Case Class vs Class

Reason is the light and the light of life.

Jerry Su Apr 17, 2019 Apr 17, 2019 2 mins
programming scala

定义

  • class的定义:
class BankAccount {
  def deposit(amount: Int): Unit = {
    if (amount > 0) balance = balance + amount
  }
  • case class的定义:
case class Note(name: String, duration: String, octave: Int)
  • 创建BankAccount和Note的实例：
val aliceAccount = new BankAccount()
val c3 = Note("C", "Quarter", 3)
  1. case class类实例化不需要new, case class有一个默认的apply方法来负责对象的创建。
  2. 创建带参case class时，参数时val类型的。 e.g： c3.name = ‘Jerry’ //does not compile

比较

val aliceAccount = new BankAccount
val bobAccount = new BankAccount

// aliceAccount == bobAccount shouldBe False

val c3 = Note("C", "Quarter", 3)
val cThree = Note("C", "Quarter", 3)

// c3 == cThree shouldBe True
  • 在Scala中，默认情况下，比较对象将比较它们的引用，即**按引用比较**。
  • 但在case class实例的情况下，重新定义相等性以比较聚合信息的值，即**按值比较**。

pattern matching

  • pattern matching不适用于class
  • 用pattern matching从case class实例中抽取信息
c3 match {
  case Note(name, duration, octave) => "The duration of c3 is duration"
}

继承

class可继承，case class不可继承（因为不可能正确地实现它们的相等）

case class的代码实现

case class只是class的一个特例，目的是将多个值聚合为一个单值。Scala显示的支持case class是因为在实践中常用。
当我们在定义一个case class时，编译器实际定义了一个使用更多方法和伴随对象的增强class。

e.g:

case class Note(name: String, duration: String, octave: Int)

编译器实际定义：

class Note(_name: String, _duration: String, _octave: Int) extends Serializable {  // Note class

  // Constructor parameters are promoted to members
  val name = _name
  val duration = _duration
  val octave = _octave

  // Equality redefinition
  override def equals(other: Any): Boolean = other match {
    case that: Note =>
      (that canEqual this) &&
        name == that.name &&
        duration == that.duration &&
        octave == that.octave
    case _ => false
  }

  def canEqual(other: Any): Boolean = other.isInstanceOf[Note]

  // Java hashCode redefinition according to equality
  override def hashCode(): Int = {
    val state = Seq(name, duration, octave)
    state.map(_.hashCode()).foldLeft(0)((a, b) => 31 * a + b)
  }

  // toString redefinition to return the value of an instance instead of its memory addres
  override def toString = "Note(name,duration,octave)"

  // Create a copy of a case class, with potentially modified field values
  def copy(name: String = name, duration: String = duration, octave: Int = octave): Note =
    new Note(name, duration, octave)

}

object Note {  // 伴随对象

  // Constructor that allows the omission of the `new` keyword
  def apply(name: String, duration: String, octave: Int): Note =
    new Note(name, duration, octave)

  // Extractor for pattern matching
  def unapply(note: Note): Option[(String, String, Int)] =
    if (note eq null) None
    else Some((note.name, note.duration, note.octave))

}

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