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Abstract class in Java

Abstraction ?

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

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

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.

  • An abstract class must be declared with an abstract keyword.
  • It can have abstract and non-abstract methods.
  • It cannot be instantiated.
  • It can have constructors and static methods also.
  • It can have final methods which will force the subclass not to change the body of the method.

Example :

  
abstract class A{}
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

  
abstract void printStatus();//no method body and abstract    
  

  
abstract class Bike{  
  abstract void run();  
}  
class Honda4 extends Bike{  
void run(){System.out.println("running safely");}  
public static void main(String args[]){  
 Bike obj = new Honda4();  
 obj.run();  
}  
}    
  

  
running safely
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.

  
abstract class Shape{  
abstract void draw();  
}  
//In real scenario, implementation is provided by others i.e. unknown by end user  
class Rectangle extends Shape{  
void draw(){System.out.println("drawing rectangle");}  
}  
class Circle1 extends Shape{  
void draw(){System.out.println("drawing circle");}  
}  
//In real scenario, method is called by programmer or user  
class TestAbstraction1{  
public static void main(String args[]){  
Shape s=new Circle1();//In a real scenario, object is provided through method, e.g., getShape() method  
s.draw();  
}  
}
output
  
drawing circle
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.

  
//Example of an abstract class that has abstract and non-abstract methods  
 abstract class Bike{  
   Bike(){System.out.println("bike is created");}  
   abstract void run();  
   void changeGear(){System.out.println("gear changed");}  
 }  
//Creating a Child class which inherits Abstract class  
 class Honda extends Bike{  
 void run(){System.out.println("running safely..");}  
 }  
//Creating a Test class which calls abstract and non-abstract methods  
 class TestAbstraction2{  
 public static void main(String args[]){  
  Bike obj = new Honda();  
  obj.run();  
  obj.changeGear();  
 }  
}
output
  
bike is created
running safely..
gear changed

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

  
class Bike12{  
abstract void run();  
}
output
  
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.

  
interface A{  
void a();  
void b();  
void c();  
void d();  
}  
  
abstract class B implements A{  
public void c(){System.out.println("I am c");}  
}  
  
class M extends B{  
public void a(){System.out.println("I am a");}  
public void b(){System.out.println("I am b");}  
public void d(){System.out.println("I am d");}  
}  
  
class Test5{  
public static void main(String args[]){  
A a=new M();  
a.a();  
a.b();  
a.c();  
a.d();  
}}
output
  
I am a
I am b
I am c
I am d
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