PK - Chapter 2

Question 1

What are the 2 keywords for
declaring variables ?

Answer 1

val - immutable

var - mutable

Question 2

Define constant foo = Test

Answer 2

val foo = “Test”

Question 3

Define variable foo with an explicit type of Number and double value of 12.3

Answer 3

var foo: Number = 12.3

Question 4

Is referential equality guaranteed
on boxed values ?

Answer 4

No

Even if same value, may have
been boxed in different locations.

Question 5

What are the built-in number types
and their memory sizes ?

Answer 5

There are 6:

Long - 64

Int - 32

Short - 16

Byte - 8

Double - 64

Float - 32

Question 6

# Define variable foo of type
Long with value = 12,333

Answer 6

var foo = 12_333L

Question 7

# Define variable foo of type
double with value 12.33

Answer 7

var foo = 12.33

Question 8

# Define variable foo with type
of Float and value of 12.33

Answer 8

var foo = 12.33F

Question 9

Create a number literal for each
built-in Number type

Answer 9

There are 6 build-in Number types:

Long
val foo = 1234L
Int
val foo = 1234
Double
val foo = 12.34
Float
val foo = 12.34F
Hexadecimal
val foo = 0xAB
Binary
val foo = 0b01010101

Question 10

What is the default type for
loating point numbers ?

Answer 10

Double

Question 11

What is the default type
for integral numbers ?

Answer 11

Int

Question 12

Convert an Int to a Long

Answer 12

val fooInt = 1234

val fooLong = fooInt.toLong()

Question 13

Convert a Float to a Double.

Answer 13

val fooFloat = 12.34F

val fooDouble = fooFloat.toDouble()

Question 14

What are the boolean operations ?

Answer 14

There are 3:

• negation !
• conjunction &&
• disjunction ||

Bonus:
conjunction and disjunction are lazy

Question 15

What are the bitwise operators ?

Answer 15

There are 6:

shl - Shift Left
shr - Shift Right
ushr - Unsigned Shift Right
and
or
xor

Question 16

What are the
Char escape sequences ?

Answer 16

There are 9:

\t
\b
\n
\r
‘
“

$
\u1234

Bonus:
Char type not treated as a number, as in Java.

Question 17

What is a raw string ?
What is the syntax ?
Typical use ?

Answer 17

• No escaping is necessary, so all characters can be included
• The string starts and ends with 3 double quotes (“””)
  val foo = “””

This is the first line
second line of the string
third line with special character / in the string”””

• Typically used for multi-line strings or regex strings

Question 18

What functions are provided
by an array ?

Answer 18

There are 4:

iterator()
size()
get(index)
set(inces, value)

Bonus:

get() and set() are also available through
bracket syntax.

val element1 = fooArray[0]

Question 19

What are the primitive array classes ? Why use them ?

Answer 19

There are 8 primitive array classes:

CharArray
BooleanArray
LongArray
IntArray
ShortArray
ByteArray
DoubleArray
FloatArray

Avoids boxing to improve performance.

Question 20

What are the max values
for built-in Number types ?

Answer 20

They can be retrieved using Type.MAX_VALUE.

Int.MAX_VALUE =
Long.MAX_VALUE =
Short.MAX_VALUE =
Float.MAX_VALUE =
Double.MAX_VALUE =

Question 21

Create an Int array of 1, 2, 3.

Answer 21

val fooIntArray = arrayOf(1, 2, 3)

Question 22

How are Kotlin arrays
different from Java arrays ?

Answer 22

Arrays are not part of the language.

Arrays are regular collection classes.

Question 23

What is the syntax for comments ?

Answer 23

Same as Java.

// line comment

/*
A block comment,
which can span many
lines.
*/

Question 24

How are packages handled in Kotlin ?

Answer 24

Same as Java.

package com.company.theapp
class Foo

Question 25

How are imports handled in Kotlin ?

Answer 25

Same as Java.

import com.company.theapp.Foo
import com.company.theapp.packagez.*

Question 26

Give an example of import renaming.

