Misc.

Question 1

How does Modulo % work?

Answer 1

How many times does the RIGHT side go into the LEFT side, then the remainder is the result.

or,

Left side DIVIDED BY Right side, return the remainder.

Question 2

String Raw multiline?

Answer 2

Use triple quotes...
E.g.
""" 
Hello,
My name is James.
How are you?
"""

Question 3

&& AND Short-circuiting

Answer 3

Do the most complex boolean check last because if the first check fails then the last one will not compute.
E.g.
if (quickCheck && complexLongCheck) {
// Do stuff
}

Question 4

|| OR Short-circuiting

Answer 4

Do the most complex boolean check last because if the first check returns true then the last one will not compute.
E.g.
if (quickCheck && complexLongCheck) {
    // Do stuff
}

Question 5

Structural Equality check

Answer 5

==
and
!=

e.g.

var name 1 = “James”
var name 2 = “Dougie”

println(name1 == name2)
// Prints false

Question 6

Referential Equality check

Answer 6

===
and
!===

e.g.

var person1 = Person("jim")
var person2 = Person("jim")

println(person1 === person2)
// Prints false

Question 7

What are the terms for the two types of equality in Kotlin?

Answer 7

Structural and Referential

Question 8

Why don’t referential checks work for ‘primitives’?

Answer 8

In Kotlin traditional primitives (numbers, chars, bools) are converted from Objects to actual primitives at runtime. So you can use the Structural equality check ‘==’ for those.
Strings use the String pool and so have their own logic for equality.
All other ‘custom’ objects work with the referential equality check ‘===’.

Question 9

What will be the value of strLength after the ‘Nullable Safe’ call if A. myString is null and B. myString is not null.

val strLength = myString?.length

Answer 9

A. null

B. The length of myString.

Question 10

What does the ‘Elvis’ operator do? ( ?: )

Answer 10

If the anything on the left-hand side of the operator is null, then return the value on the right-hand side.

It's shorthand for...
if(person != null) {
    return person.name
} else {
    return "Anon" // Some default value
}

Useful when used with Nullable Safe calls…
return person?age ?: “Anon”

Question 11

Unit vs Void vs Nothing

Answer 11

TODO

Question 12

What does invoke() do?

Answer 12

TODO

Question 13

Can you define functions within a function? From WHEN and WHERE can you call this function?

Answer 13

Yes.
You can only call it after you’ve defined the function,
The ‘inner’ function is scoped to the function it’s defined in so you can’t call it from outside the function.

Question 14

Can you store a function in a variable?

Answer 14

Yes!

Question 15

When to use a Single Line Expression?

E.g. fun myFunc = println(“hey”)

Answer 15

Whenever the body of the function can be written on a single line WITHOUT going over the line character limit.

Question 16

What is a Trailing Lambda and when can it be used?

Answer 16

When a function’s last parameter is a function then when calling that function you can pass the final function argument after the parentheses.

E.g.
someFunction(1stArg, 2ndArg) { println(“3rdArg”) }

If the only parameter a function has is a function then you can omit the parentheses entirely.

E.g.
someFunction { println(“I’m the only Arg”)

Question 17

How does you define a var args parameter?

Answer 17

With the 'vararg' keyword.
E.g.
fun myFunction(vararg authors: String)

Question 18

What can you do in Kotlin to avoid method overloading?

Answer 18

Use default arguments instead.

Question 19

How do you add an access modifier to a Class’s primary constructor?

Answer 19

You have to use the ‘constructor’ keyword (which is optional if you don’t need to change the access modifier) and then you place your access modifier before it.

E.g.
class Person internal constructor(val name: String) { }

Question 20

A class is allowed multiple init { } blocks. How do you control the order in which they are called?

Answer 20

They operate in the order in which they are defined in the class. Top-most / first init { } block is called first and so on.

Question 21

In what order do multiple class constructors bodies run?

Answer 21

The primary constructor/init blocks run first, then recursively back down to the initial constructor that the client called.

When a secondary construct calls another constructor with “: this(args)” it’s body is not run, rather it is added to the stack until the primary constructor has run (and presumably, the object is instantiated) then the stack is popped until all secondary constructors have run their bodies code.

E.g.
fun main() {
    val person = Person()
}

class Person(val a: Int, val b: Int) {
    init {
        println("first")
    }
    constructor(a: Int): this(a, 2) {
        println("Second")
    }
    constructor(): this(1) {
        println("Third")
    }
}

This will print:
first
Second
Third

Question 22

In what order to init blocks and property initialisers run?

