Abstract Class vs. Interface

[JiaPei Jen]

How do I determine an object should be an abstract class or an interface? Could the explanation be given with examples? Thanks.

[Parag Shah]

IMO an Interface must be be preffered to abstract classes unless you wish to have functionality that an Interface does not provide. This would include having non-primitive attributes (you can only define primitive data members in an interface). Also all data members in an Interface are final and static, so that may be another consideration. Also since we cannot implement methods within Interfaces, we must use abstract (non-pure) where we wish to impplement a method in the base class which will be used by the sub-classes.
Since Java does not support multiple inheritence it is always better to use Interfaces wherever possible so that we do not lock up a sub-class's ability to inherit from any other class.
Parag

[Doug Wang]

Hi JiaPei,
Nice to see you here. I composed a mail just now which invited you to participate in this forum.

[Junilu Lacar]

Parag is right about preferring interfaces over abstract classes. The main reason for choosing abstract class over interface, as given by Joshua Bloch in his book "Effective Java Programming Language Guide" is if ease of evolution is more important than flexibility and power.
You can add a method to an abstract class that provides for some basic functionality. Doing the same with an interface would require making changes to all implementing classes and adding the new method. When you have a published interface, one that has been released to the outside world, it would be practically impossible to change it at all.
Contrary to what Parag said earlier though, interfaces can have non-primitive members. Go ahead, try it.
Junilu

[Doug Wang]

Hi JiaPei,
Two articles about Abstract classes Vs. Interfaces from JavaWorld you'd like to check out: Q & A and Move from theory to practice on with a complete example.
Junilu, I am not sure I catch the statement in the above Q&A article "If you need to change your design, make it an interface." It sounds contrary to what you said.
Your thoughts?
[ March 28, 2002: Message edited by: Doug Wang ]

[JiaPei Jen]

Thanks, Doug. The information is very helpful.