Kotlin Fundamentals: Classes

1. Classes are public by default

2. Classes are Final by default

If you want to Inherit a class, You need to mark it as open indicating that it can be inherited explicitly

3. Will have a primary constructor

This primary constructor is used to create an instance ``kotlin fun primaryConstructorDemo() { val science = Science() val history = History() // val maths = Maths() // Cannot access '': it is private in 'Maths' }

// Class with default constructor(way 1)
class Science()
// Class with default constructor(way 2)
class History constructor()
// Class with private constructor
class Maths private constructor()
// Class with protected constructor constructor
class Computers protected constructor()

### `4`. `Might have secondary constructors`
### `5`. No need to use the `new` keyword to create an object of the class like we do in Java
### `6`. `Classes can be abstract like in other languages`
### `7`. `They may have properties and methods`
### `8`. `Properties of the class are private by default`
We can mark it as `val` or `var` thus it becomes public
```kotlin
    class Person(name: String)
    class Student(val name: String)

    fun test() {
        val person = Person("Donald")
        person.name // Unresolved reference: name

        val student = Student("Einstein")
        student.name // We can access the name variable
    }

9. Using the init{} block in class

• We cannot execute the code in the constructor like how we did in Java
• If we want some code to be executed when the object is created we can use init block.
• In the init block, We have access to the values passed to the constructor variables output

Before City Class construction
City is created with Name:-> New York
After City Class construction
City construction is in progress

code

    fun initBlockSignificanceDemo() {
        println("Before City Class construction")
        val demo = City("New York")
        println("After City Class construction")
        demo.demo()
    }

    class City(val name: String){
        init {
            println("City is created with Name:-> $name")
        }

        fun demo(){
            println("City construction is in progress")
        }
    }