如果密钥不存在，添加或更新映射密钥的最佳做法是什么

Question

我想知道如果密钥不存在，添加或更新地图密钥的最佳做法是什么。 例如，这段代码将抛出异常。

val states = scala.collection.mutable.Map[Int, String]()
states(1) = "Alaska"
states(2) = states(2) + " A Really Big State" // throws null pointer exeption

由于

Answer 1

要更新条目是否缺席，您可以执行以下操作：

states.getOrElseUpdate(2, " A Really Big State")

这是一个如何运作的例子

val states = scala.collection.mutable.Map[Int, String]()
val empty = states.get(2) // empty == None
val bigState = states.getOrElseUpdate(2, "A Really Big State") // bigState == A Really Big State
val stillBigState = states.getOrElseUpdate(2, "An even bigger state") // stillBigState == A Really Big State

Answer 2

这应该可以解决问题：

val states = scala.collection.mutable.Map[Int, String]()
states(1) = "Alaska"
states.get(2) match {
  case Some(e) => states.update(2, e + "A really Big State")
  case None => states.put(2, "A really Big State")
}