Why abstract class cant create instance???

[Ramesh Shanmugam]

Abstract class is also a class. It also has default constructor.
Then why we cant able to create instance for Abstract class??

Thansk in advance,
Ramesh

[Keith Lynn]

Because, by definition, an abstract class is complete. How could you create an object from a class where a method is not defined? What would happen if you tried to call the method?

[Stan James]

Keith had a finger-check where he meant to type "incomplete" and got "complete".

The whole point of Abstract classes is that you cannot make an instance. Why did the language designers think this was necessary? As Keith said it lets a class designer specify part of a class but leave other necessary parts for later. Many times an abstract class does part of a task for you in a generic way but forces you to override certain methods and provide other parts of the task customized for your particular task.

[Keith Lynn]

Oops I apologize. I typed too fast and didn't proofread. I meant incomplete.

[Z Sugiarto]

yeah, imagine if u have a rule saying: a person HAVE to be either male or female. Can't be both, can't be neither. With abstract class you can have such a design:

Abstract class Person

class Male extends Person

class Female extends Person

it means that you cant instantiate a sexless person. You have to instantiate a Male or a Female, not a Person. The concept of Abstract class allows you to enforce rules like that (note that the above is just example... dont take the design personally) and still maintain some polymorphism power on it.

Person p1 = new Male()
Person p2 = new Female()

p1.dance() -> does michael jackson dance, or something
p2.dance() -> does demi moore striptease dance, or something

[Ramesh Shanmugam]

Thanks Keith, James and Zenikko