I am looking into abstract classes and interfaces.
Can someone tell me if there is a difference between a fully abstract class and an interface?
Google is your friend, so use it.
There certainly are differences. The first few that I can think of are
You can not inherit multiple abstract classes, where you can implement multiple interfaces
You can write a decorator for an interface but not for an abstract class
You can provide default implementations in an abstract class, but not in an interface. This allows to extend an abstract class in a backwards compatible way, which is not possible with an interface
An abstract class can contain method implementation while an interface can't.
In Interface :
you can only declare the variables, Methods but no defination is written in this interface
Abstract class :
while a class is declared as Abstract class then the declarations may or may not include abstract methods.In abstract class we can write the definitions also. Unlike interfaces, abstract classes can contain fields that are not static and final.
Such abstract classes are similar to interfaces, except that they provide a partial implementation, leaving it to subclasses to complete the implementation.
READ THE JAVA DOCUMENTATION!
Abstract Classes versus Interfaces
Unlike interfaces, abstract classes can contain fields that are not static and final, and they can contain implemented methods. Such abstract classes are similar to interfaces, except that they provide a partial implementation, leaving it to subclasses to complete the implementation. If an abstract class contains only abstract method declarations, it should be declared as an interface instead.
Multiple interfaces can be implemented by classes anywhere in the class hierarchy, whether or not they are related to one another in any way. Think of Comparable or Cloneable, for example.
By comparison, abstract classes are most commonly subclassed to share pieces of implementation. A single abstract class is subclassed by similar classes that have a lot in common (the implemented parts of the abstract class), but also have some differences (the abstract methods).
There is a difference between a fully abstract class and an interface?
If all methods are abstract the behaviour is almost (not exactly) the same, but yes there are differences. Mainly because a class can't extends from multiple (abstract or not) classes but can implements many interfaces. Also the fields can't be private in an interface.
Note that the Oracle documentation suggest to turn fully abstract class into interface.
Related
An abstract class can have both abstract and non abstract method. Class have only abstract methods can called absolutely abstract class.
There is no advantage of one over the other.
One approach allows you to share implementation code.
The other approach allows you to not share implementation code.
You use whichever is more appropriate to the task that you are doing.
If abstract class still has their childs and child has their own behaviors also some behaviors are common, in that case we sud go for non absolutely abstract class
I am going through some Java code and I see lots of abstract classes which contain nothing in them.
For eg. something like this -
public abstract class Processor
{
}
They have concrete implementation classes though. In what situations would such abstract classes make sense?
IMO, an abstract class is preferred over an interface when you want to create a base class with behaviors and state common to all the subclasses. I am not sure of any situation where an empty abstract class would be more useful than an interface. Usually, the marker interfaces are the ones that are empty.
Abstract classes are used when you have some logic which would be common to all possible implementing classes.
Writing an Abstract class with nothing inside and different classes extending it is pretty much useless.The only difference is that you can use the same handle for all the concrete classes.
One of the situation would be, if you don't want your class instantiated using new keyword, then you can define it is abstract.
In some libraries, they will look for implementation of these marker abstract classes using reflection. Also this is kind of a documentation for your code by abstract class name itself.
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Abstract class and Interface class?
When i need to use abstract class and interface in Java?
My doubt is
Which situation i need to use abstract class and which situation i need to use interface.
interface satisfy the abstract class properties. so then why we need especially abstract class?
i know, that abstract class contains abstract methods and non abstract methods, but we can use abstract class as a ordinary class, then the result will be same in the both classes. the ordinary class also inherited same as abstract class. So why we need abstract class.
If anybody know the good example please reply me.
Thanks.
Another important aspect of abstract class is that unlike interface, adding new methods to it won't break binary compatibility. So, from the API evolution point of view, especially when you can expect additions to the public API, abstract classes are more preferable.
I would say that there are two types of inheritance. One I would say as Implementation Inheritance and other as Contract Inheritance.
Abstract classes are used for having Implementation Inheritance. You can extend/change the behavior of your super/parent class.
