Diff between ABSTRACT & INTERFACE.

Question

can I know the exact difference between abstract and interface
and also can I know where exactly what can be used. if possible briefly with programs.

Answer 1

An abstract class is meant to be used only when you have a class of a type that will never have an instance, but from which classes are actual types of. For example, class Fruit would demonstrait the qualities required for an abstract class. You should never, in the real world, have an instance of Fruit that isn't actually some kind of Fruit; there's no such thing as the instance of an abstract Fruit. However, there are common traits between apples, oranges, bannanas, etc. Therefor it would make sense to have an abstract class, Fruit, from which the classes Apple, Bannana, Orange, Pear, Peach, etc.. extend. This way you could treat all of these objects as Fruits by upcasting, but you wouldn't have to worry about a generic fruit being created.
An interface is different, it's not a "is a" relationship, but rather an interface between one object and another (Apropriately named isn't it?). It's meant to be used only as a communication channel between objects. For instance, an eyeball would be a good example of an object that has a communication interface. An eyeball watches for information being presented to it and turns it into Images for the Brain, for this purpose there could exist a EyeListener interface:
public interface EyeListener
public void informationReceived(BufferedImage myImage);
Now, the eyeball will just hang out and produce images for the Brain, or whatever other information processing units that will exist in the future that implement the EyeListener interface (Neural CPUs running Java Bytecode). I think it's fairly obvious how a Brain lacks any kind of relationship to the Type Eye, but has a strong relationship with the two instances of Eye it works with.
Another way to solve the problem of providing a communication interface between these two things would be to call the Brain a type of "Eye", and simply extend Eye and override the informationReceived method to get the Image information it produces. And in C++, strangely enough, this kind of use of Object Orientation is fairly common.
And I think everybody can see that there is something really extremely wrong with this.
And you should see by now that relationships that require interfaces are MUCH more common that relationships that would involve an abstract class. Thinking about the relationships this way makes the Class Eye more flexable, and allows it to hide a great deal of it's implementation behind the interface.
-Jason Thomas.