I read that "A metaclass is the class of a class. Like a class defines how an instance of the class behaves, a metaclass defines how a class behaves. A class is an instance of a metaclass."
I was wondering how different this is from interface in Java. interface too provides a blueprint for the classes to follow.
No.
About Python Metaclass:
Metaclass in Python is a class which is used to create classes. A class that you create in Python is simply an instance of its metaclass. type is the super most metaclass in Python.
In Python, a class is modeled as an object, which gives you a lot of flexibility in its usage, by attaching all the properties of objects to classes. For example, you can even create class definition dynamically, just the way you would create an instance of a class.
This is a good read.
About Interfaces in Java:
First of all, an interface is not a class. It is just another type (Both interfaces and classes are types in Java). It is true that classes and interfaces are closely related in Java (An interface is a blueprint of a class). But they are entirely different types. Interface in java is simply a mechanism to achieve abstraction.
Conclusion: In Python, you really don't have anything like java interface.
Related
As Java 9 is going to allow us to define private and private static methods too in interfaces, what would be the remaining difference in interface and class?
Moreover, is Java moving towards multiple inheritance slowly?
Private interface methods in Java 9 behave exactly like other private methods: They must have a body (even in abstract classes) and can neither be called nor overridden by subclasses. As such they do not really interact with inheritance. Talking of which (and particularly multiple inheritance), there are (at least?) three kinds of it:
Inheritance of types means that one type can be another type, e.g. String is an Object. Java allowed multiple inheritance of types from day one (via interfaces).
Inheritance of behavior means that one type can inherit the behavior of another type. Before Java 8, only classes could implement methods, so there was only single inheritance of this kind. With Java 8 came default methods, which allowed interfaces to implement methods, thus giving Java multiple inheritance of behavior.
Inheritance of state means that a type inherits another type's internal state (i.e. fields). As it stands (Java 9 and everything currently proposed for future Java versions), only classes can have state, so there is only single inheritance of this kind.
As you can see private interface methods do not add anything here.
Regarding your question of how interfaces and classes compare, there are two main differences: multiple inheritance and state. Interfaces support the former, classes can have the latter. Since state is kind-of important in typical OOP, classes will remain relevant. 😉
If there were a way for an interface to force an implementation to have a particular non-public field or straight-out define one itself, the game would change and interfaces could compete with classes.
Private methods are not inherited by subclasses, so this feature doesn't affect implementation classes. I believe the private methods in interfaces allow us to share code between default methods.
Java interfaces still cannot have non-static members. That's a big difference and not multiple inheritance IMO.
Java 9 interfaces still cannot contain fields and constructors. This makes a huge difference between classes and interfaces, so Java 9 is far from multiple inheritance.
Java Interface in version 9 have private methods but static private. The feature has been introduced to allow modular methods. One function should work with one responsibility instead of using lengthy default methods. It has nothing to do with multiple Inheritance. The more private static methods, the more you will be able to write the clean and reusable code. Anyways, static methods whether public or protected can not be overridden.
Although its an old question let me give my input on it as well :)
abstract class: Inside abstract class we can declare instance
variables, which are required to the child class
Interface: Inside interface every variables is always public static
and final we cannot declare instance variables
abstract class: Abstract class can talk about state of object
Interface: Interface can never talk about state of object
abstract class: Inside Abstract class we can declare constructors
Interface: Inside interface we cannot declare constructors as purpose of
constructors is to initialize instance variables. So what
is the need of constructor there if we cannot have instance
variables in interfaces.
abstract class: Inside abstract class we can declare instance and static blocks
Interface: Interfaces cannot have instance and static blocks.
abstract class: Abstract class cannot refer lambda expression
Interfaces: Interfaces with single abstract method can refer lambda expression
abstract class: Inside abstract class we can override OBJECT CLASS methods
Interfaces: We cannot override OBJECT CLASS methods inside interfaces.
I will end on the note that:
Default method concepts/static method concepts in interface came just to save implementation classes but not to provide meaningful useful implementation. Default methods/static methods are kind of dummy implementation, "if you want you can use them or you can override them (in case of default methods) in implementation class" Thus saving us from implementing new methods in implementation classes whenever new methods in interfaces are added. Therefore interfaces can never be equal to abstract classes.
To develop a fully functional application consisting of more than 4 classes, what is the right way of handling shared data? I have researched about Static Methods and variables and Utility classes. It's said that use of Static methods hinders the concept of Object Orientation concept. So, if anybody could help me on how to use shared data between classes without hindering Object Oriented concept, then I would be highly grateful.
