Can we instantiate an abstract class

Question

During one of my interview  I was asked  If we can instantiate an abstract class    My reply was  No  we can t   But  interviewer told me  Wrong  we can     I argued a bit on this  Then he told me to try this myself at home    abstract class my       public void mymethod             System out print  Abstract             class poly       public static void main String a              my m   new my               m mymethod              Here  I m creating instance of my class and calling method of abstract class  Can anyone please explain this to me  Was I really wrong during my interview

User · Answer

Extending a class doesn t mean that you are instantiating the class   Actually  in your case you are creating an instance of the subclass   I am pretty sure that abstract classes do not allow initiating  So  I d say no  you can t instantiate an abstract class  But  you can extend it   inherit it   You can t directly instantiate an abstract class  But it doesn t mean that you can t get an instance of class  not actully an instance of original abstract class  indirectly  I mean you can not instantiate the orginial abstract class  but you can    Create an empty class Inherit it from abstract class Instantiate the dervied class   So you get access to all the methods and properties in an abstract class via the derived class instance

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Technical Answer  Abstract classes cannot be instantiated - this is by definition and design   From the JLS  Chapter 8  Classes      A named class may be declared abstract    8 1 1 1  and must be declared   abstract if it is incompletely implemented  such a class cannot be   instantiated  but can be extended by subclasses    From JSE 6 java doc for Classes newInstance        InstantiationException - if this Class represents an abstract class  an interface  an array   class  a primitive type  or void  or if the class has no nullary constructor  or if the    instantiation fails for some other reason    You can  of course  instantiate a concrete subclass of an abstract class  including an anonymous subclass  and also carry out a typecast of an object reference to an abstract type   A Different Angle On This - Teamplay  amp  Social Intelligence   This sort of technical misunderstanding happens frequently in the real world when we deal with complex technologies and legalistic specifications      People Skills  can be more important here than  Technical Skills    If competitively and aggressively trying to prove your side of the argument  then you could be theoretically right  but you could also do more damage in having a fight   damaging  face    creating an enemy than it is worth   Be reconciliatory and understanding in resolving your differences   Who knows - maybe you re  both right  but working off slightly different meanings for terms      Who knows - though not likely  it is possible the interviewer deliberately introduced a small conflict misunderstanding to put you into a challenging situation and see how you behave emotionally and socially   Be gracious and constructive with colleagues  follow advice from seniors  and follow through after the interview to resolve any challenge misunderstanding - via email or phone call   Shows you re motivated and detail-oriented

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It s impossible to instantiate an abstract class   What you really can do  has implement some common methods in an abstract class and let others unimplemented  declaring them abstract  and let the concrete descender implement them depending on their needs   Then you can make a factory  which returns an instance of this abstract class  actually his implementer   In the factory you then decide  which implementer to choose  This is known as a factory design pattern      public abstract class AbstractGridManager           private LifecicleAlgorithmIntrface lifecicleAlgorithm                 more private fields            Method implemented in concrete Manager implementors          abstract public Grid initGrid               Methods common to all implementors         public Grid calculateNextLifecicle Grid grid               return this getLifecicleAlgorithm   calculateNextLifecicle grid                      public LifecicleAlgorithmIntrface getLifecicleAlgorithm                 return lifecicleAlgorithm                    public void setLifecicleAlgorithm LifecicleAlgorithmIntrface lifecicleAlgorithm                this lifecicleAlgorithm   lifecicleAlgorithm                           more common logic and getters-setters pairs         The concrete implementer only needs to implement the methods declared as abstract  but will have access to the logic implemented in those classes in an abstract class  which are not declared abstract   public class FileInputGridManager extends AbstractGridManager    private String filePath     Method implemented in concrete Manager implementors  abstract public Grid initGrid     public class FileInputGridManager extends AbstractGridManager        private String filePath         Method implemented in concrete Manager implementors      abstract public Grid initGrid         public Grid initGrid String filePath            List lt Cell gt  cells   new ArrayList lt  gt             char   chars          File file   new File filePath      for example foo txt                more logic         return grid            Then finally the factory looks something like this   public class GridManagerFactory       public static AbstractGridManager getGridManager LifecicleAlgorithmIntrface lifecicleAlgorithm  String    args           AbstractGridManager manager   null              input from the command line         if args length    2               CommandLineGridManager clManager   new CommandLineGridManager                clManager setWidth Integer parseInt args 0                 clManager setHeight Integer parseInt args 1                    possibly more configuration logic                             manager   clManager                        input from the file         else if args length    1               FileInputGridManager fiManager   new FileInputGridManager                fiManager setFilePath args 0                   possibly more method calls from abstract class                             manager   fiManager                           more possible concrete implementors         else              manager   new CommandLineGridManager                      manager setLifecicleAlgorithm lifecicleAlgorithm           return manager            The receiver of AbstractGridManager would call the methods on him and get the logic  implemented in the concrete descender  and partially in the abstract class methods  without knowing what is the concrete implementation he got  This is also known as inversion of control or dependency injection

