Question

I see Java has only one metaclass (the Class class), but other languages, say Smalltalk, have one metaclass for each Class.

Why is that? What’s the need for metaclasses? What difference does it make to have them one way or another?

Answer 1

The fundamental need for at least one metaclass is that if you want objects that represent classes (or want classes to be objects), then those objects must have a type.

Wikipedia says:

In early Smalltalks, there was only one metaclass called Class. This
implied that the methods all classes have were the same, in particular
the method to create new objects, i.e., new. To allow classes to have
their own methods and their own instance variables (called class
instance variables and should not be confused with class variables),
Smalltalk-80 introduced for each class C their own metaclass C class.

So the question is, do you want every class object to have the same type (and hence the same members), or do you want class objects to differ in ways that require them to have different types, so that there are type-checked operations which can be performed on the object that represents class A but not on the object that represents class B? Java and early Smalltalks answered that question differently from later Smalltalks.

So for example java.lang.Class.newInstance() takes no constructor arguments, whereas you can imagine that it might be nice to be able to call clz.newInstance(1) where clz is the class object for a class that has a constructor with takes an int. In Java you can still look through the constructors of the class yourself to find a match for the arguments you want to pass, but the type of the class object doesn’t tell you whether you will find one.

Also note that Smalltalk stops at one level. The type of C is C class, but the type of C class is Metaclass. There’s no infinite recursion of types C class class etc, because although different class objects in Smalltalk accept different messages, there’s no demand for different metaclass objects to accept different messages.