Kotlin для Android

Зачем нужен Kotlin для
Android?
• Ограничения языка Java и его проблемы
• Нельзя добавить методы для типов,
определенных платформой либо сторонней
библиотекой (ад из “*Util” классов)
• Проблемы с null
• Отсутсвие немодифицируемых коллекций

Android?
• Android поддерживает только Java 6
• Нет Java 8 Date API
• Нет Stream API для коллекций
• Нет лямбда выражений, ссылок на методы,
замыканий
• Try-with-resource (только для Android 4.4+)
• Проверка на null повсюду

Android?
• Проблемы дизайна Android API
• Сложная иерархия
• NPE повсюду
• Сложное в использование API

Variables
val sample: String = "Sample String"

Variables
val sample: String = "Sample String” 
val sample = "Sample String”
val sample: List<String> = ArrayList()

Variables
val sample: String = null
COMPILE ERROR

Variables
val sample: String? = null
sample?.trim()
sample!!.trim()

Lazy Variables
class SampleFragment : Fragment() {
val margin: Int by lazy
{ resources.getDimensionPixelSize(R.dimen.std_margin) }
}

String Templates
val student = Student(1, "Ivan Ivanov", Date()) 
println("Student '${student.name}' is on ${student.course} course.")

Functions
fun sum(a: Int, b: Int): Int { 
return a + b 
}

Single-Expression Functions
fun sum(a: Int, b: Int): Int = a + b

Single-Expression Functions
fun sum(a: Int, b: Int) = a + b

Extension Functions
public final class CalendarUtils { 
 
public static boolean isMonday(Calendar calendar) { 
return calendar.get(Calendar.DAY_OF_WEEK) ==
Calendar.MONDAY; 
} 
}
Calendar calendar = Calendar.getInstance(); 
boolean isMonday = CalendarUtils.isMonday(calendar);

Extension Functions
fun Calendar.isMonday(): Boolean { 
return get(Calendar.DAY_OF_WEEK) == Calendar.MONDAY 
}
val calendar = Calendar.getInstance()
val isMonday = calendar.isMonday()

Extension Functions
fun Calendar.isMonday() =
get(Calendar.DAY_OF_WEEK) == Calendar.MONDAY
val calendar = Calendar.getInstance()
val isMonday = calendar.isMonday()

Extension Functions
fun View.setPaddingTop(padding: Int) { 
setPadding(paddingLeft,
padding,
paddingRight,
paddingBottom) 
}

Extension Functions
String name = null; 
int nameColumnIndex = 
cursor.getColumnIndexOrThrow("nameColumn"); 
if (!cursor.isNull(nameColumnIndex)) { 
name = cursor.getString(nameColumnIndex); 
}
fun Cursor.getStringOrNull(columnName: String): String? { 
val nameColumnIndex = getColumnIndexOrThrow(columnName) 
return if (isNull(nameColumnIndex)) null else getString(nameColumnIndex) 
} 
 
fun Cursor.getString(columnName: String) = getStringOrNull(columnName)!!

Extension Functions
fun Cursor.getStringOrNull(columnName: String): String? { 
val nameColumnIndex = getColumnIndexOrThrow(columnName) 
return if (isNull(nameColumnIndex)) null else getString(nameColumnIndex) 
} 
 
fun Cursor.getString(columnName: String) = getStringOrNull(columnName)!
val firstName = cursor!!.getString("nameColumn")

Function Expressions
{ it.toString() }
{ x, y -> x + y }
{ x: Int, y: Int -> x + y }
val sum: (Int, Int) -> Int = { x, y -> x + y }
val sum: (Int, Int) -> Int = { x: Int, y: Int -> x + y }

High-order Functions
public fun <T> Array<T>.filter(predicate: (T) -> Boolean): Array<T> { 
return … 
}
val versions = arrayOf("Gingerbread", “Honeycomb", 
"Ice Cream Sandwich”,"Jelly Bean", "Lollipop") 
val versionsWithBigLength = versions.filter { it.length > 10 }

