You can do better with Kotlin

Svetlana Isakova
You can do better with Kotlin

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- modern
- pragmatic
- Android-friendly
Kotlin Programming
Language

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Ofﬁcial on Android

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Not only Android

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Multiplatform projects

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Pragmatic
- tooling
- Java interop

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From

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has good tooling

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- completion
- navigation
- refactorings
- inspections
…

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can be easily mixed
with Java code

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*.java
*.class
*.dex
compiled to Java bytecode
*.kt

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Kotlin code
Java code
You can have Java &
Kotlin code in one project

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You can gradually add
Kotlin to your existing app

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Android-friendly

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Android Studio
is based on
IntelliJ IDEA

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just another library
for your app
rxjava-2.1.2
kotlin-stdlib-1.1.4 6315
10212

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No Kotlin SDK
…just JDK + extensions
small runtime jar easy Java interop

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Modern
- concise
- safe
- expressive

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concise

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public class Person { 
private final String name; 
private final int age; 
 
public Person(String name, int age) { 
this.name = name; 
this.age = age; 
} 
 
public String getName() { 
return name; 
} 
 
public int getAge() { 
return age; 
} 
}

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- equals
- hashCode
- toString
 
data  
class Person(val name: String, val age: Int)

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this.age = age; 
} 
 
return name; 
} 
 
return age; 
} 
}
 
class Person(
val name: String,
val age: Int
)
person.name
person.getName()

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class Person(
val name: String,
val age: Int
)
person.getName()

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this.age = age; 
} 
 
return name; 
} 
 
return age; 
} 
}
person.name

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public void updateWeather(int degrees) { 
String description; 
Colour colour; 
if (degrees < 5) { 
description = "cold"; 
colour = BLUE; 
} else if (degrees < 23) { 
description = "mild"; 
colour = ORANGE; 
} else { 
description = "hot"; 
colour = RED; 
} 
// ... 
}
enum Colour { BLUE, ORANGE, RED, /*...*/; }

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fun updateWeather(degrees: Int) { 
val description: String 
val colour: Colour 
description = "cold" 
colour = BLUE 
description = "mild" 
colour = ORANGE 
} else { 
description = "hot" 
colour = RED 
} 
// ... 
}

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val (description: String, colour: Colour) = 
Pair("cold", BLUE) 
Pair("mild", ORANGE) 
} else { 
Pair("hot", RED) 
}
// ... 
}

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val (description, colour) = 
Pair("cold", BLUE) 
Pair("mild", ORANGE) 
} else { 
Pair("hot", RED) 
}
// ... 
}

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val (description, colour) = when { 
degrees < 5 -> Pair("cold", BLUE) 
degrees < 23 -> Pair("mild", ORANGE) 
else -> Pair("hot", RED) 
}
// ... 
}

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degrees < 5 -> "cold" to BLUE 
degrees < 23 -> "mild" to ORANGE 
else -> "hot" to RED 
} 
}

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degrees < 5 -> "cold" to BLUE 
degrees < 23 -> "mild" to ORANGE 
else -> "hot" to RED 
}
String description; 
Colour colour;
description = "cold"; 
colour = BLUE; 
description = "mild"; 
colour = ORANGE; 
} else { 
description = "hot"; 
colour = RED; 
}

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safe

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Billion Dollar Mistake

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Modern approach:
to make NPE
compile-time error,
not run-time error

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Nullable types in Kotlin
val s1: String = "always not null"
 
val s2: String? = null
s1.length ✓
✗
s2.length
"can be null or non-null"
null ✗

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val s: String?
if (s != null) { 
s.length 
}
Dealing with Nullable Types

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s?.length
val s: String?
Dealing with Nullable Types

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val length = if (s != null) s.length else null
val s: String?
Nullability operators
val length = s?.length

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val length = if (s != null) s.length else null
val s: String?
val length: Int? = s?.length

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val length = if (s != null) s.length else 0
val s: String?
val length: Int = s?.length ?: 0

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val s: String?
if (s == null) fail() 
s.length
Control-ﬂow analysis

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val s: String?
Making NPE explicit
s!!
throws NPE if s is null
s!!.length

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Nullable Types Under the Hood
No performance overhead
@Nullable, @NotNull annotations

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Annotate your Java types in Kotlin
Type
behaves like
regular Java type
@Nullable
@NotNull Type
Type?
Type
Type
@ParametersAreNonnullByDefault
@MyNonnullApi
Type/Type?

