Yes, this is a well-known pattern in OO. In the Objective-C Cocoa libraries you'll find it applied systematically, and is known as class clusters. The result is that the Cocoa libraries are much easier to understand than the equivalent c# or java ones. It allows the hiding of a inheritance hierarchy behind an abstract class that has class side creation methods that return subclasses.
public class A{
public B method(){
B b = new B();
return b;
}
}
class B extends A{
}
如果这就是你所说的，它是有效的。
I think you shouldn't worry too much about whether it is "clean OO" to return an instance of a subclass. It's being used and done so often because it is very helpful and makes your code very readable compared to somme kind of factories and stuff.
My biggest concern here would be that you should name your class method carefully. Don't use #new is probably the most important rule, but you should always use a name that already says: give me an instance of what is appropriate right now.
I'd say this is not limited to subclasses, such a method can even return objects that do not inherit from the class. In a dynamically typed language, this is okay. Remember, we're in a dynamically typed language, so as long as you have polymorphic interfaces, the class of an object is not really important to its users as long as it responds to your message sends...