High-order Functions
fun String.validateWith(predicate: (String) -> Boolean) =
predicate(this)
val validLength: (String) -> Boolean = { it.length > 10 } 
"Kirill".validateWith(validLength)

Extension Function Expressions
• Extension Function - Функция, которая
добавляется к типу не модифицируя его.
• Function Expression - Необъявляемая функция,
которую можно использовать в качестве
выражения (например для передачи параметров
в метод)
• High-order function - Функция, которую
используют другую функцию как параметр либо
возвращает функция

fun SQLiteDatabase.transact(
transaction: SQLiteDatabase.() -> Unit) { 
beginTransaction(); 
try { 
transaction() 
setTransactionSuccessful() 
} finally { 
endTransaction() 
} 
}

fun SQLiteDatabase.transact(
try { 
transaction() 
} finally { 
endTransaction() 
} 
}
val names = arrayOf("Ivan Invanov") 
db.transact { delete("students", "name = ?", names) }

inline fun SQLiteDatabase.transact(
try { 
transaction() 
} finally { 
endTransaction() 
} 
}

val names = arrayOf("Ivan Invanov") 
db.transact { delete("students", "name = ?", names) }
+
inline fun
=
db.beginTransaction(); 
try { 
db.transaction() 
db.setTransactionSuccessful() 
} finally { 
db.endTransaction() 
}

Default Arguments
fun setViewVisible(visible: Boolean, 
animate: Boolean = true) { 
//... 
}

public class Student { 
private int course; 
private String name; 
private final Date birthday; 
 
public Student(int course, @NotNull String name, @NotNull Date birthday) { 
this.course = course; 
this.name = name; 
this.birthday = birthday; 
} 
 
public int getCourse() { return course; } 
public void setCourse(int course) { this.course = course; } 
@NotNull public String getName() { return name; } 
public void setName(@NotNull String name) { this.name = name; } 
@NotNull public Date getBirthday() { return birthday; } 
 
@Override 
public boolean equals(Object o) { 
if (this == o) return true; 
if (o == null || getClass() != o.getClass()) return false; 
 
Student student = (Student) o; 
return course == student.course 
&& name.equals(student.name) 
&& birthday.equals(student.birthday); 
} 
 
@Override 
public int hashCode() { 
int result = course; 
result = 31 * result + name.hashCode(); 
result = 31 * result + birthday.hashCode(); 
return result; 
} 
}

Kotlin class
class Student(var course: Int,
var name: String,
val birthday: Date) { 
 
override fun equals(other: Any?): Boolean { 
if (this === other) return true 
if (other?.javaClass != javaClass) return false 
 
other as Student 
return course == other.course 
&& name == other.name 
&& birthday != other.birthday 
} 
 
override fun hashCode(): Int { 
var result = course 
result += 31 * result + name.hashCode() 
result += 31 * result + birthday.hashCode() 
return result 
} 
}

Data class
data class Student(var course: Int, 
var name: String, 
val birthday: Date)

Singleton
public final class Singleton { 
 
private static Singleton instance; 
 
public static Singleton getInstance() { 
if (instance == null) { 
synchronized (Singleton.class) { 
if (instance == null) { 
instance = new Singleton(); 
} 
} 
} 
return instance; 
} 
 
public void sayHello() { 
System.out.println("Hello!"); 
} 
 
private Singleton() {} 
}

Object
object Singleton { 
fun sayHello() = println("Hello!") 
}

Коллекции
val names = listOf<String>(..) // Список только для чтения 
names 
.filter { it.startsWith("A") } 
.sortedBy { it } 
.map { it.toUpperCase() } 
.forEach { print(it) }

Что еще
• Перегрузка операторов
• Type smart cast
• Range
• Расширенное API рефлексии
• Делегирование классов
• Делегирование свойств (Lazy, Observable и др.)
• Destructuring declarations
• Named arguments in function
• Tail recursive functions
• Нет проверяемых исключений

Полезные ссылки
• kotlinlang.org 
Официальный сайт языка
• try.kotlinlang.org 
Онлайн IDE
• github.com/JetBrains/kotlin 
Исходны код проекта (лицензия Apache 2.0)

Kotlin для Android

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