scala apply是什么

apply使得创建实例时无需new.采用该机制,创建对象就等同于调用apply方法~

object Person {
  def apply(name: String, age: Integer, favColor: String): Person = new Person(name, age, favColor)
}

class Person(name: String, age: Integer, favColor: String) {
def run(): Unit ={
  println(s"$name is running")
}
}
object Test extends App{
//不需要new关键字,传给Person的参数等价于传给了aplly方法
  val p=Person("xiaohong",23,"red")
  p.run()//xiaohong is running
}

为了理解,我们加上new关键字,此时调用的是Person的构造方法,而不是apply方法

object Person {
  def apply(name: String, age: Integer, favColor: String): Person = new Person(name, age, favColor)
}

class Person(name: String, age: Integer, favColor: String) {
def run(): Unit ={
  println(s"$name is running")
}
}
object Test extends App{
//此时调用的是Person的构造方法,而不是apply方法
  val p=new Person("xiaohong",23,"red")
  p.run()//xiaohong is running
}

apply方法也可以显示调用,一般不这么做,这里是为了演示原理

object Person {
  def apply(name: String, age: Integer, favColor: String): Person = new Person(name, age, favColor)
}

class Person(name: String, age: Integer, favColor: String) {
def run(): Unit ={
  println(s"$name is running")
}
}
object Test extends App{
//显式调用apply
  val p=Person.apply("xiaohong",23,"red")
  p.run()//xiaohong is running
}

删掉apply方法,不用new创建对象会报错

让我们把class加上case关键字,此时不报错了. 说明系统自动创建了一个参的apply方法!!这是关键. 就是说如果是case class,系统会自动在其伴生对象中创建apply方法!

object Person {
}

case class Person(name: String, age: Integer, favColor: String) {
def run(): Unit ={
  println(s"$name is running")
}
}
object Test extends App{
  val p=Person("xiaohong",23,"red")
  p.run()//xiaohong is running
}

apply在源码中的应用

比如创建Array

object ArrayDemo extends App{
  val a=Array(1,2,5)
}

参数实际上传给了object Array的apply方法!

def apply(x: Int, xs: Int*): Array[Int] = {
    val array = new Array[Int](xs.length + 1)
    array(0) = x
    var i = 1
    for (x <- xs.iterator) { array(i) = x; i += 1 }
    array
  }

创建List同理,实际调用的是apply方法

object ArrayDemo extends App{
  val a=scala.collection.immutable.List(1,2,5)
}

override def apply[A](xs: A*): List[A] = xs.toList