Difference between abstract class and interface
use of abstract class

This question has been asked here about a billion times:

http://search.java.sun.com/search/java/index.jsp?qt=interfacenh=10qp=st=11rf=0since=country=language=charset=variant=col=javaforums

hai,
I am new to java programming

I want to difference between abstract interface,interface and abstract class

public abstract interface Test{}

public interface Test{}

public abstract class Test{}

which kind of situations use the above interfaces and abstract classes

It would be helpfull if explain with example....

Thanks

shashikumar, please don't multipost and don't use the browser's back button to edit your post. This creates multiple posts. I've removed the other thread you started a minute after this one.

db

Hi,
I want to know the exact implementation differences between an abstract classes and Interfaces.
When would we use each of them..
Its seems that each does the same kind of functionality...but we need to just extend an abstract class in one and implement an Interface in another.

Do let me know.

Thanks,
Ravi

Hi,
I want to know the exact implementation differences
between an abstract classes and Interfaces.
When would we use each of them..
Its seems that each does the same kind of
functionality...but we need to just extend an abstract
class in one and implement an Interface in another.

Do let me know.

Thanks,
RaviHi Ravi,
A class must be declared abstract if one or more of its method is abstract. In case of an Interface, ALL the methods MUST be abstract. As java does not support multiple inheritence, Interface comes handy. In java, a class cannot extend from more than one class. To incorporate multiple inheritance, one can extend a class from another class and implement interface.

public TestClass extends AbsClass implements Inter {
}

Thanks,
Hope this helps,
Karthick

Hi

I was just wondering what is the advantage of using an abstract class over interface. In what situation, using an abstract class is more suitable than using interface? and vice versa?

This question has been asked and answered a million times. For instance, see this thread: http://forum.java.sun.com/thread.jsp?forum=31thread=352322

938864 wrote:
Abstract class vs interface is common question and I have been hearing from time and time that abstract class are slightly faster than interfaces Please cite a reference to this.

P.S. You will not notice any difference so who cares.

In what respect have I not just answered exactly that question?

1whats the basic difference between an interface and an abstract class?
when should one be using an interface or an abstract class?

2whats the difference between method overriding and method overlaoding?
does both of them has any relation with the instance of te class implementing them?

http://www.google.com/search?q=java+interface+%22abstract+class%22

http://www.google.com/search?q=java+method+overloading+overriding

Hi everybody!
I'm little confused abt the difference b/w abstract class and interface. Plz help me in this regard.
Bye
Mustafa

You've got a lot of postings describing the semantic differences, here's one that gives you a practical difference: use interfaces to define policy, abstract classes to define mechanism.

Assuming that got a big "huh?" ...

An interface defines a set of methods, which I think of as "capabilities." For example, java.util.List defines methods to add/remove/retrieve items to a list. Any object that implements that interface is saying that it has the capability to maintain items in a list. However, it doesn't say anything about how that object is implemented; perhaps it uses an array, perhaps a linked list.

By comparison, java.util.AbstractList defines at least part of the mechanism used to implement the List interface. It still leaves some of the implementation open to the programmer, but handles a lot of the details common to all lists.

By keeping this practical definition in mind, I'm not tempted to use an abstract class in place of an interface. Instead, I limit abstract classes to places where there's common implementation -- in other words, they become a refactoring tool.

One example: as I've started to develop Swing apps, I find that I'm implementing a lot of dialogs. Since I want a common look and feel to my dialogs, there's a lot of code that's common. However, since each dialog has a separate purpose in life, there's also a lot that differs. So I implemented an AbstractDialog class that handles common operations such as UI decorations.

Now, I could have implemented this as an instantiable class, which would have methods that are overridden by subclasses. But I went with an abstract class for two reasons: (1) there's no reason to instantiate the base dialog, and more important, (2) by defining certain methods as abstract, I force the subclass into providing implementations, thus eliminating bugs due to laziness.

