Tutorial

Abstract Class in Java

Published on August 3, 2022
author

Pankaj

Abstract Class in Java

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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Pankaj

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JournalDev
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July 14, 2013

Nice tutorial, I think you have covered everything about abstract classes and methods. Please provide some difference between abstract class and interface.

- Amit

JournalDev
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November 2, 2016

Methods of a Java interface are implicitly abstract and cannot have implementations. A Java abstract class can have instance methods that implements a default behaviour. Variables declared in a Java interface are by default final. An abstract class may contain non-final variables. Members of a Java interface are public by default. A Java abstract class can have the usual flavours of class members like private, protected, etc. A Java interface should be implemented using keyword “implements”; A Java abstract class should be extended using keyword “extends”. An interface can extend another Java interface only, an abstract class can extend another Java class and implement multiple Java interfaces. A Java class can implement multiple interfaces but it can extend only one abstract class. Happy Learning :)

- Aliasger Motiwala

    JournalDev
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    May 13, 2014

    There are two classes person and Employee. You have created the object of person class. Is it ok? and where is ‘changeName’ method?

    - Kapil

      JournalDev
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      June 8, 2014

      This is is not showing parameter value . i have use Person student= new Employee(“Deepak”,“Male”,20);

      - deepak

        JournalDev
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        October 7, 2014

        1. abstract class can’t be instantiated 2. abstract class can contain definition of function with respect to these features, my question is— can we call those function which are defined in abstract class by reference of that class? if yes then why and how?

        - Firoj Mujawar

        JournalDev
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        October 7, 2014

        You should have a concrete class extending the abstract class and provide implementation of abstract methods, you can also have anonymous class implementation. After that it’s just inheritance.

        - Pankaj

          JournalDev
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          March 10, 2015

          can we override a non abstract method of abstract class . If yes then what is the use of making a method abstract in abstract class.

          - vishnu

            JournalDev
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            April 4, 2015

            Abstract class can contain Constructor, if we cannot create an instance what is the use of this constructor.

            - Raghuveer Kurdi

            JournalDev
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            April 5, 2015

            Yes, Abstract classes can have constructors unlike Interfaces. However we cannot directly instantiate an Abstract class to create an instance. The purpose of the Abstract class constructors (we can have multiple abstract class constructors with different arguments) is to initialize the final variables from subclasses (kind of lazy initialization)

            - Vishnu Prakash

              JournalDev
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              February 17, 2016

              To Achieve Constructor Chaining Abstract Class Should have Constructor, though Object Cant be Created for it. Every Java Class Will inherit From Object class… So Even Abstract Class Will inherit From Object Class. So If There is Inheritance between Classes then Constructor Chaining is Mandatory!! Constructor Chaining means Sub Class Constructor Calling its Immediate Super Class Constructor.

              - Raj

                JournalDev
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                June 29, 2015

                Abstract class can’t be instantiated. It is wrong what about anonymous class.

                - manish

                JournalDev
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                March 22, 2016

                No. Abstract Classes cannot be instantiated. Let us say we have got an abstract class absClass with one unimplemented abstract method doSayHello. When we implement doSayHello method in the curly brackets, we are doing it under the pretence that we are extending an unnamed class but not by instantiating the abstract class itself. absClass ab = new absClass(){ @Override public void doSayHello() { System.out.println(“Hello World !”); } }; ab.doSayHello(); // prints “Hello World !”

                - Yunus Atheist

                  JournalDev
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                  November 23, 2015

                  Hi Pankaj, Can u please tell me where we can use interfaces and abstract classes in realtime,could you please give me in realtime example.

                  - Rajesh

                    JournalDev
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                    January 7, 2016

                    can i have more day to day life examoles on abstract class…

                    - shaurya gaurav mehta

                    JournalDev
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                    November 2, 2016

                    public interface LoginAuth{ public String encryptPassword(String pass); public void checkDBforUser(); } Now suppose you have 3 databases in your application. Then each and every implementation for that database needs to define the above 2 methods: public class DBMySQL implements LoginAuth{ // Needs to implement both methods } public class DBOracle implements LoginAuth{ // Needs to implement both methods } public class DBAbc implements LoginAuth{ // Needs to implement both methods } But what if encryptPassword() is not database dependent, and it’s the same for each class? Then the above would not be a good approach. Instead, consider this approach: public abstract class LoginAuth{ public String encryptPassword(String pass){ // Implement the same default behavior here // that is shared by all subclasses. } // Each subclass needs to provide their own implementation of this only: public abstract void checkDBforUser(); } Now in each child class, we only need to implement one method - the method that is database dependent. Tried my best to make you understand. Happy Learning :)

                    - Aliasger Motiwala

                      JournalDev
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                      January 25, 2016

                      i need to submit this morning. i have working out my self but i am unable to define it. define an interface that consists of methods that can operate on classes employee and student

                      - odelami emmanuel

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