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class Optional(val value: T) {
 
fun isPresent() = value != null 
 
fun get() = value ?: 
throw NoSuchElementException("No value present")
}
Nullable types ≠ Optional

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expressive

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you can avoid any repetition
you can make the code look nicer
you can create API looking like DSL
expressive

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Extension Functions

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fun String.lastChar() = get(length - 1)
this can be omitted
Extension Functions
fun String.lastChar() = this.get(this.length - 1)

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import com.example.util.lastChar
import com.example.util.*
Extension Functions
val c: Char = "abc".lastChar()

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Calling Extension Functions
from Java code
StringExtensions.kt
char c = StringExtensionsKt.lastChar("abc");
JavaClass.java
import static StringExtensionsKt.lastChar;
char c = lastChar("abc");

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No. Because it’s a regular
static method under the hood.
Extension Functions
Is it possible to call a private
member of String here?

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Extensions for Android
Toast.makeText(this, "Thank you!”, Toast.LENGTH_SHORT).show()
Activity
extension function
on Activity
toast("Thank you!")
this.toast("Thank you!")

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this.startActivity(“ANSWER" to 42)
val intent = Intent(this, NewActivity::class.java) 
intent.putExtra("ANSWER", 42) 
startActivity(intent)
Extensions for Android

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infix fun A.to(that: B) = Pair(this, that)
"ANSWER".to(42)
"hot" to RED
mapOf(0 to "zero", 1 to "one")
The to extension function

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Lambdas

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Lambdas
button.addActionListener { println("Hi") }

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{ employee: Employee -> employee.city == City.PRAGUE }
What’s an average
age of employees
working in Prague?
Working with collections with Lambdas
val employees: List
employees.filter { it.city == City.PRAGUE }.map { it.age }.average()
data class Employee(
val city: City, val age: Int
)

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What’s an average
age of employees
working in Prague?
Working with collections with Lambdas
val employees: List
data class Employee(
val city: City, val age: Int
)
extension functions
employees.filter { it.city == City.PRAGUE }.map { it.age }.average()

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Kotlin library: extensions on collections
• filter
• map
• reduce
• count
• find
• any
• flatMap
• groupBy
• …

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Under the Hood
No performance overhead
Lambdas can be inlined

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Extension Function & Lambda
Lambda with receiver

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val sb = StringBuilder() 
with (sb) { 
appendln("Alphabet: ") 
for (c in 'a'..'z') { 
append(c) 
}
toString() 
}
The with function
with is a function
sb.appendln("Alphabet: ") 
for (c in 'a'..'z') { 
sb.append(c) 
} 
sb.toString() 

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lambda
is its
second
argument
with (sb) { 
this.appendln(“Alphabet: ") 
for (c in 'a'..'z') { 
this.append(c) 
}
this.toString() 
}
with (sb, { -> 
this.appendln(“Alphabet: ") 
for (c in 'a'..'z') { 
this.append(c) 
}
this.toString() 
})
lambda
is its
second
argument
with is a function
this is
an implicit receiver
in the lambda
with (sb) { 
for (c in 'a'..'z') { 
append(c) 
}
toString() 
}
this can be omitted

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with (sb) { 
... 
}
inline fun with(
receiver: T,
block: T.() -> R
): R = receiver.block()
The with function declaration

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with (sb) { 
for (c in 'a'..'z') { 
this.append(c) 
} 
}
lambda with
implicit this

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html { 
table { 
for (product in products) { 
tr { 
td { text(product.description) } 
td { text(product.price) } 
td { text(product.popularity) } 
} 
} 
} 
}
HTML Builders
lambdas with receiver

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val db: SQLiteDatabase = …
db.beginTransaction() 
try { 
db.delete("users", "first_name = ?", arrayOf("Jake")) 
db.setTransactionSuccessful() 
} finally { 
db.endTransaction() 
}
db.inTransaction { 
delete("users", "first_name = ?", arrayOf("Jake")) 
}
Avoiding Duplication

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db.beginTransaction() 
try { 
db.delete("users", "first_name = ?", arrayOf("Jake")) 
db.setTransactionSuccessful() 
} finally { 
db.endTransaction() 
}
db.inTransaction { 
delete("users", "first_name = ?", arrayOf("Jake")) 
}
Inline functions
is declared as
inline function
generated bytecode is similar to

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ANKO-DSL
DSL for dynamic layouts

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Alerts
fun Activity.showAreYouSureAlert(process: () -> Unit) { 
alert(title = "Are you sure?", 
message = "Are you really sure?") { 
positiveButton("Yes") { process() } 
negativeButton("No") { } 
}.show() 
}
Are you sure?
Are you really sure?
No Yes

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customView { 
verticalLayout { 
val email = editText { 
hint = "Email" 
} 
 
val password = editText { 
hint = "Password" 
transformationMethod =
PasswordTransformationMethod.getInstance() 
} 
 
positiveButton("Log In") { 
logIn(email.text, password.text) 
} 
} 
}
Password
Log In
Email
Custom layouts

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You can do better with Kotlin

You can do better with Kotlin

Svetlana Isakova

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