Interfaces would go for Contract Inheritance. Where you are more interested in having the class implement some kind of a contract (service methods with arguments - more of a contract) and the behavior is different for different implementation, nothing generic that you can bundle up in an abstract class and extend the behavior.
You need to use abstract classes if you want to apply the template pattern http://en.wikipedia.org/wiki/Template_method_pattern, usually in framework code. As a rule of thumb, if you're not intending to implement the template pattern, you're better off with interfaces, which permit loose coupling, the way spring framework does. Loose coupling leads to a design open to evolutions and a better testability, with techniques like mock objects (http://easymock.org)
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Can anyone tell me what exactly the difference between an completely abstract class and an interface?
An Abstract class can also have all its methods as abstract. An interface has all its methods as abstract. What is the main difference between the two in this scenario?
If there is difference between a pure abstract Class and interface? What is the use of interface? Where interface is being used we can make use of pure abstract class?
To complete the former answers :
An interface is a "contract". If a class implements an interface it have to propose all the services listed in the interface.
An abstract class is a skeleton. It defines a certain way its extended classes will work while letting them some free space (the abstract methods) to be unique.
A pure abstract class doing the same thing as a interface but have the problem of unique extending so, for me, it have no interest
Every interface is implicitly abstract: Every method declaration in the body of interface is implicitly abstract and public.
An abstract class has methods that can contain implementation. Abstract methods can be either public, protected or default access (package visible). Unlike interfaces abstract classes can contain fields that are not static and final.
Also see:
Interfaces vs Abstract classes and the
Java tutorial
In Java and C#, one can use multiple interfaces to derive from and only a single class to inherit from,
One reason to choose pure abstract over interface is to force sub classes to implement particular methods that are implemented by a super class.
For example (in Java),
Say you want all extending classes to implement toString(), equals(), and hashCode().
You could create an interface called ForceSomeMethods for that contract, but those methods are implicitly implemented by Object.
Making ForceSomeMethods a pure abstract class with toString(), etc as abstract methods, all subclasses will be forced to implement those methods.
It's not a very theorotical explaination but, programatically it's all correct
Interface Abstract Class
Extend Class No Yes
Extend Abstract Class No Yes
Implement Interface Yes(Extend Interface) Yes
Variables Public Static Final Public/Protected/Private/static/final/transient/volatile
Contain Non-Public Method No Public/Protected/*Private
Contain Abstract Method Yes Yes
Contain No-Body, Non-Abstract Method Yes No
Contain Defined Method No Yes
Contain Main Method No Yes
*Abstract classes can have private methods, but not abstract private methods.
An abstract class can provide an implementation, i.e. (public, protected, private) method bodies. An interface can just declare public method signatures. These methods have to be realized (in the form of method bodies) by classes implementing the interface.
There are three differences:
Interfaces can only declare public methods (i.e. no protected or package-private visible methods) and can not declare any fields
Subclasses can only extend at most one abstract class, but can implement any number of interfaces
The abstract class can also have implementations for some or all of the methods
I'm just going to address one point (mainly because the other questions have been addressed already):
"Where interface is being used we can
make use of pure abstract class?"
In theory, you could. However, you will lose flexibility and loose coupling to some extent. It's far more preferable to code to interfaces and pass those around, especially in Inversion of Control (IoC) scenarios and from an integration point of view, as this allows far greater extensibility.
Since the question is about pure abstract classes then I'd say the answer is going to be related to inheritance and scope. It's something I've wondered myself many times and this is what I've come up with.
Obviously the features related to multiple inheritance have been answered previously so I won't go in to any of that. Scope is a big one though.
In an interface you can't define a member's access modifiers since they are implicitly public,...you are defining the public interface for it's eventual implementation. There's an important difference there since you can define a protected abstract member in a pure abstract class.
Inheriting from such a class definition would force the inheritor to implement the abstract member but scope it privately to consumers of the class (though it would have to be defined as protected so unless the class was marked as sealed further inheritors would have access).
In essence you can define a private interface using pure abstract classes. Whether that's a good idea is a different question altogether but one good use I've seen it used for is to enforce design patterns and standardize class designs.
HTH
You can use Interface for multiple inheritance, but you can't use abstract class for multiple inheritance.
