Class object in Objective C

Question

I am coming from Java to Objective C, and the idea of a class object has me wondering about similarities with Java. From the Objective C guide in Apple documentation:

A class definition's information is compiled and recorded in data structures made available to the runtime systems. The compiler creates just one object, a class object, to represent the class.

So my understanding is that the class object is created for all classes that are going to be used by the program, and a class object is what is used to create objects for that class.

For comparison, does the JVM have a similar object for all classes it loads?

Answer 1

A comparable structure in Java would be java.lang.Class.

Answer 2

I think there is a class object for each class.
That class object is the one that, at low level, is used for functions as class_getName(), class_getSuperclass(), class_getVersion(), class_respondsToSelector(). If there would be a single class object for all the classes, then those functions would return the same result for all the classes.

Answer 3

Given that Java was derived directly from Objective-C (no, really, it was), the runtime models of the two are quite similar.

In Java, the notion of a "Class" isn't quite as generic as it is in Objective-C.

In Objective-C, a Class is an instance of what is known as the metaclass. For all intents and purposes, each Class object in Objective-C does exactly as you say; it describes a particular class available in the Objective-C runtime.

The same is conceptually true of Java classes. There is one key difference. In Objective-C, class methods are inherited across subclasses and more significantly a subclass can override a superclass's class method(s).

For example, the NSArray class implements the +array class method (the '+' means "class method"). The NSMutableArray subclass of NSArray overrides +array to return a mutable instance instead.

java.lang.Class is more akin to the Objective-C runtime API; it is the mechanism via which you introspect the classes available in the runtime. Since Java doesn't have functional API, the API is wrapped up in an appropriately named class. java.lang.Class is kinda the runtime API and the metaclass all in one.