Abstract class and interface

Abstract class and interface

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 In object-oriented languages, interfaces generally form a hierarchy.

 1) First and major difference between abstract class and interface is that, abstract class is a class while interface is a interface, means by extending abstract class you can not extend another class because Java does not support multiple inheritance but you can implement multiple inheritance in Java.

2) Second difference between interface and abstract class in Java is that you can not create non abstract method in interface, every method in interface is by default abstract, but you can create non abstract method in abstract class. Even a class which doesn't contain any abstract method can be abstract by using abstract keyword.

3) Third difference between abstract class and interface in Java is that abstract class are slightly faster than interface because interface involves a search before calling any overridden method in Java. This is not a significant difference in most of cases but if you are writing a time critical application than you may not want to leave any stone unturned.

4) Fourth difference between abstract class vs interface in Java is that, interface are better suited for Type declaration and abstract class is more suited for code reuse and evolution perspective.

5) Another notable difference between interface and abstract class is that when you add a new method in existing interface it breaks all its implementation and you need to provide an implementation in all clients which is not good. By using abstract class you can provide default implementation in super class.


Read more: http://java67.blogspot.com/2012/09/what-is-difference-between-interface-abstract-class-java.html#ixzz41RJ4z4vL


In Java, when should you use an interface instead of an abstract class?


The main points to consider [two already mentioned here] are:
  • Interfaces are more flexible, because a class can implement multiple interfaces.  Since Java does not have multiple inheritance, using abstract classes prevents your users from using any other class hierarchy. In general, prefer interfaces when there are no default implementations or state. Java collections offer good examples of this (Map, Set, etc.).
  • Abstract classes have the advantage of allowing better forward compatibility.  Once clients use an interface, you cannot change it; if they use an abstract class, you can still add behavior without breaking existing code. If compatibility is a concern, consider using abstract classes.
  • Even if you do have default implementations or internal state, consider offering an interface and an abstract implementation of it. This will assist clients, but still allow them greater freedom if desired [1].
  • )Main difference in my perspective is that Interface must contain abstract methods. whereas in an abstract class there is no such necessity and so it can contain even some methods of its own. Security factors takes a blow in such cases.

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