一、伴生类和伴生对象

object ApplyApp {

}

class ApplyTest{

}

object ApplyTest{

｝

class ApplyTest 是 object ApplyTest 伴生类
object ApplyTest 是 class ApplyTest 伴生对象

二、初探 Apply

object ApplyApp {
  def main(args: Array[String]): Unit = {
    for (i <- 1 to 10){
      ApplyTest.incr // 直接调用ApplyTest，就是Object对象
    }
    println(ApplyTest.count) // 结果10，说明object 就是一个单例对象
  }
}

class ApplyTest{

}

object ApplyTest{
  println("Object ApplyTest enter...")

  var count = 0
  def incr = {
    count = count + 1
  }

  println("Object ApplyTest leave...")
}

输出

Object ApplyTest enter...
Object ApplyTest leave...
10

三、Apply 最佳实践

直接可以调用的就是对象：val b = ApplyTest() // ==> Object.apply
需要new，就是类： val c = new ApplyTest

object ApplyApp {
  def main(args: Array[String]): Unit = {

    // 直接调用的是 Object.apply，这里不需要再new了，因为在Object里面的apply已经new 了
    val b = ApplyTest() // ==> Object.apply

    println("--------------------------------------")
    val c = new ApplyTest
    println(c)
    c()

  }
}

class ApplyTest{
  def apply() = {
    println("class ApplyTest apply...")
  }
}

object ApplyTest{
  println("Object ApplyTest enter...")

  var count = 0
  def incr = {
    count = count + 1
  }

  // 最佳实践：在Object的apply方法中去 new Class
  def apply() = {
    println("Object ApplyTest apply...")

    // 在object中的Apply中new class
    new ApplyTest

  }

  println("Object ApplyTest leave...")
}

输出

Object ApplyTest enter...
Object ApplyTest leave...
Object ApplyTest apply...
--------------------------------------
com.hhcycj.scala.object_04.ApplyTest@735b5592
class ApplyTest apply...