Answer 26

import com.company.theapp.Foo
import com.company/theotherapp.Foo as Foo2

Question 27

Give an example of string templates.

Answer 27

val name = “Sam”
val nameHello = “Hello $name”
val nameLength = “$nameHello. Your name has ${name.length} characters.”

Bonus:

Also called string interpolation.

Question 28

Give an example of string interpolation.

Answer 28

val name = “Sam”
val nameHello = “Hello $name”
val nameLength = “$nameHello. Your name has ${name.length} characters.”

Bonus:

Also called string templates.

Question 29

Explain ranges.

Answer 29

A range is an interval that has a start value
and an end value.

Any types which are comparable can be used
to create a range, using the “..” operator

val aToZ = “a”..”z”
val oneToNine = 1..9

Question 30

Explain the “in” operator.

Answer 30

The “in” operator is used to test whether a
given value is included in the range.

val aToZ = “a”..”z”
val isTrue = “c” in aToZ

Question 31

Give an example of using step()
and reversed() with a range.

Answer 31

val oddNumbers = (1..50).step(2)

val countingDownEvenNumbers = (2..100).step(2).reversed()

Bonus:

val countDown = 100.downTo(0)
val foo = 10
val rangeTo = foo.rangeTo(20)

Question 32

Give an example of a while loop.

Answer 32

while (true) {
println(“This will print forever!”)
}

Question 33

Give an example of a for loop.

Answer 33

val fooList = listOf(1,2,3,4)
for (k in fooList) {
println(k)
}

Note the required use of the “in” keyword.

Bonus:
“for” loop can be used with any object
that implements iterator()

Question 34

What must iterator() return ?

Answer 34

An object that implements:

hasNext() : Boolean
next() : T

Question 35

Why would you use a Range
instead of a List ?

Answer 35

Ranges are compiled into an index-based
collection for loops that are supported
directly on the JVM, so the iterator
introduces no performance penalty.

Question 36

How would you iterate over
the characters in a String ?

Answer 36

val foo = “some characters”
for (char in foo) {
println(char)
}

Question 37

What is the key difference between
Java and Kotlin for exception handling ?

Answer 37

• All exceptions in Kotlin are unchecked.
• In Java, unchecked exceptions are those that
  do not need to be added to method signatures.
• In Kotlin, exceptions are never part of the
  method signature.

Question 38

Explain the rules for
coding Kotlin exceptions.

Answer 38

​There are 4 rules:

• 1 try block
• zero or more catch blocks
• zero or one finally block
• at least one catch or finally block

Question 39

Give an example of
a try-catch block.

Answer 39

try {
something()
} catch (e: SomeException) {
println(“caught SomeException”)
} finally {
cleanup()
}

Bonus:

At least one catch or finally block
is required.

Question 40

Instantiate class Foo.

Constructor takes a single Long argument.

Answer 40

val foo = Foo(100L)

Unlike Java, there is no “new” operator.

Question 41

Describe referential equality
versus structural equality.

Answer 41

Referential equality
2 objects point to the same memory address
Operators:
===
!==

Structural equality
determined by using the equals function of the class
Operators (null safe):
== (not the same as Java)
!= (not the same as Java)

Question 42

Explain the usage of “this”.

Answer 42

• “this” refers to the current receiver
• Current receiver is the instance that
  received the invocation of the function,
  or in the case of also and let, the current
  receiver is the containing object of the
  object on which they are called
• Extension functions can reference “this”

Question 43

What is the current receiver ?

Answer 43

• Current receiver is the instance that received
  the invocation of a function, or in the case of also
  and let, the current receiver is the containing
  object of the object on which they are called
• Can be referenced inside the function with “this”
• Has access to members of “this”

Question 44

How would you refer to member
variable “address” of an instance
of type Building in the object graph ?

Answer 44

inner class Foo() {
 fun printAddress() = println(this@Building.address)
}

Note the use of the “@” notation.

Question 45

What are the visibility modifiers ?