One last comment: this example defines very little in the way of policy, so it's clearly a candidate to be an abstract class. You may wonder what to do about objects that aren't so clear, but I've found that there are very few of these. In my version of a good OO design, objects either define policy or mechanism, but almost never both.

Hi to all,

can any one tel me what are abstract classes and what hapens if i check the ABSTRACT ckeck box while creating a interface in se24(class builder)?

Thanks Regards

Priya

Thanks Matthew .

Thanks Regards

Priya

Edited by: priya latha on Dec 19, 2007 11:43 AM

Hi,

Could u plz tell me the difference between Abstract class and Interface?

Thanks in advance.

Gopi

Lots.

An abstract class can contain some method implementations, or indeed all the method implementations. It may contain methods with all the various access modifiers. It cannot be instantiated. A class may inherit from only a single abstract class.

An interface contains only public method stubs and constants. A class may implement multiple interfaces. An interface cannot (obviously) be instantiated.

Abstract classes are particularly useful when you need to provide a semi-complete implementation for reuse. Interfaces are used more like types.

Look at java.util.* for some good examples of the use of both.

I am kinda new to Java but have been studying frequently. I have a question that is probably very simple: What is the difference/advantage between an abstract Class and an Interface?

Search the forum. Here's a topic that addresses it:
http://forum.java.sun.com/thread.jsp?forum=31thread=273324

Hi All,

I want to know what is the difference between Abstract class and Interface.

and what is the importance of using them.

Moderator message: please search for available information/documentation.

Edited by: Thomas Zloch on Sep 27, 2011 1:55 PM

Please refer to this thread, already this is discussed in forums

[ABAP Objects.. Abastract Interface Class;

Thanks

Pavan

is there any specific difference b/w interface and abstract class. if wht is tht? and how?

abstract class can be have methods with body

interface cannot

interface is 100% abstract class

Hey:

I'm curious, what's the difference between an interface and an abstract class? Thanks.

"You have an abstract class if you want to implement some of the methods in it, but you do not want users to make instances of the class or do not want to implement all of the methods. Take a look at some of the abstract classes in java.util. Nearly all of the methods are implemented already so that it's far easier to create new data structures than if only interfaces were provided. "

That's what I was looking for. Thanks.

hi all,

what is the difference between an ABSTRACT CLASS with full of METHOD DEFINITIONS and no implemented methods. AND an interface having the same definitions.

In which situation we use abstract class and in which situation we use interface.

what are the benefits.

Can anybody explain it clearly.

bye
-hari

http://www.google.com/search?q=java+difference+abstract+interfaceTrying to teach a man to fish, are you?

Hi ,

Please help on this ..

Regards,
vij

All the methods declared inside interface should be abstract

All the methods declared for an interface are abstract, by definition.

but in case of abstract class at least one method should be abstract

No. An abstract class is not required to have any abstract methods. It's not even a "should".

Abstract class must be extended abstract ()
methods generally contain only declaration part
without definition part.
No "generally" about it, as there are no exceptions. Abstract methods do not have a declared body, by definition.

~

can anyone pls tell me where exactly an abstract class is used and where exactly an interface is to be used.

i think the main difference between an abstract class and an interface is that an interface will have all the abstract methods whereas an abstract class can have one or more abstract methods--- if i am not wrong.
anything to add? -- pls let me know on this....

and also how they might be useful for us....

The differences are already quoted. In addition of that
the abstract class will come in the class/system hierarchy, but interface will not.

While this is by no means complete, the major
differences between an interface and an abstract
class with all abstract methods is that you don't
have to create an implementation for the abstract
methods in a class that extends the abstract class.Yes you do; unless the extending class is also abstract.

You do have to implement all methods in an
n interface. Also you can use access modifiers other
than public in an abstract class, while interface
methods must always be public. public or package-private?

