Abstract class in Java

A class that is declared with abstract keyword, is known as abstract class in java. It can have abstract and non–abstract methods (method with body)

Abstraction in Java

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

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 Abstract class

1. Abstract class
2. Interface

Abstract class in Java

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.

Example abstract class

1. abstract class A{}

abstract method

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

Example abstract method

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

Example of abstract class that has abstract method

In this example, Bike the abstract class that contains only one abstract method run. It
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();
}
} 

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

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

File: TestAbstraction1.java

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 real scenario, object is provided through method e.g. getShape
() method
s.draw();
}
}

Output

drawing circle

Abstract class having constructor, data member, methods etc

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

File: TestAbstraction2.java

 //example of abstract class that have method body
abstract class Bike{
Bike(){System.out.println("bike is created");}
abstract void run();
 void changeGear(){System.out.println("gear changed");}
}

class Honda extends Bike{
void run(){System.out.println("running safely..");}
}
class TestAbstraction2{
public static void main(String args[]){
 Bike obj = new Honda();
 obj.run();
obj.changeGear();
 }
}

bike is created
running safely..
gear changed

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

class Bike12{
abstract void run();
 }

Output

compile time error

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

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