Answer 45

4 visibility modifiers:

• public
• private
• protected
• internal

Note: public is the default visibility

Question 46

Explain the private visibility modifier.

Answer 46

• Private top-level functions, classes or
  interfaces can only be accessed from
  the same file.
• Inside a class, interface, or object, any
  private function or property is only
  visible to other members.

Bonus: Public is the default visibility.

Question 47

Explain the protected visibility modifier.

Answer 47

• Top-level functions, classes or interfaces
  cannot be protected
• Only functions or properties inside a class
  or interface can be protected. They are only
  visible to:
  
  • members of that class or interface
  • subclasses or subinterfaces
  • classes that implement the interface

Question 48

Explain the internal modifier.

Answer 48

Any code that is marked internal is
visible from other classes and
functions inside the same module.

This can be a Maven or Gradle module,
or an IntelliJ/Android module.

Question 49

Explain expression versus statement.

Answer 49

• An expression evaluates to a value.
  “hello”.startsWith(“h”)
• A statement has no resulting value returned.
  val a = 1

Bonus:

In Kotlin, if…else and try…catch blocks
are expressions.
Expressions can also be code blocks. The
last line of the block must be an expression.

Question 50

Use an if block as an expression.

Answer 50

return if (x==0) methodOne() else methodTwo()

Note: The if clause must have an else

Question 51

Use a try block as an expression.

Answer 51

val success = try {
doSomething()
true
} catch (e: IOException) {
false
}

Note that the last line of each
block returns a boolean.

Question 52

Use a code block as an expression.

Answer 52

val success = {
doSomething()
doSomethingMore() // returns a boolean
}

Note: The last line of the code block returns
a boolean.

Question 53

Explain null syntax.

Answer 53

• A variable which can be assigned null
  must be declared with a “?”.
  var fooString: String? = null

Bonus:
Extension functions defined for String
are not available for type String?

Question 54

Explain the “is” operator.

Answer 54

• Equivalent to Java’s instanceof
• Used for type checking at run time

when(x) {
is String -> println(x.toUpper())
else -> Unit
}

• “!is” can also be used

Question 55

Explain smart casts.

Answer 55

If a variable has been type checked (is),
it is implicitly cast to the more specific type
within the remainder of that scope/block.

Question 56

Explain explicit casting.

Answer 56

* Use the "as" keyword to cast an object
 fun length(fooAny: Any): Int {

val aString = fooAny as String
return aString.length
}

• Use the “as?”

Bonus:

val fooString: String? = fooAny as String
// in case fooAny is null or cannot be cast to a String

Question 57

What is the “safe cast” operator ?

Answer 57

If you want to avoid the ClassCastException
when a cast fails, use the “as?” operator.
This will return null if the cast fails.

val fooAny = “some string”
val someString: String? = fooAny as? String
val aFile: File? = fooAny as? File // aFile will equal null

Question 58

Explain the when expression.

Answer 58

There are 2 forms of when statement:

• with an argument
  a replacement for the Java
  switch statement
• without an argument
  a replacement for the Java
  if..else construct

The else clause is required, unless the
compiler can infer that all possible
conditions have been satisfied.

Question 59

Create a when statement
with an argument.

Answer 59

when (x) {
0 -> println(“x is 0”)
1 -> println(“x is 1”)
else -> println(“x is neither 0 or 1”)
}

when (x) {
0, 1 -> println(“x is 0 or 1”)
else -> println(“x is neither 0 or 1”)
}

Note the required use of else, since
the compiler cannot infer that all possible
conditions have been satisfied.

Question 60

Explain the different ways to construct
the conditions of a when statement
​which has an argument ?

Answer 60

• the condition must be the same type as the argument
• condition can be a constant
• constants can be combined into a single condition,
  ​seperated by commas
• can use any function or expression that resolves to
  ​the same type as the argument
• ranges (of the same type) also supported, using
  ​the in operator
• collections (of the same type) also supported,
  ​using the in operator
• smart casts can be used, as in
  when (any) {
  is String -> any.startsWith(“Foo”)
  else -> false
  ​}
• there must always be an else condition,
  unless the compiler can infer that all
  possible conditions have been satisfied.
• There is no restriction on combining these
  different types of conditions.

