Abstract Class

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. 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. All other functionality of the class still exists like with abstract classes, you can declare fields that are not static and final, and define public, protected, and private concrete methods and all accessed in the same manner.

Ex-

public abstract class GraphicObject {

            // declare fields 

            // declare nonabstract methods

            abstract void draw(); 

    }

Can an abstract class be final?
No a class cannot be marked "abstract" as well as "final". Marking a class "abstract" means it contains partial implementation of methods and hence depends on its subclasses to provide concrete implementation. Marking the class "final" means that no other class can
extend it and hence abstract classes cannot be marked final.


Can abstract class have constructors in Java?
Yes, abstract class can declare and define constructor in Java. Since you can not create instance of abstract class, constructor can only be called during constructor chaining, i.e. when you create instance of concrete implementation class. Also even if you don’t provide any constructor, compiler will add default no argument constructor in abstract class, without that your subclass will not compile, since first statement in any constructor implicitly calls super(), default super class constructor in Java.


Can you create instance of abstract class?
No, you can not create instance of abstract class in Java, they are incomplete. Even though, if your abstract class don’t contain any abstract method, you can not create instance of it. By making a class abstract, you told compiler that, it’s incomplete and should not be instantiated. Java compiler will throw error, when a code tries to instantiate abstract class.


What is abstract method in Java?
An abstract method is a method without body. You just declare method, without defining it and use abstract keyword in method declaration. All method declared inside Java Interface are by default abstract. Here is an example of abstract method in Java

public void abstract sampleMethod();

Can abstract class contains main method in Java ?
Yes, abstract class can contain main method, it just another static method and you can execute Abstract class with main method, until you don’t create any instance.


Is it necessary for abstract class to have abstract method?
No, It’s not mandatory for an abstract class to have any abstract method. You can make a class abstract in Java, by just using abstract keyword in class declaration. Compiler will enforce all structural restriction, applied to abstract class.