Abstract Class -Real Time Example

Question

Recently in a interview I was asked a very general question  what is abstract in java  I gave the definition and it was followed with some other question on abstract as what is abstract method and difference between abstract method and concrete method and etc  Then at last interviewer asked to give a real time example when I should use or define a class as abstract I got confused I gave some example but he was not convinced   I googled it but found no real solution   So can someone give me real time example i e  when he defined a class as abstract in his her project and why   Thanks

User · Answer

The best example of an abstract class is GenericServlet. GenericServlet is the parent class of HttpServlet. It is an abstract class.

When inheriting 'GenericServlet' in a custom servlet class, the service() method must be overridden.

User · Answer

You should be able to cite at least one from the JDK itself   Look in the java util collections package   There are several abstract classes   You should fully understand interface  abstract  and concrete for Map and why Joshua Bloch wrote it that way

User · Answer

Here  Something about abstract class      Abstract class is an incomplete class so we can t instantiate it  If methods are abstract  class must be abstract  In abstract class  we use abstract and concrete method both  It is illegal to define a class abstract and final both    Real time example--  If you want to make a new car WagonX  in which all the another car s properties are included like color size  engine etc and you want to add some another features like model baseEngine in your car Then simply you create a abstract class WagonX where you use all the predefined functionality as abstract and another functionalities are concrete  which is is defined by you   Another sub class which extend the abstract class WagonX By default it also access the abstract methods which is instantiated in abstract class SubClasses also access the concrete methods by creating the subclass s object  For reusability the code  the developers use abstract class mostly   abstract class WagonX      public abstract void model       public abstract void color       public static void baseEngine                 your logic here          public static void size              logic here        class Car extends WagonX   public void model        logic here   public void color        logic here

User · Answer

I use often abstract classes in conjuction with Template method pattern  In main abstract class I wrote the skeleton of main algorithm and make abstract methods as hooks where suclasses can make a specific implementation  I used often when writing data parser  or processor  that need to read data from one different place  file  database or some other sources   have similar processing step  maybe small differences  and different output  This pattern looks like Strategy pattern but it give you less granularity and can degradated to a difficult mantainable code if main code grow too much or too exceptions from main flow are required  this considerations came from my experience   Just a small example   abstract class MainProcess     public static class Metrics       int skipped      int processed      int stored      int error        private Metrics metrics    protected abstract Iterator lt Item gt  readObjectsFromSource      protected abstract boolean storeItem Item item     protected Item processItem Item item           do something on item and return it to store  or null to skip        return item        public Metrics getMetrics         return metrics           Main method      final public void process         this metrics   new Metrics        Iterator lt Item gt  items   readObjectsFromSource        for Item item   items          metrics processed          item   processItem item         if null    item             if storeItem item             metrics stored            else           metrics error                  else           metrics skipped                          class ProcessFromDatabase extends MainProcess     ProcessFromDatabase String query        this query   query        protected Iterator lt Item gt  readObjectsFromSource         return sessionFactory getCurrentSession   query query  list          protected boolean storeItem Item item        return sessionFactory getCurrentSession   saveOrUpdate item           Here another example

User · Answer

A good example of real time found from here -     A concrete example of an abstract class would be a class called   Animal  You see many animals in real life  but there are only kinds of   animals  That is  you never look at something purple and furry and say    that is an animal and there is no more specific way of defining it     Instead  you see a dog or a cat or a pig    all animals  The point is    that you can never see an animal walking around that isn t more   specifically something else  duck  pig  etc    The Animal is the   abstract class and Duck Pig Cat are all classes that derive from that   base class  Animals might provide a function called  Age  that adds 1   year of life to the animals  It might also provide an abstract method   called  IsDead  that  when called  will tell you if the animal has   died  Since IsDead is abstract  each animal must implement it  So  a   Cat might decide it is dead after it reaches 14 years of age  but a   Duck might decide it dies after 5 years of age  The abstract class   Animal provides the Age function to all classes that derive from it    but each of those classes has to implement IsDead on their own    A business example      I have a persistance engine that will work against any data sourcer    XML  ASCII  delimited and fixed-length   various JDBC sources    Oracle  SQL  ODBC  etc   I created a base  abstract class to provide   common functionality in this persistance  but instantiate the   appropriate  Port   subclass  when persisting my objects   This makes   development of new  Ports  much easier  since most of the work is done   in the superclasses  especially the various JDBC ones  since I not   only do persistance but other things  like table generation   I have   to provide the various differences for each database   The best   business examples of Interfaces are the Collections  I can work with a   java util List without caring how it is implemented  having the List   as an abstract class does not make sense because there are fundamental   differences in how anArrayList works as opposed to a LinkedList    Likewise  Map and Set  And if I am just working with a group of   objects and don t care if it s a List  Map  or Set  I can just use the   Collection interface

[java] Abstract Class:-Real Time Example

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