Abstract class in Java

Question

What is an  abstract class  in Java

User · Answer

Get your answers here   Abstract class vs Interface in Java  Can an abstract class have a final method   BTW - those are question you asked recently  Think about a new question to build up reputation       Edit   Just realized  that the posters of this and the referenced questions have the same or at least similiar name but the user-id is always different  So either  there s a technical problem  that keyur has problems logging in again and finding the answers to his questions or this is a sort of game to entertain the SO community

User · Answer

An abstract class is one that isn t fully implemented but provides something of a blueprint for subclasses   It may be partially implemented in that it contains fully-defined concrete methods  but it can also hold abstract methods   These are methods with a signature but no method body   Any subclass must define a body for each abstract method  otherwise it too must be declared abstract  Because abstract classes cannot be instantiated  they must be extended by at least one subclass in order to be utilized   Think of the abstract class as the generic class  and the subclasses are there to fill in the missing information

User · Answer

Solution - base class  abstract   public abstract class Place    String Name  String Postcode  String County  String Area   Place                  public static Place make String Incoming            if  Incoming length    lt  61  return  null            String Name    Incoming substring 4 26   trim            String County    Incoming substring 27 48   trim            String Postcode    Incoming substring 48 61   trim            String Area    Incoming substring 61   trim             Place created          if  Name equalsIgnoreCase Area                     created   new Area Area County Postcode             else                   created   new District Name County Postcode Area                     return  created              public String getName             return  Name              public String getPostcode             return  Postcode              public String getCounty             return  County              public abstract String getArea

User · Answer

Class which can have both concrete and non-concrete methods i e  with and without body    Methods without implementation must contain  abstract  keyword  Abstract class can t be instantiated

User · Answer

An abstract class is a class that is declared abstract     it may or may not include abstract methods  Abstract classes cannot be instantiated  but they can be subclassed   In other words  a class that is declared with abstract keyword  is known as abstract class in java  It can have abstract method without body  and non-abstract methods  method with body    Important Note - Abstract classes cannot be used to instantiate objects  they can be used to create object references  because Java s approach to run-time Polymorphism is implemented through the use of superclass references  Thus  it must be possible to create a reference to an abstract class so that it can be used to point to a subclass object  You will see this feature in the below example  abstract class Bike      abstract void run           class Honda4 extends Bike        void run            System out println  running safely                   public static void main String args              Bike obj   new Honda4             obj run

User · Answer

An abstract class is a class which cannot be instantiated   An abstract class is used by creating an inheriting subclass that can be instantiated   An abstract class does a few things for the inheriting subclass    Define methods which can be used by the inheriting subclass  Define abstract methods which the inheriting subclass must implement  Provide a common interface which allows the subclass to be interchanged with all other subclasses    Here s an example   abstract public class AbstractClass       abstract public void abstractMethod        public void implementedMethod     System out print  implementedMethod            final public void finalMethod     System out print  finalMethod            Notice that  abstractMethod    doesn t have any method body   Because of this  you can t do the following   public class ImplementingClass extends AbstractClass          ERROR      There s no method that implements abstractMethod     So there s no way for the JVM to know what it s supposed to do when it gets something like new ImplementingClass   abstractMethod     Here s a correct ImplementingClass   public class ImplementingClass extends AbstractClass       public void abstractMethod     System out print  abstractMethod            Notice that you don t have to define implementedMethod   or finalMethod     They were already defined by AbstractClass   Here s another correct ImplementingClass   public class ImplementingClass extends AbstractClass       public void abstractMethod     System out print  abstractMethod            public void implementedMethod     System out print  Overridden           In this case  you have overridden implementedMethod     However  because of the final keyword  the following is not possible   public class ImplementingClass extends AbstractClass       public void abstractMethod     System out print  abstractMethod            public void implementedMethod     System out print  Overridden           public void finalMethod     System out print  ERROR           You can t do this because the implementation of finalMethod   in AbstractClass is marked as the final implementation of finalMethod    no other implementations will be allowed  ever   Now you can also implement an abstract class twice   public class ImplementingClass extends AbstractClass       public void abstractMethod     System out print  abstractMethod            public void implementedMethod     System out print  Overridden             In a separate file  public class SecondImplementingClass extends AbstractClass       public void abstractMethod     System out print  second abstractMethod            Now somewhere you could write another method   public tryItOut         ImplementingClass a   new ImplementingClass        AbstractClass b   new ImplementingClass         a abstractMethod          prints  abstractMethod        a implementedMethod       prints  Overridden        lt -- same     a finalMethod             prints  finalMethod         b abstractMethod          prints  abstractMethod        b implementedMethod       prints  Overridden        lt -- same     b finalMethod             prints  finalMethod         SecondImplementingClass c   new SecondImplementingClass        AbstractClass d   new SecondImplementingClass         c abstractMethod          prints  second abstractMethod        c implementedMethod       prints  implementedMethod        c finalMethod             prints  finalMethod         d abstractMethod          prints  second abstractMethod        d implementedMethod       prints  implementedMethod        d finalMethod             prints  finalMethod        Notice that even though we declared b an AbstractClass type  it displays  Overriden    This is because the object we instantiated was actually an ImplementingClass  whose implementedMethod   is of course overridden   You may have seen this referred to as polymorphism    If we wish to access a member specific to a particular subclass  we must cast down to that subclass first      Say ImplementingClass also contains uniqueMethod      To access it  we use a cast to tell the runtime which type the object is AbstractClass b   new ImplementingClass      ImplementingClass b  uniqueMethod      Lastly  you cannot do the following   public class ImplementingClass extends AbstractClass  SomeOtherAbstractClass              implementation     Only one class can be extended at a time   If you need to extend multiple classes  they have to be interfaces   You can do this   public class ImplementingClass extends AbstractClass implements InterfaceA  InterfaceB              implementation     Here s an example interface   interface InterfaceA       void interfaceMethod        This is basically the same as   abstract public class InterfaceA       abstract public void interfaceMethod        The only difference is that the second way doesn t let the compiler know that it s actually an interface   This can be useful if you want people to only implement your interface and no others   However  as a general beginner rule of thumb  if your abstract class only has abstract methods  you should probably make it an interface   The following is illegal   interface InterfaceB       void interfaceMethod     System out print  ERROR           You cannot implement methods in an interface   This means that if you implement two different interfaces  the different methods in those interfaces can t collide   Since all the methods in an interface are abstract  you have to implement the method  and since your method is the only implementation in the inheritance tree  the compiler knows that it has to use your method

