问题
Is class an object in object oriented language? How are Class methods accessed by just name of the class.method name? (internal working). Is this same as a object.method?
And If the Class is same as object (belong to object class which is super class of every thing in OO) and we instantiate it (make object of it), can we make instance of an instance of an class other than Object class.
(Mainly interested in the theoretical perspective even if practically not required ever)
回答1:
Well, it depends on the language that you are using; in pure OO languages where everything is an object (e.g. Smalltalk) classes are no exception and are objects too. In other languages where classes are not considered as first class citizens they are just special language constructs or primitive types. From now on I'll use Smalltalk as a target language, due to its reflection support and homogeneous style.
How Class methods are accessed by just name of the class.method name? (internal working). Is this same as as object.method?
Since classes are objects, they are in turn instances of a class (a metaclass). Thus, sending a message to the class is just sending a message to an object whose role is to represent how classes behave. There is a lot of literature out there, you can take a look for example here and here for some introduction.
And If the Class is same as object (belong to object class which is super class of every thing in OO) and we instantiate it (make object of it), can we make instance of an instance of an class other than Object class.
I'm not sure I follow you here, but just for clarification it is not always the case that Object is the superclass of all the classes. The thing is that If you start following the relationships between classes and metaclasses, you may reach a sort of infinite loop. Different languages work this out in different ways and for example, in VisualWorks Smalltalk, Object is a subclass of nil. The thing is that nil is also an object (remember, everything is an object) and it actually represents "nothing". As you may expect, nil is an instance of a class (UndefinedObject) and it also implements some of the class protocol. As a result it can be used to represent a class form where nothing is inherited :).
Finally, I don't know if this answers your question, but yes, you can do many cool things with full reflective capabilities, like creating new classes on the fly or reshaping existing ones. I'll leave you here some documents that you may find interesting regarding this topic:
- Metaclasses
- Understanding Metaclasses
- Debugging Objects
回答2:
A class isn't an object, you can think of it as a 'blueprint' for an object. It describes the shape and behaviour of that object. Objects are instances of a class.
See here for more information.
回答3:
Is class a object in object oriented language?
The notion of class is first and foremost theoretical. You can define a thing with "class" semantics even if your language does not support the formal notion of "class" (e.g. Javascript). A class is a template for an object, from which you create instances.
How Class methods are accessed by just name of the class.method name?
I'm not quite sure what you mean. Some languages support "static" or "class" methods, which are methods that are not invoked within the context of an instance of the class. So its not the same as "object.method" which is a normal method on the class where the 'this' (or equivalent) will be the instance on which the method is invoked.
Java (from comments)
For Java, there is a class called Object, and a class called Class. See this. These Java constructs are separate from the notion of Class and Object. The former are implementation details -- they are how the designers constructed the language, the latter are general concepts. So in Java, you can have an instance of an Object, where the instance is an object(the concept) of type Object(the Java construct).
回答4:
A Basic notion of object means that has been allocated some memory. As devdigital said class is just a blueprint for an object which holds the bare-bone structure and determines how the object will behave when it's it will be instantiated.
Classes are just bodies(without soul) and objects are living bodies(having a soul) that interact with environment or in biological terms respond to external stimuli(public methods and properties) in an analogy to philosophy of life :)
Technically classes are just machine interpretable code residing in memory(not necessary main memory or registers). Objects are code loaded into executable memory(registers/main memory)
来源：https://stackoverflow.com/questions/14752948/is-class-an-object-in-object-oriented-language