- A class that is declared using “abstract” keyword is known as abstract class.
- It can have abstract methods(methods without body) as well as concrete methods (regular methods with body).
- A normal class(non-abstract class) cannot have abstract methods.
- An abstract class can not be instantiated, which means you are not allowed to create an object of it.
Why we need an abstract class?
Lets say we have a class Animal that has a method sound() and the subclasses(see inheritance) of it like Dog, Lion, Horse, Cat etc. Since the animal sound differs from one animal to another, there is no point to implement this method in parent class. This is because every child class must override this method to give its own implementation details, like Lion class will say “Roar” in this method and Dog class will say “Woof”.
So when we know that all the animal child classes will and should override this method, then there is no point to implement this method in parent class. Thus, making this method abstract would be the good choice as by making this method abstract we force all the sub classes to implement this method( otherwise you will get compilation error), also we need not to give any implementation to this method in parent class.
Since the Animal class has an abstract method, you must need to declare this class abstract.
Now each animal must have a sound, by making this method abstract we made it compulsory to the child class to give implementation details to this method. This way we ensures that every animal has a sound.
Abstract class Example
//abstract parent class
abstract class Animal{
//abstract method
public abstract void sound();
}
//Dog class extends Animal class
public class Dog extends Animal{
public void sound(){
System.out.println("Woof");
}
public static void main(String args[]){
Animal obj = new Dog();
obj.sound();
}
}
Output:
Woof
Hence for such kind of scenarios we generally declare the class as abstract and later concrete classes extend these classes and override the methods accordingly and can have their own methods as well.
Abstract class declaration
An abstract class outlines the methods but not necessarily implements all the methods.
//Declaration using abstract keyword
abstract class A{
//This is abstract method
abstract void myMethod();
//This is concrete method with body
void anotherMethod(){
//Does something
}
}
Rules
Note 1: As we seen in the above example, there are cases when it is difficult or often unnecessary to implement all the methods in parent class. In these cases, we can declare the parent class as abstract, which makes it a special class which is not complete on its own.
A class derived from the abstract class must implement all those methods that are declared as abstract in the parent class.
Note 2: Abstract class cannot be instantiated which means you cannot create the object of it. To use this class, you need to create another class that extends this this class and provides the implementation of abstract methods, then you can use the object of that child class to call non-abstract methods of parent class as well as implemented methods(those that were abstract in parent but implemented in child class).
Note 3: If a child does not implement all the abstract methods of abstract parent class, then the child class must need to be declared abstract as well.
Since abstract class allows concrete methods as well, it does not provide 100% abstraction. You can say that it provides partial abstraction. Abstraction is a process where you show only “relevant” data and “hide” unnecessary details of an object from the user.
Why can’t we create the object of an abstract class?
Because these classes are incomplete, they have abstract methods that have no body so if java allows you to create object of this class then if someone calls the abstract method using that object then What would happen?There would be no actual implementation of the method to invoke.
Also because an object is concrete. An abstract class is like a template, so you have to extend it and build on it before you can use it.
Example to demonstrate that object creation of abstract class is not allowed
As discussed above, we cannot instantiate an abstract class. This program throws a compilation error.
abstract class AbstractDemo{
public void myMethod(){
System.out.println("Hello");
}
abstract public void anotherMethod();
}
public class Demo extends AbstractDemo{
public void anotherMethod() {
System.out.print("Abstract method");
}
public static void main(String args[])
{
//error: You can't create object of it
AbstractDemo obj = new AbstractDemo();
obj.anotherMethod();
}
}
Output:
Unresolved compilation problem: Cannot instantiate the type AbstractDemo
Note: The class that extends the abstract class, have to implement all the abstract methods of it, else you have to declare that class abstract as well.
Abstract class vs Concrete class
A class which is not abstract is referred as Concrete class. In the above example that we have seen in the beginning of this guide, Animal is a abstract class and Cat, Dog & Lion are concrete classes.
Key Points:
- An abstract class has no use until unless it is extended by some other class.
- If you declare an abstract method in a class then you must declare the class abstract as well. you can’t have abstract method in a concrete class. It’s vice versa is not always true: If a class is not having any abstract method then also it can be marked as abstract.
- It can have non-abstract method (concrete) as well.
Abstract method in Java with examples
- A method without body (no implementation) is known as abstract method.
- A method must always be declared in an abstract class, or in other words you can say that if a class has an abstract method, it should be declared abstract as well.
- This is how an abstract method looks in java:
- public abstract int myMethod(int n1, int n2);
- As you see this has no body.
Rules of Abstract Method
1. Abstract methods don’t have body, they just have method signature as shown above.
2. If a class has an abstract method it should be declared abstract, the vice versa is not true, which means an abstract class doesn’t need to have an abstract method compulsory.
3. If a regular class extends an abstract class, then the class must have to implement all the abstract methods of abstract parent class or it has to be declared abstract as well.
Example 1: abstract method in an abstract class
//abstract class
abstract class Sum{
/* These two are abstract methods, the child class
* must implement these methods
*/
public abstract int sumOfTwo(int n1, int n2);
public abstract int sumOfThree(int n1, int n2, int n3);
//Regular method
public void disp(){
System.out.println("Method of class Sum");
}
}
//Regular class extends abstract class
class Demo extends Sum{
/* If I don't provide the implementation of these two methods, the
* program will throw compilation error.
*/
public int sumOfTwo(int num1, int num2){
return num1+num2;
}
public int sumOfThree(int num1, int num2, int num3){
return num1+num2+num3;
}
public static void main(String args[]){
Sum obj = new Demo();
System.out.println(obj.sumOfTwo(3, 7));
System.out.println(obj.sumOfThree(4, 3, 19));
obj.disp();
}
}
Output:
10
26
Method of class Sum
Example 2: abstract method in interface
All the methods of an interface are public abstract by default. You cannot have concrete (regular methods with body) methods in an interface.
//Interface
interface Multiply{
//abstract methods
public abstract int multiplyTwo(int n1, int n2);
/* We need not to mention public and abstract in interface
* as all the methods in interface are
* public and abstract by default so the compiler will
* treat this as
* public abstract multiplyThree(int n1, int n2, int n3);
*/
int multiplyThree(int n1, int n2, int n3);
/* Regular (or concrete) methods are not allowed in an interface
* so if I uncomment this method, you will get compilation error
* public void disp(){
* System.out.println("I will give error if u uncomment me");
* }
*/
}
class Demo implements Multiply{
public int multiplyTwo(int num1, int num2){
return num1*num2;
}
public int multiplyThree(int num1, int num2, int num3){
return num1*num2*num3;
}
public static void main(String args[]){
Multiply obj = new Demo();
System.out.println(obj.multiplyTwo(3, 7));
System.out.println(obj.multiplyThree(1, 9, 0));
}
}
Output:
21
0