: From the language perspective, there are several
differences, few of them are:-
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- An abstract class may contain fields, which are not ‘static’ and ‘final’ as is the case with interfaces.
- It may have few (or all) implemented methods as well, whereas Interfaces can’t have any implementation code. All the methods of an interface are by default ‘abstract’. Methods/Members of an abstract class may have any visibility: public, protected, private, none (package). But, those of an interface can have only one type of visibility: public.
- An abstract class automatically inherits the Object class and thereby includes methods like clone(), equals(), etc. There is no such thing with an interface. Likewise, an abstract class can have a constructor, but an interface can’t have one…
- Another very famous difference is that Interfaces are used to implement multiple inheritance in Java as a class in Java can explicitly have only one super class, but it can implement any number of interfaces… blah blah… :-)
From the performance perspective, the different is
that Interfaces may be little slower as they require extra indirection to find
the corresponding method in the actual class. Though, modern JVMs have already
made that difference very little.
If you want to add a new method to an interface, then you either need to track all the classes implementing that interface or you’ll extend that interface to make a new interface having that extra method(s). In case of an abstract class, you’ll simply add the default implementation of that method and all the code will continue to work.
Many differences are listed already, but the main difference lies in the usage of the two. They are not rivals, but in most of the cases they are complimentary. We need to understand when to use what.
When to use an Interface: it asks you to start everything from scratch. You need to provide implementation of all the methods. So, you should use it to define the contract, which you’re unsure of how the different vendors/producers will implement. So, you can say that Interfaces can be used to enforce certain standards.
When to use an Abstract Class: it is used mostly when you’ve partial implementation ready with you, but not the complete. So, you may declare the incomplete methods as ‘abstract’ and leave it to the clients to implement it the way they actually want. Not all the details can be concrete at the base class level or different clients may like to implement the method differently.
If you want to add a new method to an interface, then you either need to track all the classes implementing that interface or you’ll extend that interface to make a new interface having that extra method(s). In case of an abstract class, you’ll simply add the default implementation of that method and all the code will continue to work.
Many differences are listed already, but the main difference lies in the usage of the two. They are not rivals, but in most of the cases they are complimentary. We need to understand when to use what.
When to use an Interface: it asks you to start everything from scratch. You need to provide implementation of all the methods. So, you should use it to define the contract, which you’re unsure of how the different vendors/producers will implement. So, you can say that Interfaces can be used to enforce certain standards.
When to use an Abstract Class: it is used mostly when you’ve partial implementation ready with you, but not the complete. So, you may declare the incomplete methods as ‘abstract’ and leave it to the clients to implement it the way they actually want. Not all the details can be concrete at the base class level or different clients may like to implement the method differently.
When to use both: if you want to implement multiple inheritance where you have the luxury
of providing partial implementation as well. You’ll then put all that code in
an abstract class (this can be a concrete class as well… but here we assume
that the class is also only partially implemented and hence an abstract class),
extend that class, and implement as may interfaces as you want.
Update[June 25, 2008]: Do interfaces in Java really inherit the Object class? If NOT then how
do we manage to call Object methods on the references of an interface type?
Read more in this article - Do Interfaces inherit the Object class in Java?
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