正如我在我的评论中所说，收集应该做你想要的：

As I state in my comment, collect should do what you want:

list.collect{
  case x if x % 2 == 0 => x*2
}

收集方法允许您同时指定匹配元素（ filter ）的条件，并修改匹配的值（ map ）

The collect method allows you to both specify a criteria on the matching elements (filter) and modify the values that match (map)

而且，在@TravisBrown建议的情况下，您也可以使用 flatMap ，特别是在条件为更复杂，不适合作为防卫条件。这样的例子如下：

And as @TravisBrown suggested, you can use flatMap as well, especially in situations where the condition is more complex and not suitable as a guard condition. Something like this for your example:

list.flatMap{
  case x if x % 2 == 0 => Some(x*2)
  case x => None
}