There are two primary axes of "inheritance" in object-oriented languages like Java. "Implementation" inheritance is where the sub-class inherits the actual code implementation from the parent. "Interface" inheritance is where the "sub-class" adheres to the public interface of the "parent".
Alas, Java actually mixes the two notions together a bit... Java interfaces are nice and clean -- when you "implement" an interface, you are stipulating that your class adheres to the "contract" of the interface that you specified. Java class inheritance isn't so clean -- when you sub-class in Java you are getting both the code inheritance but you are also stipulating that your sub-class adheres to the "contract" of the interface of the parent class.
Abstract classes in Java are just like regular Java classes but with the added constraint that you cannot instantiate them directly. In terms of that added constraint, they are basically classes which don't actually implement all of the code specified by their "contract".
So, it's generally considered good OO practice to specify the "contract" which you want to adhere to via Java interfaces. Then use normal Java class inheritance primarily for code reuse purposes. Use abstract Java classes when you want to provide some standard base code but want/need to force the user's of your class to complete the implementation (i.e., you create a skeleton implementation and the sub-classes must flesh it out).
I'm working on a project that makes really heavy use of the javax.script.* packages. I have a situation where I would like to create JavaScript objects that extend an Abstract Java Class, much like you can use Invocable.getInterface to make JavaScript objects that implement Java interfaces. Is this possible? And, if so, how do you do it?
Yes, you can; previous poster is wrong. See the documentation for JavaAdapter.
Unless you want to go the route of generating bytecode at runtime (using BCEL as below) then no. You can do it with interfaces using proxy classes but there is no equivalent for abstract classes.
If you really want to try BCEL, your best strategy is to do this:
Write a method that uses BCEL to generate a byte[] of bytecode for a new class that extends the abstract class and delegates every abstract method to JavaScript.
Define a naming convention that relates abstract classes to the wrapper, e.g. foo.MyAbstractClass corresponds to foo.MyAbstractClassDynamicLangWrapper.
Roll a ClassLoader that implements findClass to recognize that naming convention and to generate the class bytes and calls defineClass
Make sure your scripting language uses your custom classloader to resolve class names in scripts. I think in Rhino you use setApplicationClassLoader but I'm not sure.
What is the difference between an abstract class and an interface?
Interfaces are stateless. They cannot gave variables, though they can have constants.
Also, interfaces provide the 'design by contract ' capability.
Abstract classes force a concrete implementation, where interfaces allow more flexibility because any class that implements that interface can be substituted at run time.
Also, since interfaces simply describe behavior that is exposed, not the implementation, then thet allow for multiple inheritance.
Also abstract classes are more a design convenience as they provide for compiler enforcement in that subclasses must implement abstract methods.
Interfaces and abstract classes are related, but serve different purposes.
At runtime the type of object is checked and the corresponding class method invoked.
This is also called late binding.
This is done by the runtime VM not by the programmer thus taking that If Else test out of your program code. So, your code is more flexible and does not depend on the class type to resolve the correct method to call. Thus us also called polymorphism.
An abstract class can have methods implemented. An interface cannot. Also a class can only extend one abstract class, but can implement many interfaces.
Use of interface : There are a number of situations in software engineering when it is important for disparate groups of programmers to agree to a "contract" that spells out how their software interacts. Each group should be able to write their code without any knowledge of how the other group's code is written. Generally speaking, interfaces are such contracts.
One benefit of using interfaces is that they simulate multiple inheritance. All classes in Java (other than java.lang.Object, the root class of the Java type system) must have exactly one base class; multiple inheritance of classes is not allowed. Furthermore, a Java class may implement, and an interface may extend, any number of interfaces; however an interface may not implement an interface.
Another use of interfaces is being able to use an object without knowing its type of class, but rather only that it implements a certain interface.
Difference bw abstract class and interface : Abstract class is a class which contain one or more abstract methods, which has to be implemented by sub classes. An abstract class can contain no abstract methods also. A Java Interface can contain only method declarations and public static final constants and doesn't contain their implementation. The classes which implement the Interface must provide the method definition for all the methods present. An abstract class means the class must be extended. An abstract class must be extended by first concrete class in the inheritance tree. In the abstract class we can have both declaration and definition of a method but in interfaces there are only method signatures, no definition at all. An interface is like a 100% pure abstract class. A class can extend only one class but can implement multiple interfaces. Interfaces provides multiple inheritance without causing deadly diamond of death problem.
Extensive discussions here :
Abstract classes and interfaces in C#
Do/can abstract classes replace interfaces?
Disadvantage : when you have an 1000 class implementing an interface in your library, tomorrow if you want to have an additional method in interface , then changes should be reflected everywhere
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?
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.
A comparable structure in Java would be java.lang.Class.
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.