The abstract modifier can be used with classes, methods, properties, indexers, and events. Use the abstract modifier in a class declaration to indicate that a class is intended only to be a base class of other classes. Members marked as abstract, or included in an abstract class, must be implemented by classes that derive from the abstract class.
Examples and practices described in this page don't take advantage of improvements introduced in later releases. Inheritance In the preceding lessons, you have seen inheritance mentioned several times.
In the Java language, classes can be derived from other classes, thereby inheriting fields and methods from those classes.
A class that is derived from another class is called a subclass also a derived class, extended class, or child class. The class from which the subclass is derived is called a superclass also a base class or a parent class.
Excepting Object, which has no superclass, every class has one and only one direct superclass single inheritance. In the absence of any other explicit superclass, every class is implicitly a subclass of Object.
Classes can be derived from classes that are derived from classes that are derived from classes, and so on, and ultimately derived from the topmost class, Object.
Such a class is said to be descended from all the classes in the inheritance chain stretching back to Object. The idea of inheritance is simple but powerful: When you want to create a new class and there is already a class that includes some of the code that you want, you can derive your new class from the existing class.
In doing this, you can reuse the fields and methods of the existing class without having to write and debug! A subclass inherits all the members fields, methods, and nested classes from its superclass. Constructors are not members, so they are not inherited by subclasses, but the constructor of the superclass can be invoked from the subclass.
In the Java platform, many classes derive directly from Object, other classes derive from some of those classes, and so on, forming a hierarchy of classes. Classes near the bottom of the hierarchy provide more specialized behavior. An Example of Inheritance Here is the sample code for a possible implementation of a Bicycle class that was presented in the Classes and Objects lesson: Except for the constructor, it is as if you had written a new MountainBike class entirely from scratch, with four fields and five methods.
However, you didn't have to do all the work. This would be especially valuable if the methods in the Bicycle class were complex and had taken substantial time to debug.
What You Can Do in a Subclass A subclass inherits all of the public and protected members of its parent, no matter what package the subclass is in.
If the subclass is in the same package as its parent, it also inherits the package-private members of the parent. You can use the inherited members as is, replace them, hide them, or supplement them with new members: The inherited fields can be used directly, just like any other fields.
You can declare a field in the subclass with the same name as the one in the superclass, thus hiding it not recommended.
You can declare new fields in the subclass that are not in the superclass. The inherited methods can be used directly as they are. You can write a new instance method in the subclass that has the same signature as the one in the superclass, thus overriding it. You can write a new static method in the subclass that has the same signature as the one in the superclass, thus hiding it.
You can declare new methods in the subclass that are not in the superclass. You can write a subclass constructor that invokes the constructor of the superclass, either implicitly or by using the keyword super. The following sections in this lesson will expand on these topics. Private Members in a Superclass A subclass does not inherit the private members of its parent class.Abstract Class in Java: It is like a template, so you have to extend it and build on it before you can use it.
You may also want to read this: Uday unfortunately you cannot access the concrete methods of an abstract class unless you extend the class. An abstract class is actually almost useless on their own. You can see an abstract. And you may argue that the we can make an abstract class and make the MSDocReader class extend that abstract class.
But it can happen that the MSDocReader class may be extending some other class and since java does not support multiple inheritance., it can create problems.
A class which is declared with the abstract keyword is known as an abstract class in Java. It can have abstract and non-abstract methods (method with the body).
Before learning the Java abstract class, let's understand the abstraction in Java first.
If we were able to extend an interface using a class, the result could no longer be called an interface, but it must be called an abstract class. As of today I don’t know any programming language that allows us to do something like that. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods." CRUD " Create, Retrieve, Update, Delete".
I never understood the use of abstract classes until we wrote a CMS at work. Having an Abstract class for blocks or plugins seemed to make perfect sense as anyone trying to extend it would have to used a standard for example and all the code was a cleaner and there was no unearthing of OO madness.