Abstract Class

Abstract Class

An abstract class is a class that cannot be instantiated. The class may contain any or none of the following:

• Ordinary methods. That is, class methods and/or instance methods.
• Abstract methods
• Variables - class variables and/or instance variables.
• Constants.

The purpose of an abstract class is to list items that are common to its sub-classes. An abstract class must be declared with the keyword abstract. An abstract cannot be instantiated. However, you can declare a variable of such a class.

The format for declaring an abstract class is as follows:

	public abstract  class name
	{
		 	:
		public abstract return_type method();
		//Definition cannot be supplied.
			:
	}

An abstract method is one that is declared but its body is not implemented. The following rules govern the way one uses an abstract method:

• An abstract method cannot be implemented within the class that it is declared.
• Abstract methods are not declared in ordinary classes.
• An abstract method is declared using the keyword abstract preceding the method name.

  The format for declaring an abstract method is:
  
  	public abstract return_type method();
  
  

  Note: No java statements must be supplied.

Relationship between an Abstract class and its direct sub-class

• The direct sub-class of an abstract class promises implement all of the abstract methods that it has inherited from its direct super-class, the abstract class. In other words, once a class has been declared to be an abstract class, its direct subclass is expected to implement its abstract method(s).
• If a direct subclass does not implement all of the abstract methods that it inherited from an abstract class, then that sub-class too is an abstract class. In this case the sub-class must be declared an abstract class following the format given above.
• It is an error if a direct subclass fails to implement all of the abstract methods that it inherited from its direct superclass, when at the same time the subclass was not declared to be an abstract class.
• If a sub-class does not require all of the methods in an abstract class, then one could form a concrete class from the abstract class, by supplying dummy bodies for each method. From this point any class that inherits this new class can use only those methods that are necessary for that class.
• A non-abstract method that is defined within an abstract class can be redefined in subsequent sub-class(s). These sub-classes do not necessarily have to be direct subclass of the parent abstract class.

Demo Program No. 3