Abstract keyword

Abstract keyword used for method declaration declares the methods without implementations.

Abstract class in java have abstract methods that is not implemented in abstract class, but implemented  in subclasses in java program. If  the class in java program is not required to get instantiated than that class use the abstract keyword but this class rather is available for other classes to extend by other classes.

Abstract keyword will be used in  method declaration to declare that method without providing the implementation in that java program. 

In other words we can say that, it formally unfinished class as well as  method,  that marked with the help of keyword abstract.
 
Defining abstract is a way of preventing someone from instantiating a class that is supposed to be extended first. In java program abstract class is deliberately missing similar to like an interface which will missing all method bodies in the program. Abstract class provides a way to extend an actual class. We will not use new on abstract classes but will use abstract references in the java program, that always point to objects of the class that extends an abstract class.

In java program for practical use of an abstract class, we will define a non-abstract class that extends an abstract one. This will use any of the inherited non-abstract methods.

Most of the time abstract class may extend another abstract class. In that condition it need not implement all in the non-abstract methods. An abstract keyword used both on classes and methods.  In case of class declared with an abstract keyword may not be instantiated that is the only thing that abstract keyword doing. 

Syntact: Abstract Class Syntax

abstract class ClassName { ... ? abstract Type MethodName(); ? ? Type Method() { / method body } }

Example: Declaring an abstract class:

abstract StudentSystem (String name);

If a class is declared abstract type then its methods will be also declared abstract type.

When a method is declared abstract, the method can not have a definition. This is the only effect the abstract keyword has on method.

Here's an example of an abstract method:

abstract int student_id (String name);

Both the abstract classes and abstract methods are skeletons shaped. Abstract keyword defines a structure while without any implementation. In java program an abstract classes have abstract methods but  an abstract class will not necessarily require that methods will also abstract type.