Shulou(Shulou.com)06/02 Report--

Big data, a good programmer, shares the generics of the Scala series. Classes with one or more type parameters are generic.

Definition of generic classes:

/ / Class definition with type parameter A

Class Stack [A] {

Private var elements: List [A] = Nil

/ / generic method

Def push (x: a) {elements = x:: elements}

Def peek: a = elements.head

Def pop (): a = {

Val currentTop = peek

Elements = elements.tail

CurrentTop

}

}

For the use of generic classes, the type parameter An is replaced by a specific type.

Val stack = new Stack [Int]

Stack.push (1)

Stack.push (2)

Println (stack.pop) / / prints 2

Println (stack.pop) / / prints 1

1. Covariation

Define a type List [+ A], if An is covariant, meaning: for type An and BMagee An are subtypes of B, then list [A] is a subtype of List [B].

Abstract class Animal {

Def name: String

}

Case class Cat (name: String) extends Animal

Case class Dog (name: String) extends Animal

The Scala standard library has a generic class sealed abstract class List [+ A], because the type parameters in it are covariant, so the following program is called successfully.

Object CovarianceTest extends App {

/ / define parameter type list [Animal]

Def printAnimalNames (animals: List [Animal]): Unit = {

Animals.foreach {animal = >

Println (animal.name)

}

}

Val cats: List [Cat] = List (Cat ("Whiskers"), Cat ("Tom"))

Val dogs: List [Dog] = List (Dog ("Fido"), Dog ("Rex"))

/ / input parameter type is List [Cat]

PrintAnimalNames (cats)

/ / Whiskers

/ / Tom

/ / input parameter type is List [Dog]

PrintAnimalNames (dogs)

/ / Fido

/ / Rex

}

two。 Inverse transformation

Define a type Writer [- A], if An is inverted, meaning: for type An and BMagee An is a subtype of B, then Writer [B] is a subtype of Writer [A].

Abstract class Animal {

Def name: String

}

Case class Cat (name: String) extends Animal

Case class Dog (name: String) extends Animal

Define the print information class that operates corresponding to the above class

Abstract class Printer [- A] {

Def print (value: a): Unit

}

Class AnimalPrinter extends Printer [Animal] {

Def print (animal: Animal): Unit =

Println ("The animal's name is:" + animal.name)

}

Class CatPrinter extends Printer [Cat] {

Def print (cat: Cat): Unit =

Println ("The cat's name is:" + cat.name)

}

Inversion test

Object ContravarianceTest extends App {

Val myCat: Cat = Cat ("Boots")

/ / define the parameter type as Printer [Cat]

Def printMyCat (printer: Printer [Cat]): Unit = {

Printer.print (myCat)

}

Val catPrinter: Printer [Cat] = new CatPrinter

Val animalPrinter: Printer [Animal] = new AnimalPrinter

PrintMyCat (catPrinter)

/ / you can pass the parameter type as Printer [Animal]

PrintMyCat (animalPrinter)

}

3. Upper bound

Upper bound definition: t: B] (elem: U): Node [U]

}

Case class ListNode [+ B] (h: B, t: Node [B]) extends Node [B] {

Def prepend [U >: B] (elem: U): ListNode [U] = ListNode (elem, this)

Def head: B = h

Def tail: Node [B] = t

}

Case class Nil [+ B] () extends Node [B] {

Def prepend [U >: B] (elem: U): ListNode [U] = ListNode (elem, this)

}

test

Trait Bird

Case class AfricanSwallow () extends Bird

Case class EuropeanSwallow () extends Bird

Val africanSwallowList= ListNode [AfricanSwallow] (AfricanSwallow (), Nil ())

Val birdList: Node [Bird] = africanSwallowList

BirdList.prepend (new EuropeanSwallow)

5 event horizon (view bounds)

Note: it is out of date, just understand it.

Horizon definition: a

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.