Abstract Classes in Java | Example

→ A class is declared with the abstract keyword it is known as an abstract class .

Meaning of Abstraction in Java

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

Example:- sending SMS where you type the text and send the message. You don't know the internal processing about the message delivery.

Rule for Java Abstract class

An abstract class must be declared with an abstract keyword.
Abstract class that has an abstract method.
It is used for abstraction
Abstract classes are never 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.
An abstract class may or may not have an abstract method. But if any class has even a single abstract method, then it must be declared abstract.

Syntax:

Example1:

 abstract class A{
  abstract void callme();
}
class B extends A{
  void callme()  {
    System.out.println("Calling...");
  }
  public static void main(String[] args)  {  
    B b = new B();
    b.callme();
  }
}

Output:

Example2: Upcasting example in abstract class

abstract class Java{
 abstract void run();
}
class Php extends Java{
 void run(){
    System.out.println("Php Example");
 }}
class Python extends Java{
  void run(){
 System.out.println("Python Example");
   }}
class AbstractionEx{
public static void main(String args[]){
Java j= new Php(); // upcasting abstraction
j.run();
 }}

Output:

Abstract Method in Java

→ A method declared without any body (no implementation) within an abstract class are called abstract method..

→ A method which is declared as abstract and does not have implementation is known as an abstract method..

→ Abstract method can never be final and static.

→ Any class that extends an abstract class must, implement all the abstract methods.

Syntax:

Example1 :-

Abstract class having constructor, data member and methods

abstract class Java{
     Java(){
          System.out.println("Java is Object Oriented Language");}
 abstract void run();
      void show(){
       System.out.println("Hello How r u");}
}
class Php extends Java{
 void run(){
    System.out.println("Php Example");
 }
}
class Python extends Java{
  void run(){
 System.out.println("Python Example");
   }
}
class AbstractEx{
public static void main(String args[]){
Java j= new Php(); // upcasting abstraction
j.run();
j.show();
 }
}

Output :-

Example2 :-

 abstract class A{  
   A(){System.out.println("A for Apple");}  
   abstract void run();  
   void show(){System.out.println("A for Alphabet");}  
 }  
  
 class B extends A{  
 void run(){System.out.println("B for Ball");}  
 }  
 class TestAbstraction{  
 public static void main(String args[]){  
  A obj = new B();  
  obj.run();  
  obj.show();  
 }  
}  
 

Output :-

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OOPs Concepts

Abstract Classes in Java | Example

Meaning of Abstraction in Java

Rule for Java Abstract class

Abstract Method in Java

Abstract class having constructor, data member and methods