object like this val student = Student("hari","c") in output we receive default school name
If you want to override the default value call like this val student1 = Student("Ram","R","VIDYAA VIKAS");

2. Secondary Constructor
Secondary constructors are prefixed with the constructor keyword and declared inside the class body

class Student(firstName: String, lastName: String,schoolName: String="GHSS") {
  var firstName: String = firstName
  var lastName: String = =lastName
   constructor(firstName: String, lastName: String,schoolName: String="GHSS")this(firstName,lastName){
  println("Student data firstName = $firstName and lastName = $lastName" and school name $schoolName)
  }
}

One thing to note here is that every secondary constructor must call the primary constructor, either directly or indirectly via another secondary constructor
The secondary constructor calls the primary constructor using this keyword
Getter && Setter
If you use val in constructor then there was no setter properties only getter properites
If you use var in constructor then you could use both setter && getter properties in accessing from object.

class Student (var firstName: String, var lastName: String,val schoolName: String="GHSS")
val student = Student("hari", "c")   
student.firstName="Ram"// ok
student.schoolName="VIDYAA VIKAS"// Error

NoteKotlin internally calls the default (implicit) getters/setters of the properties