User · Answer

Here  i m creating instance of my class   No  you are not creating the instance of your abstract class here  Rather you are creating an instance of an anonymous subclass of your abstract class  And then you are invoking the method on your abstract class reference pointing to subclass object   This behaviour is clearly listed in JLS - Section   15 9 1  -     If the class instance creation expression ends in a class body  then   the class being instantiated is an anonymous class  Then           If T denotes a class  then an anonymous direct subclass of the class    named by T is declared  It is a compile-time error if the   class    denoted by T is a final class    If T denotes an interface  then an anonymous direct subclass of    Object that implements the interface named by T is declared    In either case  the body of the subclass is the ClassBody given in    the class instance creation expression    The class being instantiated is the anonymous subclass       Emphasis mine   Also  in JLS - Section   12 5  you can read about the Object Creation Process  I ll quote one statement from that here  -     Whenever a new class instance is created  memory space is allocated   for it with room for all the instance variables declared in the class   type and all the instance variables declared in each superclass of the   class type  including all the instance variables that may be hidden       Just before a reference to the newly created object is returned as the   result  the indicated constructor is processed to initialize the new   object using the following procedure    You can read about the complete procedure on the link I provided     To practically see that the class being instantiated is an Anonymous SubClass  you just need to compile both your classes  Suppose you put those classes in two different files   My java   abstract class My       public void myMethod             System out print  Abstract              Poly java   class Poly extends My       public static void main String a              My m   new My               m myMethod              Now  compile both your source files   javac My java Poly java   Now in the directory where you compiled the source code  you will see the following class files   My class Poly 1 class     Class file corresponding to anonymous subclass Poly class   See that class - Poly 1 class  It s the class file created by the compiler corresponding to the anonymous subclass you instantiated using the below code   new My         So  it s clear that there is a different class being instantiated  It s just that  that class is given a name only after compilation by the compiler   In general  all the anonymous subclasses in your class will be named in this fashion   Poly 1 class  Poly 2 class  Poly 3 class      so on   Those numbers denote the order in which those anonymous classes appear in the enclosing class

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The technical part has been well-covered in the other answers  and it mainly ends in    He is wrong  he doesn t know stuff  ask him to join SO and get it all cleared        I would like to address the fact which has been mentioned in other answers  that this might be a stress-question and is an important tool for many interviewers to know more about you and how do you react to difficult and unusual situations  By giving you incorrect codes  he probably wanted to see if you argued back  To know whether you have the confidence to stand up against your seniors in situations similar to this     P S  I don t know why but I have a feeling that the interviewer has read this post

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Actually we can not create an object of an abstract class directly  What we create is a reference variable of an abstract call  The reference variable is used to Refer to the object of the class which inherits the Abstract class i e  the subclass of the abstract class

