difference between abstract class and interface

vijayanand · #1 (**permalink**) 09-21-2007, 12:22 AM

Difference between abstract class and interface

a) All the methods declared inside an interface are abstract whereas abstract class must have at least one abstract method and others may be concrete or abstract.

b) In abstract class, key word abstract must be used for the methods
Whereas interface we need not use that keyword for the methods.

c) Abstract class must have subclasses whereas interface can't have subclasses.

$enthil · #2 (**permalink**) 11-23-2007, 10:41 PM

interface contains methods that must be abstract; abstract class may contain concrete methods.
interface contains variables that must be static and final; abstract class may contain non-final and final variables.
members in an interface are public by default, abstract class may contain non-public members.
interface is used to "implements"; whereas abstract class is used to "extends".
interface can be used to achieve multiple inheritance; abstract class can be used as a single inheritance.
interface can "extends" another interface, abstract class can "extends" another class and "implements" multiple interfaces.
interface is absolutely abstract; abstract class can be invoked if a main() exists.
interface is more flexible than abstract class because one class can only "extends" one super class, but "implements" multiple interfaces.
If given a choice, use interface instead of abstract class.

shaalini · #3 (**permalink**) 11-27-2007, 09:19 PM

the main differences between an interface class and abstract class are

1.abstract classes must be given by keyword abstract,where as in interface class with key word interface

2.every method in interface class should be abstract,but this is not necessary in abstract class

3.by default what ever variables we decalre in interface are public static final,where as in abstract class they can be default and instance variables also

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