Pro Android with Kotlin

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276
CHAPTER 10: Development
While this is valid code, the repetition of 
person.
is annoying. Besides, the first four lines are 
about constructing a person, while the next line has nothing to do with a person. It would be 
nice if this could be visually expressed, and the repetition could also be avoided. This is a 
construct in Kotlin, and it reads as follows:
val person = Person().apply {
lastName = "Smith"
firstName = "John"
birthDay = "2011-01-23"
}
company = Company("ACME")
This looks more expressive compared to the original code. It clearly says construct a person, 
do something with it, and then do something else. There are five such constructs, and 
despite being similar, they differ in meaning and usage: 
also
, 
apply
, 
let
, 
run
, and 
with
.
Table 
10-3
describes them.
Table 10-3. 
Scoping Functions
Syntax
What Is 
this
What is it
Returns
Use
a.also {
... }
this
of 
outer 
context
a
a
Use for some 
crosscutting concern, 
for example to add 
logging.
a.apply {
... }
a
-
a
Use for 
postconstruction 
object forming.
a.let {
... }
this
of 
outer 
context
a
Last expression
Use for 
transformations.
a.run {
... }
a
-
Last expression
Do some computation 
using an object, with 
only side effects. For c 
better clarity, don’t use 
what it returns.
with(a) {
... }
a
-
Last expression
Group operations on 
an object. For better 
clarity, don’t use what 
it returns.
Using scoping functions greatly improves the expressiveness of your code. I use them often 
in this book.

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277
CHAPTER 10: Development
Nullability
Kotlin addresses the problem of nullability on a language level, to avoid annoying 
NullPointerException
throws. For any variable or constant, the assignment of 
null
values 
is not allowed by default; you have to explicitly declare nullability by adding a 
?
at the end as 
follows:
var name:String? = null
The compiler then knows that 
name
from the example can be 
null
and takes various 
precautions to avoid 
NullPointerException
s. You, for example, cannot write 
name.
toUpperCase()
, but you have to use 
name?.toUpperCase()
instead, which does the 
capitalization only if 
name
is not 
null
and otherwise returns 
null
itself.
Using the scoping functions we described earlier, there is an elegant method to avoid 
constructs like 
if( x != null ) { ... }
. You can instead write the following:
x?.run {
...
}
This does the same but is more expressive; by virtue of the 
?.
, the execution of 
run{}
happens only if 
x
is not 
null
.
The 
elvis
operator 
?:
is also quite useful because it handles cases where you want to 
calculate an expression only if the receiver variable is 
null
, as follows:
var x:String? = ...
...
var y:String = x ?: "default"
This is the same as 
String y = (x != null) ? x : "default");
in Java.
Data Classes
Data classes are classes whose responsibility is to carry structured data. Actually doing 
something with the data inside the data class usually is not necessary or at least not 
important.
The declaring of data classes in Kotlin is easy; all you have to do is write the following:
data class Person(
val fName:String,
val lName:String,
val age:Int)
Or, if you want to use default values for some arguments, use this:
data class Person(
val fName:String="",
val lName:String,
val age:Int=0)

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