Scala 中的私有和受保护构造函数
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Private and protected constructor in Scala
提问by Don Mackenzie
I've been curious about the impact of not having an explicit primary constructor in Scala, just the contents of the class body.
我一直很好奇在 Scala 中没有显式主构造函数的影响，只有类体的内容。
In particular, I suspect that the private or protected constructor pattern, that is, controlling construction through the companion object or another class or object's methods might not have an obvious implementation.
特别是，我怀疑私有或受保护的构造函数模式，即通过伴随对象或另一个类或对象的方法控制构造可能没有明显的实现。
Am I wrong? If so, how is it done?
我错了吗？如果是这样，它是如何完成的？
回答by Aleksander Kmetec
You can declare the default constructor as private/protected by inserting the appropriate keyword between the class name and the parameter list, like this:
您可以通过在类名和参数列表之间插入适当的关键字来将默认构造函数声明为私有/受保护的，如下所示：
class Foo private () {
/* class body goes here... */
}
回答by Daniel C. Sobral
Aleksander's answer is correct, but Programming in Scalaoffers an additional alternative:
Aleksander的回答是正确的，但Programming in Scala提供了另一种选择：
sealed trait Foo {
// interface
}
object Foo {
def apply(...): Foo = // public constructor
private class FooImpl(...) extends Foo { ... } // real class
}
