learning scala generic classes

package com.aura.scala.day01

object genericClasses {
  def main(args: Array[String]): Unit = {
    val stack = new Stack[Int]
    stack.push(1)
    stack.push(2)
    println(stack.pop())
    println(stack.pop())
  }
}

class Stack[A] {
  private var elements: List[A] = Nil
  def push(x: A){ elements = x :: elements }
  def peek():A = elements.head
  def pop():A = {
    val currentTop = peek()
    elements = elements.tail
    currentTop
  }
}

result:

"D:Program FilesJavajdk1.8.0_201injava.exe" "-javaagent:D:Program FilesJetBrainsIntelliJ IDEA 2019.1.3libidea_rt.jar=11061:D:Program FilesJetBrainsIntelliJ IDEA 2019.1.3in" -Dfile.encoding=UTF-8 -classpath "D:Program FilesJavajdk1.8.0_201jrelibcharsets.jar;D:Program FilesJavajdk1.8.0_201jrelibdeploy.jar;D:Program FilesJavajdk1.8.0_201jrelibextaccess-bridge-64.jar;D:Program FilesJavajdk1.8.0_201jrelibextcldrdata.jar;D:Program FilesJavajdk1.8.0_201jrelibextdnsns.jar;D:Program FilesJavajdk1.8.0_201jrelibextjaccess.jar;D:Program FilesJavajdk1.8.0_201jrelibextjfxrt.jar;D:Program FilesJavajdk1.8.0_201jrelibextlocaledata.jar;D:Program FilesJavajdk1.8.0_201jrelibext
ashorn.jar;D:Program FilesJavajdk1.8.0_201jrelibextsunec.jar;D:Program FilesJavajdk1.8.0_201jrelibextsunjce_provider.jar;D:Program FilesJavajdk1.8.0_201jrelibextsunmscapi.jar;D:Program FilesJavajdk1.8.0_201jrelibextsunpkcs11.jar;D:Program FilesJavajdk1.8.0_201jrelibextzipfs.jar;D:Program FilesJavajdk1.8.0_201jrelibjavaws.jar;D:Program FilesJavajdk1.8.0_201jrelibjce.jar;D:Program FilesJavajdk1.8.0_201jrelibjfr.jar;D:Program FilesJavajdk1.8.0_201jrelibjfxswt.jar;D:Program FilesJavajdk1.8.0_201jrelibjsse.jar;D:Program FilesJavajdk1.8.0_201jrelibmanagement-agent.jar;D:Program FilesJavajdk1.8.0_201jrelibplugin.jar;D:Program FilesJavajdk1.8.0_201jrelib
esources.jar;D:Program FilesJavajdk1.8.0_201jrelib
t.jar;C:UsersadminIdeaProjectsScalaDayoutproductionScalaDay;C:Program Files (x86)scalalibscala-actors-2.11.0.jar;C:Program Files (x86)scalalibscala-actors-migration_2.11-1.1.0.jar;C:Program Files (x86)scalalibscala-library.jar;C:Program Files (x86)scalalibscala-parser-combinators_2.11-1.0.4.jar;C:Program Files (x86)scalalibscala-reflect.jar;C:Program Files (x86)scalalibscala-swing_2.11-1.0.2.jar;C:Program Files (x86)scalalibscala-xml_2.11-1.0.4.jar" com.aura.scala.day01.genericClasses
2
1

实例对象 stack 只能接受整型值。然而，如果类型参数有子类型，子类型可以被传入：

class Fruit
class Apple extends Fruit
class Banana extends Fruit

val stack = new Stack[Fruit]
val apple = new Apple
val banana = new Banana

stack.push(apple)
stack.push(banana)