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Abstract Class in Java

Published on August 3, 2022
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By Pankaj
Developer and author at DigitalOcean.
Abstract Class in Java

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Abstract class in Java is similar to interface except that it can contain default method implementation. An abstract class can have an abstract method without body and it can have methods with implementation also. abstract keyword is used to create a abstract class and method. Abstract class in java can’t be instantiated. An abstract class is mostly used to provide a base for subclasses to extend and implement the abstract methods and override or use the implemented methods in abstract class.

Abstract Class in Java

abstract class in java, java abstract class example Here is a simple example of an Abstract Class in Java.

package com.journaldev.design;

//abstract class
public abstract class Person {
	
	private String name;
	private String gender;
	
	public Person(String nm, String gen){
		this.name=nm;
		this.gender=gen;
	}
	
	//abstract method
	public abstract void work();
	
	@Override
	public String toString(){
		return "Name="+this.name+"::Gender="+this.gender;
	}

	public void changeName(String newName) {
		this.name = newName;
	}	
}

Notice that work() is an abstract method and it has no-body. Here is a concrete class example extending an abstract class in java.

package com.journaldev.design;

public class Employee extends Person {
	
	private int empId;
	
	public Employee(String nm, String gen, int id) {
		super(nm, gen);
		this.empId=id;
	}

	@Override
	public void work() {
		if(empId == 0){
			System.out.println("Not working");
		}else{
			System.out.println("Working as employee!!");
		}
	}
	
	public static void main(String args[]){
		//coding in terms of abstract classes
		Person student = new Employee("Dove","Female",0);
		Person employee = new Employee("Pankaj","Male",123);
		student.work();
		employee.work();
		//using method implemented in abstract class - inheritance
		employee.changeName("Pankaj Kumar");
		System.out.println(employee.toString());
	}

}

Note that subclass Employee inherits the properties and methods of superclass Person using inheritance in java. Also notice the use of Override annotation in Employee class. Read more for why we should always use Override annotation when overriding a method.

Abstract class in Java Important Points

  1. abstract keyword is used to create an abstract class in java.
  2. Abstract class in java can’t be instantiated.
  3. We can use abstract keyword to create an abstract method, an abstract method doesn’t have body.
  4. If a class have abstract methods, then the class should also be abstract using abstract keyword, else it will not compile.
  5. It’s not necessary for an abstract class to have abstract method. We can mark a class as abstract even if it doesn’t declare any abstract methods.
  6. If abstract class doesn’t have any method implementation, its better to use interface because java doesn’t support multiple class inheritance.
  7. The subclass of abstract class in java must implement all the abstract methods unless the subclass is also an abstract class.
  8. All the methods in an interface are implicitly abstract unless the interface methods are static or default. Static methods and default methods in interfaces are added in Java 8, for more details read Java 8 interface changes.
  9. Java Abstract class can implement interfaces without even providing the implementation of interface methods.
  10. Java Abstract class is used to provide common method implementation to all the subclasses or to provide default implementation.
  11. We can run abstract class in java like any other class if it has main() method.

That’s all for an abstract class in Java. If I missed anything important, please let us know through comments.


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About the authors
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Pankaj

author

Developer and author at DigitalOcean.

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Was this helpful?

how can we invoke any static method from abstract class?

- Priyanka

    please explain me about abstract with easy example because i didn’t it in better way. i request you and please also explain me all those points in better ways

    - ANJALI THAKUR

      Can u please explain one Real World Use Case for Abstract class and abstract Method b

      - Anshul Daksh

        By using the static keyword we can access the concrete methods in the abstract class. Can any one tell without using the static how can we access the concrete methods in the abstract class?

        - Udaya Bhargavi

          Important points are easy to understand about the abstract class thank you for your explanation ;)

          - Venkatesh

            Respected admin, i would like to point out an improvement in point number 5 of the *Important points* written at the end, i believe the correct statement should be : It’s not necessary FOR AN abstract class to have abstract method. We can mark a class as abstract even if it doesn’t declare any abstract methods. instead of It’s not necessary TO HAVE abstract class to have abstract method. We can mark a class as abstract even if it doesn’t declare any abstract methods… It’s really ambiguous otherwise . i hope the admin would act promptly.

            - Hissaan

              what is the main role of abstract class in java?

              - aryan

                Does abstract class contains Constructor? If yes, then what is the use of it?

                - Bhagyashri Chaudhari

                  you ain’t mentioned what’s the use of it.

                  - Deepak gupta

                    The subclass of abstract class in java must implement all the abstract methods unless the subclass is also an abstract class. On a similar note, If Class3 extends abstract Class2 and abstract Class2 extends abstract Class1, then Class3 should implement all the abstract methods of Class1 and Class2. Class 2 need not implement methods of Class1 since Class2 is also abstract.

                    - Praveen