Java Abstraction - Java

What is Java Abstraction?

As per dictionary, abstraction is the quality of dealing thru concepts rather than events. For instance, when you consider the case of e-mail, difficult details such as what occurs as soon as you send an e-mail, the protocol your e-mail server uses are unseen from the user. So, to send an e-mail you just need to type the content, mention the address of the receiver, and click send.

Similarly in Object-oriented programming, abstraction is a procedure of hiding the operation details from the user, only the functionality will be provided to the user. In simple terms, the user will have the evidence on what the object does instead of how it does it.

In Java, abstraction is achieved using Abstract classes and interfaces.

Abstract Class

A class which contains the abstract keyword in its declaration is known as abstract class.

  • Abstract classes may or may not contain abstract methods, i.e., methods without body ( public void get(); )
  • But, if a class has at least one abstract method, then the class must be declared abstract.
  • If a class is declared abstract, it cannot be instantiated.
  • To use an abstract class, you have to inherit it from another class, provide implementations to the abstract methods in it.
  • If you inherit an abstract class, you have to provide implementations to all the abstract methods in it.

Example

This section provides you an example of the abstract class. To create an abstract class, just use the abstract keyword before the class keyword, in the class declaration.

You can observe that except abstract methods the Employee class is same as normal class in Java. The class is now abstract, but it still has three fields, seven methods, and one constructor.

Now you can try to instantiate the Employee class in the following way –

When you compile the above class, it gives you the following error –

Inheriting the Abstract Class

We can inherit the properties of Employee class just like concrete class in the following way −

Example

Here, you cannot instantiate the Employee class, but you can instantiate the Salary Class, and using this instance you can access all the three fields and seven methods of Employee class as shown below.

This produces the following result −

Output

Abstract Methods

If you want a class to contain a particular method but you want the actual implementation of that method to be determined by child classes, you can declare the method in the parent class as an abstract.

  • abstract keyword is used to declare the method as abstract.
  • You have to place the abstract keyword before the method name in the method declaration.
  • An abstract method contains a method signature, but no method body.
  • Instead of curly braces, an abstract method will have a semoi colon (;) at the end.

Following is an example of the abstract method.

Example

Declaring a method as abstract has two consequences −

  • The class containing it must be declared as abstract.
  • Any class inheriting the current class must either override the abstract method or declare itself as abstract.

Note − Eventually, a descendant class has to implement the abstract method; otherwise, you would have a hierarchy of abstract classes that cannot be instantiated.

Suppose Salary class inherits the Employee class, then it should implement the computePay() method as shown below −

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