Question 61

What is the default visibility modifier ?

Answer 61

public

Question 62

Explain the different ways to construct
the conditions of a when statement
which does not have an argument ?

Answer 62

• each condition must resolve to a boolean
• the else clause is required, unless the
  compiler can infer that all possible conditions
  (true / false) have been satisfied.

Question 63

Create a when statement that uses
multiple constants in the condition.

Answer 63

when (x) {
0, 1 -> true
else -> false
}

Question 64

Create a when statement that
uses a range in the condition.

Answer 64

when (s) {
in 10..49 -> true
else -> false
}

Question 65

Create a when statement that
uses a collection in the condition.

Answer 65

which (x) {
in someCollectionOfX -> true
else -> false
}

Note: you can specify that the argument
is in any object that implements iterator()

Question 66

Create a when statement that
uses a smart cast in the condition.

Answer 66

which (anyFoo) {
is String -> anyFoo.startsWith(“Foo”)
else -> false
}

Question 67

Give an example of
an implicit label.

Answer 67

list.forEach {
 if (it \< 50) return@forEach
 else println(it)
}

Question 68

Give an example of
an explicit label.

Answer 68

list.forEach next@ {
 if (it \< 50) return@next
 else println(it)
}

Question 69

Describe the Any type.

Answer 69

Any is the uppermost type; analogous to Java’s Object type.

The more commonly used methods include:

• toString
• hashCode
• equals

Question 70

Describe the Unit type.

Answer 70

• The Unit type is equivalent to void in Java
• Unit is a proper type, with a singleton instance
• The singleton instance of Unit is returned when
  a function specifies a return type of Unit, or
  does not specify any return type.

Question 71

Describe the Nothing type.

Answer 71

• Nothing is a type that has no instances
• Nothing is a bottom type, the subtype of
  all classes
• can be used to inform the compiler that a
  function never completes normally (it might
  loop forever, or always throw an exception)
• an empty list of Nothing could be assigned
  to a reference expecting a list of Strings.
  This is used in emptyList(), emptySet() and etc.

Question 72

Explain the
return-at-label syntax.

Answer 72

• return@someLabel
• someLabel@
• can label a code block or any
  statement inside the current scope
• can also use an implicit label
  (the containing iterator ?)

Question 73

Explain a lambda.

Answer 73

• defined inside brackets
• its parameters, if any, are declared before
  the right arrow
• when there are multiple arguments, use the
  underscore to indicate unused arguments
• arrow notation and arguments are optional
  if not used
• if only 1 parameter, arrow and argument can
  be omitted; referring the the parameter with “it”
• if the last parameter to a function is a function,
  and you are passing a lambda expression, you
  can specify it outsde the parenthesis
• if the method only has one function parameter, the
  parenthesis may be omitted

val result = someMethod(firstParam, { someArg: String -> someOtherMethod(someArg) } )

val result = someMethod(firstParam) { someOtherMethod() }

val result2 = methodSimple { someOtherMethod() }

Question 74

How can you make
number literals more readable ?

Answer 74

1_000_000L

1_254.00F

22_158.78

Question 75

Write a lambda with multiple
arguments, that does not
use them all.

Answer 75

someMethod(firstArg) { _, secondArg ->
someOtherMethod(secondArg) }

map.forEach { (_, value) -> println($value!!!”) }

Note the destructuring of the pair in the
second example.

Question 76

Summarize:

• Any
• Unit
• Nothing

Answer 76

• Any
  “top type” for all objects
  equivalent to Java Object
• Unit
  a singleton
  returned by functions without a return type
  equivalent to Java null
• Nothing
  “bottom type” for all objects
  indicates that a function never returns
  used to create empty collection

Question 77

What is a function receiver ?

Answer 77

• the instance that corresponds to the
• this* keyword inside the function body
• also referred to as the current receiver