Abstract class & Method

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.

Abstract Method in Java abstract void printStatus();//no method body and abstract

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

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();  
}  
}  

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();
}
}

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();  
 }  
}