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Interface vs Abstract Class (general OO)
(36 answers)
Closed 9 years ago.
In Java every method in an interface is implicitly abstract. But when at least one method in a class is abstract, that means that this class is abstract. Does that mean, that every interface is an abstract class?
I guess in some sort of way, yes. An abstract class can be used as an interface.
The issue is that a class can only extends on other class. A class can implements any number of interfaces.
So while abstract classes are similar to interfaces they are very different in reality.
Does that mean, that every interface is an abstract class?
No. An interface (in Java 7 and earlier) can have no method implementations and can define no instance fields. An abstract class can do both. These are just some of the substantive differences.
But when at least one method in a class is abstract.
But its not a class,Its interface.
Instead you can say it's an abstract interface.
And finally :An interface is abstract by definition.
Yes when i decompiled a interface it had the modifier public abstract interface, and by definition they must be abstract to contain abstract methods.
Related
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Why an abstract class implementing an interface can miss the declaration/implementation of one of the interface's methods?
(7 answers)
Closed 7 years ago.
While reading Herbert Schildt I came across partial implementation where overriding is'nt mandatory , But I fail to understand why do we implement such an interface where we don't override its methods :
interface CallBack{
void callback();
}
abstract class Incomplete implements Callback { //Legal
void someMethod();
}
Is there any practical use of such a class or it's just theoretical ?
One use case is a family of classes which all have to implement the callback interface in the same way. So it could be implemented in the abstract superclass and you don't have to handle it in every subclass.
Abstracted classes can't be instantiated, so you would make sure that all subclasses that you will instantiate in you system later handle the callbacks in the same way.
The name it self showing that it's abstract, it need not to implement. Where as the subclasses of that abstract class must and must full fill that definition and needs to implement that methods in that interface.
Every subclass of Incomplete now have to implement Callback.
This question already has answers here:
Why use an abstract class without abstract methods?
(11 answers)
Closed 7 years ago.
I am still confused when abstract class doesn't contain any abstract method, what a purpose of it? why don't use regular class rather than abstract class if it doesn't contain any abstract method ? In fact, I was saw this situation is applied on java and libgdx library or perhaps for every library.
So, because this situation, I was thinking is it very important to know why use abstract class without abstract method rather than regular class.
When you make a class abstract (either with or without abstract methods), you are forcing the users of this class to create concrete sub-classes of it, since they can't instantiate it.
A user of an abstract class must create a concrete derived class.
This can be useful since it allows the author of an abstract class to introduce abstract functions at a later date. The amount of refactoring necessary at that time is then significantly reduced.
This question already has answers here:
Java abstract interface
(9 answers)
Closed 9 years ago.
As we know, interface is to define some method, but some interface defined as a abstract interface, but why ?
for example:
public abstract interface Test{
}
what is the advantage for using abstract to define a interface?
There is no need to add that abstract key word. It's redundant there.
Interfaces are implicitly abstract.
Language Spec JLS# 9.1.1.1 abstract Interfaces
Every interface is implicitly abstract.
This modifier is obsolete and should not be used in new programs.
refer to http://docs.oracle.com/javase/tutorial/java/IandI/createinterface.html for the same.
Also see https://stackoverflow.com/a/18778307/805378 to get the difference between abstract and interface.
to make a class 100% abstract without using 'abstract'.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Interface or abstract class?
Abstract and Interface in java
I am still a student in Java and i came across abstract class and interface.
now when you create an abstract class you write methods like this:
public abstract void something();
but when you create a method in your interface it looks like this:
public void something();
Now im not blind i can see that there is some difference also i do know that you extend an abstract class and you implement an interface.
But can someone tell me what the difference is maybe an example of where you would use one over the other?
There's no difference between those two routines. The difference lies in that abstract classes can contain common logic used for all implementations while interfaces cannot.
"Abstract" basically means that the method has no implementation. The implementation has to be provided by subclasses. As a consequence, one cannot create instances of classes having abstract methods.
Now interfaces in Java are just a collection of method signatures. They cannot contain implementations by design. Thus, the abstract keyword would be redundant. One cannot create instances of interfaces, only of concrete classes that implement the interface.
This question already has answers here:
Implements vs extends: When to use? What's the difference?
(19 answers)
Closed 8 years ago.
What's the difference between the following keywords in Java: implements, extends?
An interface is an abstract specification of how a class should behave whilst a class is a concrete implementation of such a specification.
Therefore, when you write implements you're saying that you are fulfilling some abstract specification in the implementation you've written.
extends means that you take either an implementation (class) or specification (interface) and add to it with different or new functionality (or change the specification of its behaviour), thus modifying its behaviour and extend-ing it.
a class extends another class and implements interface. interface extends another interface.
Interface hasn't any implemented methods all defined methods are empty so if class inherits from the interface it should implement it's methods. But if Class1 inherits from Class2 then it already have some working methods (from Class2) and just extends Class2.