What is an abstract class? [duplicate]

Answer 1

Okay, now you made an interface with all the methods that change in each implementation. As you program, you notice that some code blocks are shared by ALL implementations of the interface.

These code blocks should go in an abstract class, instead of being repeated in every implementation. That way, when something changes, you only fix the code in the abstract class instead of in every implementation.

It's only abstract because you want all the inheriting classes to have their own implementations.

Answer 2

1. most commonly to serve as a base-class or interface (some languages have a separate interface construct, some don't) - it doesn't know the implementation (that is to be provided by the subclasses / implementing classes)
2. abstraction and re-use
3. when the base-class can provide no meaningful default-implementation for a method (but allowing subclasses to re-use the non-abstract parts of the implementation; any fields, non-abstract methods, etc)

For example:

public abstract class Stream { /* lots of code, some abstract methods */ }

What the heck is a stream by itself? What kind of stream? a stream to a file? a network? a memory buffer? Each may have different and unrelated ways of reading / writing, but provide a common API. It makes no sense to create just a Stream, but via the abstract class, you can code to the Stream API without knowing the details:

Stream s = CreateStream(...); // I don't *care* what kind of stream
s.Write(new byte[] {1,2,3,4,5});
s.Close();

Answer 3

Same as with interfaces, an abstract class is a contract in which you have provided some general (or abstract) functionality that is applicable to many scenarios, but expect an implementer to provide some functionality that is specific and/or different for each scenario. Someone mentioned the Stream example - very good example.

Answer 4

The point is to specify methods that derived classes must implement, same as an interface, but also provide some of the implementation (which an interface cannot do). Abstract classes are never, strictly speaking, "necessary" - but they are useful. Just search the .NET or Java standard library for them and see for yourself how they are used. You will find there are plenty of examples.

Answer 5

Abstract classes serve an useful purpose by being able to create a class that embodies a concept rather than something concrete. E.g, Animal can be a class, but no one just an animal, it is either a Bird, Dog, or Cat etc which are different kinds of animals. Hope this helps. It also goes in conjunction with concepts like inheritance and polymorphism.

Answer 6

Abstract (Base) classes give you semi-concrete classes to implement your class hierarchy against. They allow you to do several things:

1. Consolidate common behavior (unlike an interface which only defines the contract)
2. Provide default (and optionally override-able) implementations for functions
3. Provide well defined branch points for inheritance hierarchies
4. Control IoC injection points

The list goes on.