scala上界

Variance refers as how subtyping between complex types relates to subtypes of their components. Scala supports variances annotations of type parameters of a generic class.

差异是指复杂类型之间的子类型如何与其组成部分的子类型相关。 Scala支持泛型类的类型参数的方差注释。

The type of variance annotations supported in Scala are;

Scala支持的方差注释的类型是；

class Array[+X] {  def add[Y >: X](elem: Y): Array[Y] = new Array[Y] {    override def first: Y = elem    override def retrieve: Array[Y] = Array.this    override def toString() = elem.toString() + "\n" + Array.this.toString()  }  def first: X = sys.error("No elements in the Array")  def retrieve: Array[X] = sys.error("Array is empty")  override def toString() = ""}object ArrayTest extends App {  var a: Array[Any] = new Array().add("US");  //a = a.add(new Object())  a = a.add(56)  a = a.add(67.89)  println("Array elements added are: " + a)}

The annotation +X indicates that X can be used in covariant positions only. The annotation -X indicates that X can be used in contravariant positions only. Array[X] is a subtype of S if X is a subtype of S.

注释+ X表示X只能在协变位置使用。 注释-X表示X只能在相反位置使用。 如果X是S的子类型，则Array [X]是S的子类型。

In this example we are defining a covariant type parameter X to define the method add. We have a polymorphic method in which we use the element type X as a lower bound of add method type variable thereby bringing the variance of X in sync with its declaration as covariant parameter. We are inserting elements of String,Integer and float types.

在此示例中，我们定义了协变类型参数X来定义方法add。 我们有一个多态方法，其中我们使用元素类型X作为add方法类型变量的下限，从而使X的方差与其声明为协变参数同步。 我们正在插入String，Integer和float类型的元素。

上限类型界限 (Upper type Bounds)

An upper type bound X <: B declares that type variable X refers to a subtype of type B.

类型上限X <：B声明类型变量X引用类型B的子类型。

Consider an example below.

考虑下面的示例。

trait Employee {  def EmployeeIdexists(x: Any): Boolean}case class EId(a: Int) extends Employee {  def EmployeeIdexists(x: Any): Boolean =    x.isInstanceOf[EId] &&      x.asInstanceOf[EId].a == a}object TestEmployee extends App {  def findEId[A <: Employee](d: A, ls: List[A]): Boolean =    if (ls.isEmpty) false    else if (d.EmployeeIdexists(ls.head)) true    else findEId[A](d, ls.tail)  val elist: List[EId] = List(EId(25), EId(26), EId(27), EId(28))  if ((findEId[EId](EId(28), elist)))    println("Employee Id exists")  else println("Employee Id does not exist")}

This example find whether the employee id exists or not. We are defining a method findEId which creates an integer list and checks whether the id specified is contained in the list.

本示例查找员工ID是否存在。 我们正在定义一个findEId方法，该方法创建一个整数列表并检查指定的ID是否包含在列表中。

下界 (Lower type Bounds)

The term X >: B expresses that the type parameter X or the abstract type X refer to a supertype of type B. Lower type bounds declare a type to be a supertype of another type.

术语X>：B表示类型参数X或抽象类型X引用类型B的超类型。类型下限声明一个类型为另一种类型的超类型。

Create the scala class ListNode as;

将scala类ListNode创建为；

case class ListNode[+T](h: T, t: ListNode[T]) {  def head: T = h  def tail: ListNode[T] = t  def prepend[U >: T](elem: U): ListNode[U] =    ListNode(elem, this)}

We are creating a class ListNode and defining the prepend method where T only appears in covariant positions. U >: T specifies the abstract type U is a supertype of T.

我们正在创建一个类ListNode并定义prepend方法，其中T仅出现在协变位置。 U>：T指定抽象类型U是T的超类型。

The type variable T appears as a parameter type of method prepend, this rule is broken. With the help of a lower type bound, though, we can implement a prepend method.

类型变量T作为方法前缀的参数类型出现，此规则已损坏。 但是，借助于下限类型的边界，我们可以实现prepend方法。

Now create a scala object as;

现在创建一个scala对象为；

object LowerBoundTest extends App {  val empty: ListNode[Null] = ListNode(null, null)  val strList: ListNode[String] = empty.prepend("hello")    .prepend("world")  val anylist: ListNode[Any] = strList.prepend(12345)  println(strList)  println(anylist)}

That’s all for now, we will look into more scala features in future posts.

到目前为止，仅此而已，我们将在以后的文章中研究更多scala功能。

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scala上界