User · Answer

Simply speaking  you can think of an abstract class as like an Interface with a bit more capabilities    You cannot instantiate an Interface  which also holds for an abstract class    On your interface you can just define the method headers and ALL of the implementers are forced to implement all  of them  On an abstract class you can also define your method headers but here - to the difference of the interface - you can also define the body  usually a default implementation  of the method  Moreover when other classes extend  note  not implement and therefore you can also have just one abstract class per child class  your abstract class  they are not forced to implement all of your methods of your abstract class  unless you specified an abstract method  in such case it works like for interfaces  you cannot define the method body    public abstract class MyAbstractClass    public abstract void DoSomething        Otherwise for normal methods of an abstract class  the  inheriters  can either just use the default behavior or override it  as usual   Example   public abstract class MyAbstractClass     public int CalculateCost int amount          do some default calculations        this can be overriden by subclasses if needed          this MUST be implemented by subclasses   public abstract void DoSomething

User · Answer

It do nothing  just provide a common template that will be shared for it s subclass

User · Answer

A class that is declared using the abstract keyword is known as abstract class  Abstraction is a process of hiding the data implementation details  and showing only functionality to the user  Abstraction lets you focus on what the object does instead of how it does it   Main things of abstract class   An abstract class may or may not contain abstract methods There can be non abstract methods   An abstract method is a method that is declared without an  implementation  without braces  and followed by a semicolon   like this   ex   abstract void moveTo double deltaX  double deltaY   If a class has at least one abstract method then that class must be abstract  Abstract classes may not be instantiated  You are not allowed to create object of Abstract class  To use an abstract class  you have to inherit it from another class  Provide implementations to all the abstract methods in it   If you inherit an abstract class  you have to provide implementations to all the abstract methods in it    Declare abstract class Specifying abstract keyword before the class during declaration makes it abstract  Have a look at the code below   abstract class AbstractDemo      Declare abstract method Specifying abstract keyword before the method during declaration makes it abstract  Have a look at the code below    abstract void moveTo     no body   Why we need to abstract classes   In an object-oriented drawing application  you can draw circles  rectangles  lines  Bezier curves  and many other graphic objects  These objects all have certain states  for ex -  position  orientation  line color  fill color  and behaviors  for ex -  moveTo  rotate  resize  draw  in common  Some of these states and behaviors are the same for all graphic objects  for ex   fill color  position  and moveTo   Others require different implementation for ex  resize or draw   All graphic objects must be able to draw or resize themselves  they just differ in how they do it     This is a perfect situation for an abstract superclass  You can take advantage of the similarities  and declare all the graphic objects to inherit from the same abstract parent object  for ex   GraphicObject  as shown in the following figure    First  you declare an abstract class  GraphicObject  to provide member variables and methods that are wholly shared by all subclasses  such as the current position and the moveTo method  GraphicObject also declared abstract methods  such as draw or resize  that need to be a implemented by all subclasses but must  be implemented in different ways  The GraphicObject class can look something like this    abstract class GraphicObject      void moveTo int x  int y           Inside this method we have to change the position of the graphic         object according to x y             This is the same in every GraphicObject  Then we can implement here          abstract void draw       But every GraphicObject drawing case is                             unique  not common  Then we have to create that                             case inside each class  Then create these                                methods as abstract    abstract void resize        Usage of abstract method in sub classes Each non abstract subclasses of GraphicObject  such as Circle and Rectangle  must provide implementations for the draw and resize methods   class Circle extends GraphicObject     void draw           Add to some implementation here       void resize           Add to some implementation here          class Rectangle extends GraphicObject     void draw           Add to some implementation here       void resize           Add to some implementation here         Inside the main method you can call all methods like this   public static void main String args        GraphicObject c   new Circle       c draw       c resize       c moveTo 4 5          Ways to achieve abstraction in Java   There are two ways to achieve abstraction in java   Abstract class  0 to 100   Interface  100     Abstract class with constructors  data members  methods  etc  abstract class GraphicObject      GraphicObject         System out println  GraphicObject  is created          void moveTo int y  int x           System out println  Change position according to    x    and     y         abstract void draw       class Circle extends GraphicObject     void draw         System out println  Draw the Circle           class TestAbstract      public static void main String args         GraphicObject  grObj   new Circle        grObj draw       grObj moveTo 4 6          Output   GraphicObject  is created Draw the Circle Change position according to 6 and 4   Remember two rules    If the class has few abstract methods and few concrete methods  declare it as an abstract class  If the class has only abstract methods  declare it as an interface    References      TutorialsPoint - Java Abstraction BeginnersBook - Java Abstract Class Method Java Docs - Abstract Methods and Classes JavaPoint - Abstract Class in Java

