scala - 谁能给我一个关于 "What makes an object stateful"这种情况的好例子？

Question

我不明白这句话的意思(来自 Scala-Threading/Odersky/18-stateful-objects.txt line 88 ):

a class might be stateful without defining or inheriting any vars because it forwards method calls to other objects that have mutable state.

谁能给我一个很好的例子来说明 Scala 中的这种情况？

Answer 1

class Account {
  private var balance = 0
  def getBalance = balance
  def deposit(amount: Int): Unit = balance += amount
}

class NoVarAccount {
  val account = new Account()
  def balance = account.getBalance
  def deposit(amount: Int) = account.deposit(amount)
}

现在，NoVarAccount没有任何var在其中，但它仍然是有状态的，因为它将调用转发到 Account这确实是有状态的。

事实上，您无法保证调用 balance在同一个对象上两次会得到相同的结果。

val account = new NoVarAccount()
account.balance // 0
account.deposit(42)
account.balance // 42

在本例中，account.balance引用不透明，即不能替换 account.balance与它的返回值，因为它可能会有所不同。

相反，无状态帐户将如下所示:

class StatelessAccount(val balance: Int = 0) {
  def deposit(amount: Int) = new StatelessAccount(balance + amount)
}

甚至更惯用:

case class StatelessAccount(balance: Int = 0) {
  def deposit(amount: Int) = this.copy(balance = balance + amount))
}

在这种情况下 balance是参照透明的:

val account = StatelessAccount()
account.balance // 0
val account2 = account.deposit(42)
account2.balance // 42
account.balance // still 0