What are abstract classes and abstract methods

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Possible Duplicate    Abstract class in Java       I got several explanations but so far I m not able to understand that what are the abstract classes and methods in Java   Some said it has to do something with the security of the program  other said it isn t anything like that   Even from the Dietel  amp  Dietel s book I don t get it s purpose   When where and why do we use it   Kindly explain it like you re teaching a beginner  I would really appreciate your help

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abstract method do not have body A well defined method can t be declared abstract   A class which has abstract method must be declared as abstract   Abstract class can t be instantiated

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Once you get what abstract means in Java  you would ask  why they put this in   Java may work without abstract stuff  BUT it makes part of a certain OO style or vocabulary  There exists really situations where an abstract class or method is an elegant way to express the program authors intention  Most when you are programming a framework or a library that will be used by others

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ABSTRACT CLASSES AND ABSTARCT METHODS FULL DESCRIPTION GO THROUGH IT  abstract method do not have body A well defined method can t be declared abstract   A class which has abstract method must be declared as abstract   Abstract class can t be instantiated

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- Abstract class is one which can t be instantiated  i e  its object cannot be created   - Abstract method are method s declaration without its definition   - A Non-abstract class can only have Non-abstract methods   - An Abstract class can have both the Non-abstract as well as Abstract methods   - If the Class has an Abstract method then the class must also be Abstract   - An Abstract method must be implemented by the very first Non-Abstract sub-class   - Abstract class in Design patterns are used to encapsulate the behaviors that keeps changing

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An abstract class is a class that you can t create an object from  so it is mostly used for inheriting from  I am not sure if you can have static methods in it   An abstract method is a method that the child class must override  it does not have a body  is marked abstract and only abstract classes can have those methods

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An abstract class is a class that can t be instantiated  It s only purpose is for other classes to extend   Abstract methods are methods in the abstract class  have to be declared abstract  which means the extending concrete class must override them as they have no body   The main purpose of an abstract class is if you have common code to use in sub classes but the abstract class should not have instances of its own   You can read more about it here  Abstract Methods and Classes

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An abstract method is a method signature declaration with no body  For instance   public abstract class Shape                  public abstract double getArea        public abstract double getPerimeter        The methods getArea   and getPerimeter   are abstract  Because the Shape class has an abstract method  it must be declared abstract as well  A class may also be declared abstract without any abstract methods  When a class is abstract  an instance of it cannot be created  one can only create instances of  concrete  subclasses  A concrete class is a class that is not declared abstract  and therefore has no abstract methods and implements all inherited abstract methods   For instance   public class Circle extends Shape       public double radius                 public double getArea             return Math PI   radius   radius             public double getPerimeter             return 2 0   Math PI   radius            There are many reasons to do this  One would be to write a method that would be the same for all shapes but that depends on shape-specific behavior that is unknown at the Shape level  For instance  one could write the method   public abstract class Shape                  public void printArea PrintStream out            out println  The area is     getArea               Admittedly  this is a contrived example  but it shows the basic idea  define concrete behavior in terms of unspecified behavior   Another reason for having an abstract class is so you can partially implement an interface  All methods declared in an interface are inherited as abstract methods by any class that implements the interface  Sometimes you want to provide a partial implementation of an interface in a class and leave the details to subclasses  the partial implementation must be declared abstract

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With abstract classes you can have some kind of skeleton for other classes to extend   You can t instantiate them but you can put some common implementation which you can use in the classes that extend them

[java] What are abstract classes and abstract methods?

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