If you don't know why either of these things would be useful then you
probably don't care, and could just use an interface.And seeings as people are answering here desipte the laziness; abstract classes can also have implemented methods, and member variables. Interfaces cannot.

While this is by no means complete, the major
differences between an interface and an abstract
class with all abstract methods is that you don't
have to create an implementation for the abstract
methods in a class that extends the abstract class.
You do have to implement all methods in an
n interface. No.

The rules for what must be implemented are the same regardless of whether it's a class or an interface.

any one can please tell me Difference between
abstract class and interface. From a typing perspecitive there's no difference (you can declare variables of both). The only difference in principle is that an abstract class also can have concrete methods so it allows you ti inherit implementation along with the type. You can only inherit from one class but from as many interfaces you whish.

Which one is best ?The short answer is that interface is best.

Hi,please kindly help meWell, maybe if you'd post an actual question...

what is the difference between Difference between a Abstract class with 4 abstract methods and a Interface with 4 abstract methods.

Your question is better rephrased as: What is the difference between an abstract class and a interfase?
This has been discused here
http://social.msdn.microsoft.com/Forums/en-US/csharpgeneral/thread/67451957-f0bd-4b9b-a2c0-16f604ce8643
Regards

hi,recently i went for an interview they ask me what is abstract class ,interface and its difference
I said abstract class contain abstact method ,abstract method is a method with no body.Abstract class cannot be instantiated.Abstract class is a base class it required derived class for the implementation of method.
Interface is a syntactical contract that all derived class should follow
it define properties ,method,events which are known as member of interface.
Then They asked me what is the difference between them. I said
abstract class interface
1.abstact class can implement method 1.interface cant
2.abstact class can contain constructor, 2.interface cant
destructor
3.abstract class cannot support multiple 3.interface support
inheritance
etc
Then they said some different answer I said dont no.
Then they ask me when i should make abstract class for an project and when
i should make interface.
I said if suppose there is two class which must be having method with different logic then we sholud make abstract class.
and if suppose we have two class having method .with different logic then we can make interface .
Am i correct with my explaination.if not correct me .please provide me that when should we create abstract class and interface and what is difference .please help me

I would focus the answer on the distinction that you cant add an abstract class to an existing class like you can an interface.

Hi,
Can nybody tell me the difference between abstract class and interface from design point of view. This questions waz asked 2 me in an interview.

Tx.

Naseem Khan

Well. if you really need to know the details check
out page 118 of the Java Language Spec. The summary
is:

- more than one interface can be implemented by a
class.
- an abstract class can contain implementation.That is not the big difference, but one will see it if one contemplates on this a little.

hi,
i just want to know the difference between pure abstract class(as same as interface- no implementation of methods) and interface

Thanks,

A very nice and detailed article on this code project link (posted by Magnus).

Hi

Could someone tell me what the difference is between an abstract class, interface class and a concerte class. Cant figure it out.

Thank you
David

Thanks Lady

Pls give me some practical example of choosing among abstract class and interface.

Search these forums using the box on the left. This question is asked about 50 times a week, there are tons of answers around

I have following scenario where my MDB class extends an abstract class which implements the MessageListener interface.
public abstract class MDBBase implements MessageListener {
public void onMessage(Message message) {
....
processJob(some-params)
}
public abstract void processJob (some-params);
}

public class MyMDB extends MDBBase {
public abstract void processJob (some-params) {

}
}

I have following scenario where my MDB class extends an abstract class which implements the MessageListener interface.
public abstract class MDBBase implements MessageListener {
public void onMessage(Message message) {
....
processJob(some-params)
}
public abstract void processJob (some-params);
}

public class MyMDB extends MDBBase {
public abstract void processJob (some-params) {

}
}

when i use Abstract class or Interface.Which one is better?

Depends on what you want to do. An interface is more general (in that it doesn't define any actual behavior) and allows you to mimic multiple inheritance (a class can implement any number of interfaces whereas it can only extend one class (abstract or otherwise)). Abstract is good if you have a common base of functionality that all of the implementers will be using.

