Abstract class in Java
A class which is declared with the abstract keyword is known as an abstract class in Java. It can have abstract and non-abstract methods (method with the body).
Before learning the Java abstract class, let's understand the abstraction in Java first.
Abstraction in Java
Abstraction is a process of hiding the implementation details and showing only functionality to the user.
Another way, it shows only essential things to the user and hides the internal details, for example, sending SMS where you 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 Abstraction
There are two ways to achieve abstraction in java
Abstract class in Java
A class which is declared as abstract is known as an abstract class. It can have abstract and non-abstract methods. It needs to be extended and its method implemented. It cannot be instantiated.
Points to Remember
Example of abstract class
Abstract Method in Java
A method which is declared as abstract and does not have implementation is known as an abstract method.
Example of abstract method
Example of Abstract class that has an abstract method
In this example, Bike is an abstract class that contains only one abstract method run. Its implementation is provided by the Honda class.Test it Now
Understanding the real scenario of Abstract class
In this example, Shape is the abstract class, and its implementation is provided by the Rectangle and Circle classes.
Mostly, we don't know about the implementation class (which is hidden to the end user), and an object of the implementation class is provided by the factory method.
A factory method is a 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.
File: TestAbstraction1.javaTest it Now
Another example of Abstract class in java
File: TestBank.javaTest it Now
Rate of Interest is: 7 % Rate of Interest is: 8 %
Abstract class having constructor, data member and methods
An abstract class can have a data member, abstract method, method body (non-abstract method), constructor, and even main() method.
File: TestAbstraction2.javaTest it Now
bike is created running safely.. gear changed
Rule: If there is an abstract method in a class, that class must be abstract.Test it Now
compile time error
Rule: If you are extending an abstract class that has an 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.Test it Now
Output:I am a I am b I am c I am d