PK - Chapter 6

Question 1

What is a backing field?

Answer 1

Generated for a property in a class
if it uses the default implementation
of at least one of the accessors.

Question 2

How do you access a backing field?

Answer 2

The backing field can be accessed
inside a custom accessor using the field keyword.

var firstName : String
get() = field
set(value) {field = value}

Question 3

Why does a Property
need a Backing Field?

Answer 3

object.SomeProperty = “value”
is the same as
object.setSomeProperty(“value”)

so the following would be recursive
and generate a StackOverflow
———————-

var firstName : String
get() = firstName
set(value) {firstName = value}

Question 4

How would you create a public property
with a private setter?

Answer 4

var firstName : String
get() = firstName
set(value) {firstName = value}

Question 5

What is the purpose of lateinit?

Answer 5

Non-null properties have to be
initialized in the constructor, unless
they are defined with the lateinit keyword.

lateinit var delayedInitProperty : DelayedInstance

Question 6

What are the restrictions for
using Late Initialization?

Answer 6

The property:

• cannot be a primitive type
• cannot have custom getters or setters

Question 7

What happes if you access a lateinit
property before it has been initialized ?

Answer 7

kotlin.UninitializedPropertyAccessException

Question 8

Why use Late Initialization?

Answer 8

You do not initialize a non-null
variable in the constructor:

• dependency injection
• value not available during object construction

Question 9

Why use a delegated property?

Answer 9

• to write the getters and setters only once (DRY) for a common type of property in the application domain
• to guarantee that data constraints are applied uniformly across the application/library

var eventTimestamp : Long by
TimestampPropertyDelegate()

Question 10

Write the code for TimestampPropertyDelegate.

Answer 10

class TimestampPropertyDelegate {
private var timestamp = 0L
operator fun getValue(
ref: Any?, // object reference
property: KProperty<*>): Long {
return timestamp;
}

operator fun setValue(
ref: Any?, // object reference
property: KProperty<*>,
value: Long) {
timestamp = value
}
}

Question 11

How are property delegates
similar to database domain constraints?

Answer 11

You can create a data dictionary of
property delegates that enforce
data integrity for the logical data types
in your application domain.

A change to a property delegate in the
data dictionary will consistently
propogate across the application.

Question 12

What interface(s) should a
property delegate implement ?

Answer 12

• ReadOnlyProperty
• ReadWriteProperty

Question 13

Why use lazy initialization?

Answer 13

To improve performance or resource utilization:

• you don’t want to incur the cost of initialization
  unless the object is actually needed
• you may want to delay initialization until more
  critical processing is complete
• you may want to delay initialization until
  adequate resources are available

Question 14

Create a lazy initialization property.

Answer 14

val foo: Int by lazy {
getSomeInteger()
}

Question 15

What are the
LazyThreadSafeteyMode enums?

Answer 15

• SYNCHRONIZED
  uses locks
• PUBLICATION
  initializer can be called several times,
  but only the first value will be used
• NONE
  no locks are used.
  high performance when it can only
  be initialized by one thread

Question 16

Compare lateinit versus lazy.

Answer 16

1. lazy can only be used for val
   lateinit can oly be used for var
2. a lateinit cannot be immutable
3. lateinit has a backing field
4. lateinit cannot be used for nullable properties
5. lateinit can be initialized from anywhere,
   the lazy property must define its initializer