Kotlin Programming

Null safety is a Kotlin feature that eliminates NullPointerException at compile time.
By default, variables cannot hold null. You must explicitly declare a type as nullable using ?, e.g. var name: String? = null.

Append a ? to the type declaration:
var name: String? = null
Without the ?, the compiler will reject any attempt to assign null.

The safe call operator ?. allows you to access a property or call a method on a nullable object without risking a NullPointerException:
val length = name?.length
If name is null, the expression returns null instead of throwing.

The Elvis operator ?: provides a default value when an expression is null:
val len = name?.length ?: 0
If the left side is null, the right side is used.

The !! operator converts a nullable type to a non-null type and throws a KotlinNullPointerException if the value is null:
val len = name!!.length
Use it only when you are absolutely certain the value is not null.

A data class automatically generates equals(), hashCode(), toString(), copy(), and componentN() functions:
data class User(val name: String, val age: Int)
It is designed to hold immutable data.

A data class auto-generates:equals() and hashCode()toString() in the form User(name=John, age=30)copy() for creating modified copiescomponentN() functions for destructuring

copy() creates a new instance with the same values, allowing you to override specific properties:
val user2 = user1.copy(age = 25)
This is useful for working with immutable data objects.

Extension functions let you add new functions to existing classes without modifying their source code:
fun String.addExclamation(): String = this + "!"
Called as: "Hello".addExclamation() returns "Hello!".

No. Extension functions are resolved statically and do not actually modify the class. They cannot access private or protected members of the receiver class.

A sealed class restricts which classes can inherit from it. All subclasses must be defined in the same file (or same package in Kotlin 1.5+):
sealed class Result
data class Success(val data: String) : Result()
data class Error(val msg: String) : Result()

Sealed classes allow the compiler to verify that a when expression covers all possible cases, eliminating the need for an else branch:
when (result) {
  is Success -> ...
  is Error -> ...
}