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Abstract class in Java

A class that is declared with abstract keyword, is known as abstract class. Before learning abstract class, let's understand the abstraction first.

Abstraction

Abstraction is a process of hiding the implementation details and showing only functionality to the user.

Another way, it shows only important things to the user and hides the internal details for example sending sms, you just type the text and send the message. You don't know the internal processing about the message delivery.

Abstraction lets you focus on what the object does instead of how it does it.

Ways to achieve Abstaction

There are two ways to achieve abstraction in java

  1. Abstract class (0 to 100%)
  2. Interface (100%)

Abstract class

A class that is declared as abstract is known as abstract class. It needs to be extended and its method implemented. It cannot be instantiated.

Syntax to declare the abstract class

abstract method

A method that is declared as abstract and does not have implementation is known as abstract method.

Syntax to define the abstract method

Example of abstract class that have abstract method

In this example, Bike the abstract class that contains only one abstract method run. It implementation is provided by the Honda class.

Output:running safely..

Understanding the real scenario of abstract class

In this example, Shape is the abstract class, its implementation is provided by the Rectangle and Circle classes. Mostly, we don't know about the implementation class (i.e. hidden to the end user) and object of the implementation class is provided by the factory method.

A factory method is the method that returns the instance of the class. We will learn about the factory method later.

In this example, if you create the instance of Rectangle class, draw method of Rectangle class will be invoked.

Output:drawing circle

Abstract class having constructor, data member, methods etc.

Note: An abstract class can have data member, abstract method, method body, constructor and even main() method.

Output:running safely..
       gear changed
Output:constructor is invoked
running safely..
it has two wheels
30

Rule: If there is any abstract method in a class, that class must be abstract.

Output:compile time error

Rule: If you are extending any abstact class that have abstract method, you must either provide the implementation of the method or make this class abstract.

Another real scenario of abstract class

The abstract class can also be used to provide some implementation of the interface. In such case, the end user may not be forced to override all the methods of the interface.

Note: If you are beginner to java, learn interface first and skip this example.

Output:I am a
       I am b
       I am c
       I am d
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