Answer 22

In the order they are defined.

E.g.

init {
println(myValue)
}

val myValue = ‘x’

The init block will not know about myValue as it is initialised after the init block.

Take-away: init blocks should be defined after property initialisers.

Question 23

How do you override a the get() method of a field?

Answer 23

Define a get() method under the field.

E.g.
var name = “James”
get() = “Mr. $field”

Question 24

What is the ‘field’ of a get() override?

Answer 24

It is the field’s backing field. Basically it contains the contents of the field and the ‘field’ is the way to reference and use that contents.

Question 25

How do you override a the set() method of a field?

Answer 25

Define a set() method under the field.

E.g.
var name = "James"
    set(value) {
        field = value
    }

Question 26

What is ‘value’ in a field’s set() override?

Answer 26

It is the value the client passed in to set the new value to.

Question 27

What’s a good alternative to consider for list.first()?

Answer 27

list.firstOrNull() If the list is empty it will return null, potentially saving you having to check the list’s size first.

Question 28

How should you format constants?

Answer 28

Using the const keyword and the name should be in all uppercase characters like in Java.

Question 29

Where/how should you define a constant in the following scenarios?

1. The value needs to be accessed anywhere in the application and has cross-cutting concerns in many places.
2. The value is very specific to a certain class but other parts of the app need to use it.
3. The value is very specific to a certain class and other classes don’t/shouldn’t need to access it.

Answer 29

1. In a constants Singleton class. E.g.
Filename: Constants.kt
object Constants {
    const val MY_CONST = 1
}

2. In a Companion Object of the class.
3. In a Companion Object of the class with the access modifier marked as private.

Question 30

How do you create a Singleton?

Answer 30

Using ‘object’ instead of ‘class’. E.g.

object MySingleton { }

Question 31

What happens if a lateinit property is accessed before it is initialised?

Answer 31

A UnititalizedPropertyAccessException will be thrown.

Question 32

What does the lateinit modifier do?

Answer 32

It tells the compiler that you won’t know the value of the property when the object is created however you promise to initialise it before you use it.

Question 33

When might you want to use a nested class?

Answer 33

When the class relates directly to the containing class and doesn’t make sense to be used anywhere else. You can also make private to the containing class to enforce this.

Question 34

Do nested classes have access to the properties of their outer class?

Answer 34

No. You must use an inner class instead.

Question 35

How can you allow a nested class to access the properties of its enclosing class?

Answer 35

Change the nested class to be an inner class. E.g.

inner class MyInner Class { }

Question 36

Why can you not statically access an inner class like you can a nested class? E.g.

OuterClass.InnerClass()

Answer 36

Because the inner class has access to the outer class’s properties this means it must have a reference to the outer class. Referencing a inner class like so… Outerclass.InnerClass doesn’t make sense. Which instance of OuterClass should the InnerClass be referencing? The compiler doesn’t know. That’s why you must use it via an object instance and not via a static class reference. E.g. OuterClass().InnerClass().

Question 37

Nested class vs Inner class?

Answer 37

A nested class is simply an alternative approach to storing a class (that is somehow related to its outer class) in its own file within the project whereas an inner class is a property of the outer class.

Question 38

How can you get an Enum type from String/Number?

Answer 38

Use valueOf().
E.g.
enum class AccountType {
BRONZE, SILVER, GOLD
}
val valueFromApi = "gold"
val accountType = AccountType.valueof(valueFromApi.toUpper)

Question 39

Example of Enum constructor parameters.

Answer 39

enum class AccountType(val discountPercent: Int) {
    BRONZE(10),
    SILVER(15),
    GOLD(20)
}

Question 40

Example of Enum Abstract Functions.

Answer 40

enum class AccountType(val discountPercent: Int) {
    BRONZE {
        override fun calculateDiscountPercent = 5
    };

abstract fun calculateDiscountPercent(): Int }

Question 41

How to get a list of all type in an Enum?

Answer 41

YourEnum.values()

Question 42

Can an Enum have a companion object? And therefore, static like function behaviour?

Answer 42

Yes.

Question 43

Does the condition check in a when statement alway have to be defined on the first line?

Answer 43

No. You provide any number of conditional checks wihtin each case of your when statement. E.g.

when {
user.firstName == “James” -> println(“Hey James”)
user.lastName == “Baggins” && isShort -> println(“Hey Frodo”)
}

Question 44

var vs val

Answer 44

var can be changed whereas val is final. You prefer val where possible.

Question 45

How to get the class type of a variable?

Answer 45

myVar::class