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.
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
keyword is used to create an abstract class in java.abstract
keyword to create an abstract method, an abstract method doesn’t have body.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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Nice tutorial, I think you have covered everything about abstract classes and methods. Please provide some difference between abstract class and interface.
- Amit
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
There are two classes person and Employee. You have created the object of person class. Is it ok? and where is ‘changeName’ method?
- Kapil
This is is not showing parameter value . i have use Person student= new Employee(“Deepak”,“Male”,20);
- deepak
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
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
abstract class MyAbstractClass { public void definedMethod() { System.out.println(“This is defined in Abstract Base Class”); } public abstract void declaredMethod(); } class SubClass extends MyAbstractClass { public void declaredMethod() { System.out.println(“This is defined in Subclass of Abstract”); } } public class AbstractMethodCall { public static void main(String asds[]) { MyAbstractClass abs=new SubClass(); abs.declaredMethod(); abs.definedMethod(); // here is the problem. how this is done?? } }
- Firoj Mujawar
There is no problem in fact. SubClass extends MyAbstractClass. Hence, definedMethod() will also be available in MyAbstractClass. It calls the definedMethod() method and prints “This is defined in Abstract Base Class”
- Prashanth
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
Abstract class can contain Constructor, if we cannot create an instance what is the use of this constructor.
- Raghuveer Kurdi
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
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
Abstract class can’t be instantiated. It is wrong what about anonymous class.
- manish
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
Nice example for creating object for abstract class
- kalanidhi
Isn’t this example creating an instance of absClass?
- Frank Silano
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
can i have more day to day life examoles on abstract class…
- shaurya gaurav mehta
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
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
Hey could you please explain that , How do you use “employee.changeName(“Pankaj Kumar”);” ? What is changeName ?
- lahiru
I feel like the Person, Employee relationship is better represented by an interface rather than an abstract class because a Person can be an Employee but it is not a necessity. A person can be more than one thing therefore A Person should implement an interface called Employee because they can also be something else like a manager or a supervisor.
- Stanley
Every Employee "is a " Person. So Person has to be superclass. It’s your choice whether you want to have it abstract class or interface. With Java 8 interface default and static methods, there is not much difference between abstract class and interface.
- Pankaj
Why Abstract Class is used in Java? I want to know the sole purpose of Abstract Class other than its providing Data hiding and forces the programmer to implement all the methods in Abstract class.
- Yoganathan
you can have default implementation and abstract methods that you want all the implementation classes to implement. However with Java 8 default methods in interfaces, Interface and Abstract class are almost same.
- Pankaj
Abstract class points super
- Siva
Confused. Where does “changeName” come from??
- James
While doing some editing, I mistakenly deleted changeName method from Person class, I have corrected it.
- Pankaj
when to use abstract class and when to use interface?
- salman
if you want to object + abstraction then go for abstract class. if you want to only abstraction then go for interface.
- Deepak Tiwari
we can create a object for abstract class…
- CHINTUL
No … We can not create object of abstract class. It is anonymous object.
- patil s
As it has been mentioned clearly, an abstract class can’t be instantiated, we can not create an object of it.
- Fravashi Kaustav
where is the Encapsulation and polymorphism in oops concept?
- nirmala
It was a wonderful recap of what i learned java docs from various resources in internet. Thanks for covering entire picture of core java. God bless you!!
- sathish
Nice bro
- Alwin
Hi, Thank you in advace for yout awsome tutorials and topis. in the section of ‘Abstract class in Java Important Points’. point number 5 I guess is not correct. because to have an abstract method, its class must be abstract also. If I`m wrong it would be greate to clear me. Thanks
- Morteza
You got it another way around, it says “It’s not necessary to have the abstract class to have abstract method.” That means we can mark a class as abstract even if it doesn’t have any abstract methods.
- Pankaj
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
Yes, if it class3 is not an abstract class.
- VIJAY
you ain’t mentioned what’s the use of it.
- Deepak gupta
…“10. Java Abstract class is used to provide common method implementation to all the subclasses or to provide default implementation.” Did you even read the article?
- Kenneth
Does abstract class contains Constructor? If yes, then what is the use of it?
- Bhagyashri Chaudhari
Abstract classes cannot be instantiated, so they do not need constructors.
- Frank
abstarct class have a constructer for instantiate child class object .
- umakant chaudhary
what is the main role of abstract class in java?
- aryan
abstract class mainly use to restrict the object creation of that class.
- Ketul Chauhan
if you want to object + abstraction then go for abstract class. if you want to only abstraction then go for interface.
- Pulkit Sharma
basically it is an Partially implemented class means some methods have body where some have only Signature(Only declaration).
- ancd
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
Sorry for silly grammar error, I have fixed it.
- Pankaj
happy to hear such a prompt response, keep going !
- Hissaan
Important points are easy to understand about the abstract class thank you for your explanation ;)
- Venkatesh
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
We can access the concrete class’s method by making the class object. For example, public class ConcreteClass{ public void fun(){ } } public abstract class AbstractClass{ //Constructor public AbstractClass(){ ConcreteClass obj = new ConcreteClass(); obj.fun(); } }
- Umar Bilal
Can u please explain one Real World Use Case for Abstract class and abstract Method b
- Anshul Daksh
A concrete example of an abstract class would be a class called Animal. You see many animals in real life, but there are only kinds of animals. It might also provide an abstract method called “Is Dead” that, when called, will tell you if the animal has died. Since Is Dead is abstract, each animal must implement it…
- singhtinku
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
how can we invoke any static method from abstract class?
- Priyanka