User · Answer

An abstract class can not be directly instantiated  but must be derived from to be usable  A class MUST be abstract if it contains abstract methods  either directly  abstract class Foo       abstract void someMethod        or indirectly  interface IFoo       void someMethod       abstract class Foo2 implements IFoo       However  a class can be abstract without containing abstract methods  Its a way to prevent direct instantation  e g   abstract class Foo3      class Bar extends Foo3       Foo3 myVar   new Foo3       illegal  class is abstract Foo3 myVar   new Bar       allowed    The latter style of abstract classes may be used to create  interface-like  classes  Unlike interfaces an abstract class is allowed to contain non-abstract methods and instance variables  You can use this to provide some base functionality to extending classes   Another frequent pattern is to implement the main functionality in the abstract class and define part of the algorithm in an abstract method to be implemented by an extending class  Stupid example   abstract class Processor       protected abstract int   filterInput int   unfiltered        public int process int   values            int   filtered   filterInput values              do something with filtered input          class EvenValues extends Processor       protected int   filterInput int   unfiltered               remove odd numbers          class OddValues extends Processor       protected int   filterInput int   unfiltered               remove even numbers

User · Answer

Little addition to all these posts      Sometimes you may want to declare a   class and yet not know how to define   all of the methods that belong to that   class  For example  you may want to   declare a class called Writer and   include in it a member method called   write    However  you don t know how to code write   because it is   different for each type of Writer   devices  Of course  you plan to handle   this by deriving subclass of Writer    such as Printer  Disk  Network and   Console

User · Answer

It s a class that cannot be instantiated  and forces implementing classes to  possibly  implement abstract methods that it outlines

User · Answer

From oracle documentation Abstract Methods and Classes   An abstract class is a class that is declared abstract   it may or may not include abstract methods Abstract classes cannot be instantiated  but they can be subclassed  An abstract method is a method that is declared without an implementation  without braces  and followed by a semicolon   like this  abstract void moveTo double deltaX  double deltaY    If a class includes abstract methods  then the class itself must be declared abstract  as in  public abstract class GraphicObject         declare fields       declare nonabstract methods    abstract void draw       When an abstract class is subclassed  the subclass usually provides implementations for all of the abstract methods in its parent class  However  if it does not  then the subclass must also be declared abstract  Since abstract classes and interfaces are related  have a look at below SE questions  What is the difference between an interface and abstract class  How should I have explained the difference between an Interface and an Abstract class

User · Answer

A Java class becomes abstract under the following conditions   1  At least one of the methods is marked as abstract   public abstract void myMethod     In that case the compiler forces you to mark the whole class as abstract   2  The class is marked as abstract   abstract class MyClass   As already said  If you have an abstract method the compiler forces you to mark the whole class as abstract  But even if you don t have any abstract method you can still mark the class as abstract   Common use   A common use of abstract classes is to provide an outline of a class similar like an interface does  But unlike an interface it can already provide functionality  i e  some parts of the class are implemented and some parts are just outlined with a method declaration    abstract    An abstract class cannot be instantiated  but you can create a concrete class based on an abstract class  which then can be instantiated  To do so you have to inherit from the abstract class and override the abstract methods  i e  implement them

[java] Abstract class in Java

Examples related to java

Examples related to oop

Examples related to interface

Examples related to abstract-class