Abstract class in Java programming language

Abstract class

In this tutorial, we will discuss Abstract class in Java programming language

In Java programming language, Abstraction is one of the OOP concepts. A class that is declared as abstract is called as an abstract class.

The abstract class can have abstract and non-abstract methods

Why need abstract

Abstraction is a process of hiding the implementation details( internal detail) from user and user can using only its functionality

Declaration

The syntax of abstract class

abstract class class_name{   
}

Example of abstract class

abstract class Calculation{   
}

//abstract is a keyword

//abstract classes have abstract and non-abstract methods

//implementation of abstract methods is in another class

abstract method

A method that is declared as abstract and does not have an implementation. It is just a signature.

Declaration

The syntax of the abstract method

abstract return_type method_Name{
}

Example of an abstract method

abstract void get_Display{   
}

Points to remember

An abstract class must be declared with an abstract keyword
the abstract method has no implementation.
A class contains at least one abstract method.
it cannot be instantiated
An abstract class can have a final method, constructor and static method
For the class to be an abstract class, one or more methods must be abstract inside the class.

What is the usage of the abstract class

Abstraction hides implementation details and only presents the features to users.

Examples

Program 1

abstract class abstract_1{
abstract void run();
//abstract method only declaration no definition

void print()
{
    System.out.println("non abstract method in Java");
}
}// abstract_1 class end

class Example_1 extends abstract_1{
    void run()//definition of abstract method
    {
    System.out.println("abstract method in Java");
    }
    
    public static void main(String args[]){
        Example_1 obj=new Example_1();
        obj.run();
        obj.print();
    }
}

When the above code executed, it will produce the following output

abstract method in Java
non abstract method in Java

Program 2

abstract class myschool{
myschool(){
System.out.println("This is my first school");
}
abstract void subject();//abstract method
void books()      //Normal method
{
System.out.println("This is my books");
}
}

class newstudent extends myschool{
    void subject(){   //implement abstract method
        System.out.println("There are ten subject");
    }
public static void main(String args[]){
myschool obj=new newstudent();  //create an instance of the class
obj.books();
obj.subject();

}

}

When the above code executed, will be produced following output

This is my first school
This is my books
There are ten subject

Program 3

abstract class my_college{	// abstract class
    my_college(){			//constructure
    System.out.println("This is my college");
    }//end of constructure
    abstract void maths();//Absrract method
    void medicine(){      //Non abstract method
    System.out.println("medicine is my favorite course");
    }
}
class my_course extends my_college{
    //my_course inherit property of my_college
    void maths()
        {//implement of abstract method
    System.out.println("maths courses are two divisions");

        }
}

class Test_college
{
    public static void main(String args[]){//main method
    my_college obj=new my_course();// create object
    obj.maths();     // calling method using object
    obj.medicine();
    }
}

When the above code executed, will produce the following output

This is my college
Maths courses are two divisions
medicine is my favorite course

Abstract class can be contains ,data members and methods

An abstract class can have data members, abstract method, non-abstract method, constructor.

abstract class School{
School()//constructor
{
System.out.println("This is a constructor inside the abstract class");
}
abstract void teachers();//abstract method with no implementation

void students(){//non abstract method
System.out.println("We are students inside the school");
}
}

class Principal extends School{//inherit properties from parent class
void teachers(){//implements the abstract method
System.out.println("We are teachers inside the school");
}
}

class Society{//this is a test class 
    public static void main(String args[]){//main method
    School obj=new Principal();//create instance for class
    obj.teachers();//call method teachers(abstract method)
    obj.students();//call method students(non abstract method)
    }


}

When the above code executed, will produce the following output

This is a constructor inside the abstract class
We are teachers inside the school
We are students inside the school

There are other Java language keywords that are similar to this post

extends keyword in Java

implements keyword in Java

interface keyword in Java

Abstract keyword in Java

class keyword in Java

public keyword in Java

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