All the methods in Interface is public by default, by in abstract class, only the methods which you've set as an abstract need to be declared public.
A class can implement multiple interfaces, but only extend from one class (abstract or otherwise). If you need to specify an interface, then you should use an interface, so that classes may implement multiple of your interfaces.
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I'm working through some homework and a question on a previous exam paper asks to name all of the abstract classes in a given UML diagram. Fairly straightforward, I suppose. There are one abstract class and three interfaces. Do these interfaces qualify as abstract classes, in general?
Thing is, while technically interfaces may be represented as classes in languages like Java, I wouldn't consider them classes.
Abstract? Hell yes. Class? No.
Interfaces cannot have constructors, neither properties, fields, function bodies, etc. Interfaces cannot be inherited, they are implemented (again, technically it might be true that implementing an interface is actually inheriting it in specific languages, but that's not my point.) Interfaces are more like 'contracts' as they do not define any behaviour whatsoever like classes.
Now if this is a homework then you shouldn't really argue about these sort of stuff with the teacher. Just check your lecture notes and see if the word "class" is mentioned anywhere in the your teacher's definition of interface.
All interface are indeed abstract
Actually, you can declare an method as abstract within an interface... except any 'checkstyle' tool will tell you the abstract keyword is redundant. And all methods are public.
If a class implements an interface and does not implement all its methods, it must be marked as abstract. If a class is abstract, one of its subclasses is expected to implement its unimplemented methods.
To echo other answers, an interface is not a class.
An interface is a reference type, similar to a class, that can contain only constants, method signatures, and nested types. There are no method bodies. Interfaces cannot be instantiated—they can only be implemented by classes or extended by other interfaces.
Interfaces are not part of the class hierarchy, although they work in combination with classes.
When you define a new interface, you are defining a new reference data type. You can use interface names anywhere you can use any other data type name. If you define a reference variable whose type is an interface, any object you assign to it must be an instance of a class that implements the interface
To better explain why an interface is not a class, consider the following:
1/ an interface is a type used by values
2/ a class is for Objects
3/:
Object a = new Date();
String s = a.toString();
The type of the variable 'a' is Object (which is actually a type notation in Java source code meaning a reference to an Object),
but the class of the object it points to is Date.
The type (Object) only affects what code is valid according to the compiler's type-checking, but not what the code actually does.
The class of the object affects what the code does, so that the a.toString() call in the second line returns a String that looks like a Date, not one that looks like "java.lang.Object#XXXXXXXX".
Since an Interface is a type, it is used for values only, and will not represent what objects will actually do in term of runtime.
In Java though, theres a twist to the tale - all Interfaces in Java extend java.lang.Object! Try adding a method:
public void notify();
in an interface and see what happens..
An Interface extending a Class - Does that make the Interface a Class? Or the Class an Interface?? Uhh-huh.. Guess it was a hack that had to be done to prevent interfaces overriding the definitions in java.lang.Object that implementations of the interface had to extend anyway.
You've only asked about the abstract side, but don't forget the class side - I wouldn't call an interface a class, so even though interfaces are abstract (as per the specification), I still don't think they count as abstract classes. It may be worth explicitly explaining that though :)
Yes, an Interface is implicitly Abstract. Look behind the scenes as to the way it is encoded to a .class file.
Semantics are a funny thing though; under exam conditions "abstract class" would have to literally be compiled from a .java source file using abstract class in the Class' declaration.
An interface contains prototype of methods (i.e Declaration ) not defination but Abstract class can contain defination of method & atleast one Abstract method (method with only prototype)
Interfaces are used to break the Object Inheritance.
They could hold two or more objects of several classes
and classes hierarchies.
Look at an interface as an outlet plug. All classes
implementing an Interface need to have one, the same
way a computer, a coffee machine, a ventilator and a
refrigerator need to have the same device to get
power.
Abstract class looks like interface. Abstract classes can have implementations where as interface can't have any implementations.
Then, there is a question. Can we call abstract class as interface if they only have method signatures?
I think, abstract classes, unlike interfaces, are classes. They are expensive to use because there is a lookup to do when you inherit a class from abstract class.