Classes

Question 1

What is a sealed class?

Answer 1

All subclasses of a sealed class must be defined in the same file as the sealed class.
All subclasses are therefore known at compile time (similar to enums)
A sealed class is abstract by itself, it cannot be instantiated directly and can have abstract members.

Question 2

What are kotlin’s 3 uses of the object keyword?

Answer 2

object declaration - defines a Singleton class
object Singleton { }

companion object - a nested class defining 'static' members related to the outer class
companion object { }

object expression (anonymous object) - Creates an instance of the object on the fly, the same as Java's anonymous inner classes
Thread (object : Runnable {
    override fun run( ) {
        println("I'm created with anonymous object")
    }
} ).run( )

Question 3

What is the difference between enum constants and subclasses of a sealed class?

Answer 3

Each enum constant exists only as a single instance, whereas a subclass of a sealed class can have multiple instances, each with its own state.

Question 4

What is the effect of placing val in front of a parameter being passed into a constructor?

Answer 4

The property is declared AND initialised when the constructor is called.
It is therefore available to class members without having to be initialised within the class body.
class Foo(val bar: Bar) {
is the same as
class Foo(bar:Bar){
val bar = bar

Question 5

What is a data class?

Answer 5

A class whose main purpose is to hold data.
Utility functions are mechanically derivable (eg equals(); toString())
The rules for creating a data class are stricter (eg at least one param in constructor)

Question 6

What are the four visibility modifiers and their purposes?

Answer 6

public - visible everywhere
private - visible inside the same class only
internal - visible inside the same module
protected - visible inside the same class and its subclasses