When and Why to use abstract classes methods

Question

I have some basic questions about abstract classes methods  I know the basic use of abstract classes is to create templates for future classes  But are there any more uses for them  When should you prefer them over interfaces and when not  Also  when are abstract methods useful

User · Answer

read the following article http   mycodelines wordpress com 2009 09 01 in-which-scenario-we-use-abstract-classes-and-interfaces      Abstract Classes           When you have a requirement where   your base class should provide default   implementation of certain methods   whereas other methods should be open   to being overridden by child classes   use abstract classes       For e g  again take the example of the   Vehicle class above  If we want all   classes deriving from Vehicle to   implement the Drive   method in a   fixed way whereas the other methods   can be overridden by child classes  In   such a scenario we implement the   Vehicle class as an abstract class   with an implementation of Drive while   leave the other methods   properties   as abstract so they could be   overridden by child classes            The purpose of an abstract class is   to provide a common definition of a   base class that multiple derived   classes can share       For example a class library may define   an abstract class that is used as a   parameter to many of its functions and   require programmers using that library   to provide their own implementation of   the class by creating a derived class       Use an abstract class      When creating a class library which   will be widely distributed or   reused   especially to clients  use an   abstract class in preference to an   interface  because  it simplifies   versioning  This is the practice used   by the Microsoft team which developed   the Base Class Library    COM was   designed around interfaces   Use an   abstract class to define a common base   class for a family of types  Use an   abstract class to provide default   behavior  Subclass only a base class   in a hierarchy to which the class   logically belongs

User · Answer

Abstract classes methods are generally used when a class provides some high level functionality but leaves out certain details to be implemented by derived classes   Making the class method abstract ensures that it cannot be used on its own  but must be specialized to define the details that have been left out of the high level implementation   This is most often used with the template method pattern   http   en wikipedia org wiki Template method pattern

User · Answer

I know basic use of abstract classes   is to create templates for future   classes  But are there any more uses   of them    Not only can you define a template for children  but Abstract Classes offer the added benefit of letting you define functionality that your child classes can utilize later    You could not provide a default method implementation in an Interface prior to Java 8      When should you prefer them over   interfaces and when not    Abstract Classes are a good fit if you want to provide implementation details to your children but don t want to allow an instance of your class to be directly instantiated  which allows you to partially define a class     If you want to simply define a contract for Objects to follow  then use an Interface      Also when are abstract methods useful    Abstract methods are useful in the same way that defining methods in an Interface is useful  It s a way for the designer of the Abstract class to say  any child of mine MUST implement this method

User · Answer

At a very high level   Abstraction of any kind comes down to separating concerns    Client  code of an abstraction doesn t care how the contract exposed by the abstraction is fulfilled   You usually don t care if a string class uses a null-terminated or buffer-length-tracked internal storage implementation  for example   Encapsulation hides the details  but by making classes methods etc  abstract  you allow the implementation to change or for new implementations to be added without affecting the client code

User · Answer

Typically one uses an abstract class to provide some incomplete functionality that will be fleshed out by concrete subclasses   It may provide methods that are used by its subclasses  it may also represent an intermediate node in the class hierarchy  to represent a common grouping of concrete subclasses  distinguishing them in some way from other subclasses of its superclass   Since an interface can t derive from a class  this is another situation where a class  abstract or otherwise  would be necessary  versus an interface   A good rule of thumb is that only leaf nodes of a class hierarchy should ever be instantiated   Making non-leaf nodes abstract is an easy way of ensuring that

[abstract-class] When and Why to use abstract classes/methods?

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