It's also fairly common to use both (i.e. the interface TableModel and the abstract class AbstractTableModel), so that someone who wants to completely write their own implementation is free to, while someone who wants to just extend or change the standard functionality could do that as well.

Hi Experts, What is Difference Between Abstract Class And Sealed Class in C#? Thanks.

Knowledge Shared Is Knowledge Gained

Hi Geert van Horrik, Thanks.Knowledge Shared Is Knowledge Gained

Hi all,
i'm new to java, can somebody help to explain on abstract and interface method. As i know that abstract or interface does not provide implementation in the superclass and interface class respectively. All the implementation are done in subclass.. why not we use the inheritances and abstract method to solve the problem instead of using the interface... i can't see the difference between the abstract and interface.. can somebody provide me a good example to understand the interface well..

p/s: pls correct me if i wrong... ;)
thanks

public class abstract Mainclass{
abstract double area();
abstract double meter();
}

public class Subclass1 extends Mainclass{
double area(){
return 5+1;
}
}

public class Subclass2 extends Mainclass{
double area(){
return 8+10;
}
}Let's say you have
public class abstract AreaClass {
abstract double area();
abstract double meter();
}And then when you are working on your second iteration and you want to use Dimension class for this purpose for your new code but you want to leave the old code alone. If you have made AreaClass an abstract class, this is impossible.
public class AreaDimension extends AreaClass, Dimension {
}will not compile. However if you had defined AreaClass as and interface you can do this:
public class AreaDimension extends Dimension implements AreaClass {
double area() {
return height * width;
}
}Then you can pass this to any method that accepts an AreaClass as a parameter and you can also pass it to any method that takes a Dimension.

I have been asked a question: how to decide to use Abstract class or Interface?
We know that the Interface is special Abstract. All definitationsare default public and without implemetion. Classed inherit from interface has to implement all definitions.

Abstract class does not need to implement all the methods and it can not be instantiated.
Classes inheritfrom abstract classmay sharecommon logic (properties or methods) with abstract class and also override some methods' behavior.
How to clearily answer this question? Thx!

JaneC

Hello JJChen,
Interface class can be implement in small units of data but abstraction class can be implemented in large units of data.
1. Here is explanation of
Interface class
2. Here is explained
Abstraction class.
3.
Difference betweenAbstraction andInterface class.
Hope it will help you a lot.
thanks,

why we have constructor for abstract class and not for interface,

Simpler reason: interfaces have no implementation. Period.

See reply #44 and then please find something more useful to do.

Passionate_Java_Lover_Vijay wrote:
hi tigers,
i am very new to java.
i have a doublt iin collection.
at wht situation what interface wil be best than other.

where should i use List Set, Map and all.
Advance thanks 2 guys who help me.

Thanks
ThinkGodJebus. Are you for real? Get a grip will you

why we go for interface? what is the difference between interface and abstract class?

can anyone plz explain with clear picture.

Look at his profile. He's nothing but a homework dumper anyway. Someone to be ignored.

Hi Recently

I attended a Interview ,Interviewer asked me what is difference b/w abstract and interface.

I cud give answer ,,he said what if abstract class has all abstract methods.in this situation is ther any difference between abstract class and interface..

i said there will be no difference in making all methods abstract and it is similar to interface.the only difference is at one point of time u can have a class can inherit only one abstarct class whereas interface u can have more than one interface....

I want to know is my answer is right r not..please suggest is it correct and if any other ans please do reply

With Regards
Mahender
Mahender

Stepping away from purely technical differences, there are some more fundamental ones.First there is a semantic difference. A base class says that any derived classes have an is a relationship with the base. For example you could have a base class such as Stream because derived classes are streams, but you wouldn't have a base class such as Serializable because you couldn't say that a derived type is a Serializable.Secondly there is the versioning problem. With an abstract base class you can add non-abstract members in future versions without breaking any existing functionality (save for possibly name collisions) because the base class provides the implementation. With an interface you can never add members because you will break any existing implementors. http://gregbeech.com/blogs/tech

when do we need interface and when abstract class?

