Scala中的python @classmethod或@staticmethod等效于什么？

Question

当我不想使用隐式但希望有一个将（例如） Int转换为MyClass的函数时，这很有用。

Answer 1

In Scala, an object is used to define static methods. What you are describing sounds like you want this:

class MyClass(n: Int)

object MyClass {
  def apply(n: Int) = new MyClass(n)
}

So that you can do either of these:

scala> val a = MyClass.apply(3)
a: MyClass = MyClass@70d1c9b5

scala> val b = MyClass(3)       // works allowed because method is called "apply"
b: MyClass = MyClass@1b4b2db7

I would call this a "factory method".

Answer 2

You can put a method into the companion object to call it in a static way from Scala (ie without an object instance):

// Scala
class A
object A {
  def staticMethod = println("Foo!")
}
A.staticMethod // prints "Foo!"

note that the Scala compiler actually creates a new synthetic class called A$ for the singleton object A , so the line A.staticMethod calls the method on a single instance of A$ which exists at runtime.

Noteworthy: The Scala compiler also creates a static method staticMethod in A on the bytecode level which forwards the call to A$.staticMethod - in case you're mixing Java and Scala code in your project, this means that you will also be able to call this static method from Java like this:

// Java
A.staticMethod(); // works

However, if you declare the method with the same name in the companion class A , while everything still works in the same way in Scala, the static forwarder method in A will not be generated, and you will not be able to call staticMethod from Java:

// Scala
class A {
  def staticMethod = println("I'm doing something else.")
}
object A { // no companion class
  def staticMethod = println("Foo!")
}
A.staticMethod // prints "Foo!"

// Java
A.staticMethod(); // won't work