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It is a well-established fact that abstract class can not be instantiated as everyone answered   When the program defines anonymous class  the compiler actually creates a new class with different name  has the pattern EnclosedClassName n where n is the anonymous class number   So if you decompile this Java class you will find the code as below   my class  abstract class my        public void mymethod                   System out print  Abstract                poly 1 class  the generated class of the  anonymous class    class poly 1 extends my         ploly cass  public class poly extends my       public static void main String   a                my m   new poly 1       instance of poly 1 class NOT the abstract my class          m mymethod

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Just observations you could make    Why poly extends my  This is useless    What is the result of the compilation  Three files  my class  poly class and poly 1 class If we can instantiate an abstract class like that  we can instantiate an interface too    weird        Can we instantiate an abstract class   No  we can t  What we can do is  create an anonymous class  that s the third file  and instantiate it     What about a super class instantiation   The abstract super class is not instantiated by us but by java   EDIT  Ask him to test this  public static final void main final String   args        final my m1   new my                final my m2   new my                System out println m1    m2        System out println m1 getClass   toString         System out println m2 getClass   toString          output is   false class my 1 class my 2

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About Abstract Classes   Cannot create object of an abstract class Can create variables  can behave like datatypes  If a child can not override at least of one abstract method of the parent  then child also becomes abstract Abstract classes are useless without child classes   The purpose of an abstract class is to behave like a base  In inheritance hierarchy you will see abstract classes towards the top

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my       means that there s an anonymous implementation  not simple instantiation of an object  which should have been     my    You can never instantiate an abstract class

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You can say  we can t instantiate an abstract class  but we can use new keyword to create an anonymous class instance by just adding    as implement body at the the end of the abstract class

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Abstract classes cannot be instantiated  but they can be subclassed  See This Link The best example is Although Calender class has a abstract method getInstance    but when you say Calendar calc Calendar getInstance    calc is referring to the class instance of class GregorianCalendar as  quot GregorianCalendar extends Calendar  quot  Infact annonymous inner type  allows you to create a no-name subclass of the abstract class and an instance of this

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No  you can t instantite an abstract class We instantiate only anonymous class In abstract class we declare abstract methods and define concrete methods only

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You can simply answers  in just one line     No  you can never instance Abstract Class    But  interviewer still not agree  then you can tell him her     all you can do is  you can create an Anonymous Class    And  according to Anonymous class  class declared and instantiate at the same place line   So  it might be possible that  interviewer would be interested to check your confidence level and how much you know about the OOPs

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No  we can t create the object of abstract class  but  create the reference variable of the abstract class  The reference variable is used to refer to the objects of derived classes  Sub classes of Abstract class   Here is the example that illustrates this concept  abstract class Figure         double dim1        double dim2        Figure double a  double b              dim1   a            dim2   b                   area is now an abstract method       abstract double area                  class Rectangle extends Figure            Rectangle double a  double b             super a  b                  override area for rectangle      double area              System out println  Inside Area for Rectangle              return dim1   dim2             class Triangle extends Figure        Triangle double a  double b             super a  b                  override area for right triangle      double area              System out println  Inside Area for Triangle              return dim1   dim2   2             class AbstractAreas        public static void main String args                  Figure f   new Figure 10  10      illegal now          Rectangle r   new Rectangle 9  5            Triangle t   new Triangle 10  8            Figure figref     this is OK  no object is created          figref   r           System out println  Area is     figref area              figref   t           System out println  Area is     figref area                 Here we see that we cannot create the object of  type Figure but we can create a reference variable of type Figure  Here we created a reference variable of type Figure and Figure Class reference variable is used to refer to the objects of  Class Rectangle and Triangle

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The above instantiates an anonymous inner class which is a subclass of the my abstract class  It s not strictly equivalent to instantiating the abstract class itself  OTOH  every subclass instance is an instance of all its super classes and interfaces  so most abstract classes are indeed instantiated by instantiating one of their concrete subclasses   If the interviewer just said  wrong   without explaining  and gave this example  as a unique counterexample  I think he doesn t know what he s talking about  though

[java] Can we instantiate an abstract class?

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