2.All the abstract methods must be overrided and
implemented by the child class.by definition, you can't override a method that has no implementation

3.Real Examples:

when we implement the ActionListener interface for
Button click event then we have to override the
actionPerformed method, we dont have any other
chices, so it is a way of forcing the inherited class
to override certain methods.as before, we implement an abstract method. you're not forcing an override, this can't be done, nor would it be a good idea. as discussed at length recently :-)

n java you can see all the classes that starts with
Abstract is abstract.not so. there's nothing at all to stop you writing a concrete class named AbstractClass

Hi all , I want to know about

1.the usage of an abstract keyword.

2.Difference between abstract and interface

3.When to use abstract keyword.

Hi all , I want to know about Give it a try yourself first, post it here and you get feedback.

Many time, I just confuse the purpose of interface and abstract.
When should use interface or abstract in real world?
Please give me the difference point alike point of both interface and abstract.

Thank you.

Hi
Take a look on this
http://www.c-sharpcorner.com/UploadFile/yougerthen/104232008193919PM/1.aspx
Reguards

The complexity resides in the simplicity Follow me at: http://smartssolutions.blogspot.com

Hi All,

I am new to java...

I am confused lot about the difference between interface and abstract class....

Can anybody tell me the difference between interface and abstract class with some good and simple exmple...

Kindly help me...

Thanks in advance...

phdk wrote:
georgemc wrote:
public class AbstractClassThatIsntAnInterface {

public abstract void doSomething();
public abstract void doSomethingElse();

}Nice naming for a nonabstract class! ;-)

@OP: read this: [http://java.sun.com/docs/books/tutorial/java/IandI/abstract.html]Er, you, um, spotted my deliberate mistake.....

someone ask me that he had a abstarct interface class, such as :
public abstract interface Book{
}
I was comfused, who can tell me why use it?

http://java.sun.com/docs/books/tutorial/java/interpack/interfaceDef.html

Scroll down to the bottom.

Drake

hi

i am new in java.

plz tell me abstract class support which type of polymorphisam and
interface support which type of polymorphisam
plz explain

bye
siva

There's only one type of polymorphism.

Does that make it mono-polymorphism? ;)

Indeed - do a search for the forums.

The short answer is that both abstract classes and interfaces define required method signatures for subclasses that extend/implement them, but abstract classes can have implementation for some or all methods that are not abstract; interfaces have no implementation whatsoever for any methods.

%

Java allows to put 'abstract' keyword while declaring an interface, but by definition interfaces are support to be abstract, so whether there is any specific meaning of declaring an interface abstract. I mean whats the difference between an interface and an abstract interface?

No difference whatsoever. I believe the "abstarct" keyword in interfaces is an ingheritance from previous versions

hi i am looking for the difference between abstract classes and interfaces. i have refered a couple of books but nothing seems to satisfy me. and another thing when to use an abstract class and when to use an interface( other than multiple inheritence)

thanks

In my opinion, Java sun programmers construct
abstract classes in order to prevent some of the
disadvantages of interfaces such that using only
empty bodied method.It is a joke do not take it serious but it is really true that they construct abstract classes in order to prevent some of the disadvantages of interfaces

hi,

Can anyone help me regarding the difference between interfaces and abstract classes. What is the advantage of interface over abstract classes and viceversa. Obout the possibities of multple inheritance I know. I found allmost all people are talking some external answers.
Why an abstract class cant we do it with concrete class.
regards,

Jayaprasad Viswanathan

Abstarct class can never be instantiated. Same as interfaces.

Can be
subclassed.Same as interfaces.

What is the exact difference between abstract classes and interface in real scenario???

Plz could you give a practical example???

