Abstract Class In Java

Java Abstract classes are used to declare common characteristics of subclasses. An abstract class cannot be instantiated. It can only be used as a superclass for other classes that extend the abstract class. Abstract classes are declared with the abstract keyword. Abstract classes are used to provide a template or design for concrete subclasses down the inheritance tree.
Like any other class, an abstract class can contain fields that describe the characteristics and methods that describe the actions that a class can perform. An abstract class can include methods that contain no implementation. These are called abstract methods. The abstract method declaration must then end with a semicolon rather than a block. If a class has any abstract methods, whether declared or inherited, the entire class must be declared abstract. Abstract methods are used to provide a template for the classes that inherit the abstract methods.

Abstract classes cannot be instantiated; they must be subclassed, and actual implementations must be provided for the abstract methods. Any implementation specified can, of course, be overridden by additional subclasses. An object must have an implementation for all of its methods. You need to create a subclass that provides an implementation for the abstract method.
To declare that your class is an abstract class, use the keyword abstract before the class keyword in your class declaration:

abstract class Number {
. . .
}

If you attempt to instantiate an abstract class, the compiler displays an error similar to the following and refuses to compile your program:
AbstractTest.java:6: class AbstractTest is an abstract class.
It can't be instantiated.
new AbstractTest();
^
1 error

Points of abstract class :
1    Abstract class contains abstract methods.
2    Program can't instantiate an abstract class.
3    Abstract classes contain mixture of non-abstract and abstract methods.
4    If any class contains abstract methods then it must implements all the abstract methods of the abstract class.

Here is a simple example of a class with an abstract method, followed by a class which implements that method:
// A Simple demonstration of abstract.
abstract class A {
abstract void callme();
// concrete methods are still allowed in abstract classes
void callmetoo() {
System.out.println("This is a concrete method.");
}
}
class B extends A {
void callme() {
System.out.println("B's implementation of callme.");
}
}
class AbstractDemo {
public static void main(String args[]) {
B b = new B();
b.callme();
b.callmetoo();
}
}
Notice that no objects of class A are declared in the program. As mentioned, it is not possible to instantiate an abstract class. One other point: class A implements a concrete method called callmetoo( ). This is perfectly acceptable. Abstract classes can include as much implementation as they see fit.
 
Advantage of Abstract Classes

The advantage of using an abstract class is that you can group several related classes together as siblings. Grouping classes together is important in keeping a program organized and understandable. 

An Abstract class is a way to organize inheritance, sometimes it makes no since to implement every method in a class (i.e. a predator class), it may implement some to pass to its children but some methods cannot be implemented without knowing what the class will do (i.e. eat() in a Tiger class). Therefore, abstract classes are templates for future specific classes.

Disadvantage of Abstract Classes

A disadvantage is that abstract classes cannot be instantiated, but most of the time it is logical not to create a object of an abstract class.
 

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