46 Tutorial : abstract classes In Java with Programming Example Part 2: Hindi смотреть онлайн

46 Tutorial : abstract classes In Java with Programming Example Part 2: Hindi
45 : Tutorial : abstract classes In Java with Programming Example Part 1: Hindi

abstract class is a template definition of methods and variables of a class that contains one or more abstracted methods.

abstract class can not be instantiated directly

class inheriting the abstract class needs to override all the abstracted methods

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:

bstract classes are similar to interfaces. You cannot instantiate them, and they may contain a mix of methods declared with or without an implementation. However, with abstract classes, you can declare fields that are not static and final, and define public, protected, and private concrete methods. With interfaces, all fields are automatically public, static, and final, and all methods that you declare or define (as default methods) are public. In addition, you can extend only one class, whether or not it is abstract, whereas you can implement any number of interfaces.
Although 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
Any class that contains one or more abstract methods must also be declared abstract. To
declare a class abstract, you simply use the abstract keyword in front of the class keyword
at the beginning of the class declaration. There can be no objects of an abstract class. That is,
an abstract class cannot be directly instantiated with the new operator. Such objects would
be useless, because an abstract class is not fully defined. Also, you cannot declare abstract
constructors, or abstract static methods. Any subclass of an abstract class must either implement
all of the abstract methods in the superclass, or be itself declared abstract.