Thanks

At least your head doesn't make a plonk sound.Mine just makes a splutchy bursting watermelon type sound.

To the original poster - to make things simple, if there is any doubt about which you should use, use an interface. If the situation calls for an abstract class, you will know that an interface is the wrong choice, because it will be impossible to do what you want to do with an interface.

Drake

Hi,
This is ramana, I got a doubt
1. I have one interface
interface Intabs {
abstract void method1();
abstract void method2();
abstract void method();

}
2. Iam trying to convert that interface into abstract class like below
interface Intabs {
abstract void method1();
abstract void method2();
abstract void method();

}
3. Again iam trying to extend that abstract class into class like below
interface Intabs {
abstract void method1();
abstract void method2();
abstract void method();

}

Iam getting errors

Can i do it in multiple steps: Interface------abstract class----- class

Cannot reduce the visibility of the inherited method
from IntabsDerabst1.java
Right. As mentioned, you can't assign weaker access priveleges to inherited methods. If you define a method as "public void foo() {}", the "foo()" method must have at least public access in all subtypes. In your example, your interface methods are public, but you're trying to give them "package-private" (default) access in the abstract and concrete implementation classes.

~

can any one tell me the difference between an abstract class an an interface. which is more useful ?

You could also try searching this site for your question. It has been discussed in other threads as well (I posted a simialar (how do you spell that?) question).

What is most important thing about abstract class and interface.

When to use abstract class and interface.

The interface is like an agreement where the class
which implements the interface promises to write the
implementation code for all the functions in the
interface.Same for an abstract class, only, rather than "all the methods" it's "all the abstract methods." Of course, you could turn around and usethat wording for interfaces too, but in that case it means "all the methods" since interfaces' methods are all abstract.

I want to make an abstract class with methods that need interfacing, but I don't want to make an entire other class for the abstract class to implement. Is there any way to make an abstract class and interface combined into one class with no implementations on that class?

if you want a class with no implementations that what you need is an Interface

hi all,

A method in an interface consists of access specifiers as public or abstract. what is the difference if the access specifiers for the method is public and abstract.

regards,
priya

That is correct. They must be because interface methods must be overridden when subclassed.

Why can't we declare a static method in Interface but it's possible in abstract class?

Because an interface is simpler than an abstract class.
Normally you would put a static method like this into a utility class with a private constructor. see the Math class.

hi all, i would like to know the difference between abstract class and interface....could anybody enlighten me on this....

search the forums, lot's of times answered this one has been.

what is the difference between interface and Abstract class?
What are the situations we have to go for interface and Abstract class?

Neither an abstract class nor an interface can be instantiated (meaning you cannot create the object directly with the "new" keyword). They are both similar in that they have abstract methods. (Technically, you can create an abstract class with no abstract methods, but I would question the utility of making that class abstract in the first place).

The two main differences are the following:

Implementation: An abstract class can implement methods, since it is a class. An interface can never implement any methods, it only declares their signatures.Inheritance: A given class can implement an unlimited number of interfaces. A class can only extend one class. In general, I use the following rules of thumb when deciding (other developers may have differing opinions):

Given the choice, use an interface. Since you can implement an unlimited number of interfaces, they are not as restricted, from a class hierarchy perspective. If you have several classes with the same instance (class) variables declared and several methods that all accomlplish the same thing (normally, they will even have segments of code copy-pasted between classes), then re-factor the common variables and code into an abstract class. When using interfaces, always consider delegation. Delegation is something like the following:interface IFoo {
abstract public void bar();
}

public class FooDelegate {
final public void bar() {
// Default implementation goes here
}
}

public class FooImpl implements IFoo {

final public void bar() {
new FooDelegate().bar();
}
}

The advantage of delegation is that you can write a "helper" that contains the implementation, and then the classes that actually implement the interface can simply use the helper (e.g., delegate the implementation to another class).

- Saish

"My karma ran over your dogma." - Anon