When do we use abstract class and interface.

[jose chiramal]

ABSTRACT CLASS:
Use abstract class in places where you want to use implementation inheritance.
Abstract classes let you define some default behaviour and force subclasses to provide any specific behaviour. ( what does behaviour mean here ?)

INTERFACE :
Use interface when we want to avoid "diamond problem"

Any other points that can be added to the above?

[Sebastian Janisch]

There is more to interfaces and abstract classes than just a few points.

I consider abstract classes a mixture between an interface and a concrete class.

You can both provide an implementation and also declare abstract methods that need to be implemented by any subclass.

Interfaces only provide a contract of what a class can do, no matter how it is implemented.

[Campbell Ritchie]

Look at this FAQ

[Rajkamal Pillai]

Hi,

Since this is a design related question, I'd think talk about one specific such pattern which deals with both Abstract classes and Interfaces requires mention. I'm indeed talking about the Strategy Design Pattern!

According to this pattern:-

All common functionality is segregating all common functionality in a hierarchy higher up to an Abstract class and then make the individual classes implement Interfaces for adding independent functionality. An example for this in real world are Vehicles. E.g. Bus, Plane, Ship e.t.c. The common functionality is implemented in the super Abstract class with the differences by implementing Interfaces.

Hope this is helpful. Any comments on this is welcome.

Cheers,
Raj.

[Seetharaman Venkatasamy]

jose chiramal wrote:
INTERFACE :
Use interface when we want to avoid "diamond problem"

Why do you worry about *Deadly Diamond of Death* ?

By the way: Java doesnt support multiple inheritence

[Seetharaman Venkatasamy]

Raj Kamal wrote:Hi,

Since this is a design related question, I'd think talk about one specific such pattern which deals with both Abstract classes and Interfaces requires mention. I'm indeed talking about the Strategy Design Pattern!

According to this pattern:-

All common functionality is segregating all common functionality in a hierarchy higher up to an Abstract class and then make the individual classes implement Interfaces for adding independent functionality. An example for this in real world are Vehicles. E.g. Bus, Plane, Ship e.t.c. The common functionality is implemented in the super Abstract class with the differences by implementing Interfaces.
Hope this is helpful. Any comments on this is welcome.

Hmmm. another example of Abstract class + Interface = AbstractInterface example: java.util.AbstractList