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Here is the MSDN article on abstract classes, but I really don't get it...
When should I really use abstract classes? What are the advantages of using abstract classes?
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Abstract classes should be used primarily for objects that are closely related, whereas interfaces are best suited for providing common functionality to unrelated classes. If you are designing small, concise bits of functionality, use interfaces. If you are designing large functional units, use an abstract class.
Abstract class: is a restricted class that cannot be used to create objects (to access it, it must be inherited from another class). Abstract method: can only be used in an abstract class, and it does not have a body. The body is provided by the subclass (inherited from).
If both the Java interface and Java abstract class are used to achieve abstraction, when should an interface and abstract class be used? An abstract class is used when the user needs to define a template for a group of subclasses. An interface is used when a user needs to define a role for other classes.
The only reason for declaring a class as abstract is so that it can't be instantiated. There are situations where you will have common functionality that is shared between a number of classes, but by itself that common functionality does not represent an object or represents an incomplete object.
Abstract classes are useful when you need a class for the purpose of inheritance and polymorphism, but it makes no sense to instantiate the class itself, only its subclasses. They are commonly used when you want to define a template for a group of subclasses that share some common implementation code, but you also want to guarantee that the objects of the superclass cannot be created.
For instance, let's say you need to create Dog, Cat, Hamster and Fish objects. They possess similar properties like color, size, and number of legs as well as behavior so you create an Animal superclass. However, what color is an Animal? How many legs does an Animal object have? In this case, it doesn't make much sense to instantiate an object of type Animal but rather only its subclasses.
Abstract classes also have the added benefit in polymorphism–allowing you to use the (abstract) superclass's type as a method argument or a return type. If for example you had a PetOwner class with a train() method you can define it as taking in an object of type Animal e.g. train(Animal a) as opposed to creating a method for every subtype of Animal.
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