Abstract Methods and Abstract Classes

Abstract Method

An abstract method is a method that is declared without an implementation. An abstract method is declared using the keyword abstract.

Syntax

abstract returnType methodName(parameters)

Example

abstract int sum(int operand1, int operand2);

Abstract Class

An abstract class is declared using the keyword abstract—it may or may not include abstract methods. Abstract classes cannot be instantiated, but they can be subclassed.

Syntax

    abstract class ClassName{

        /* may or may not contain abstract methods */

    }

If a class extends the abstract class, then it must provide the implementation for all the abstract methods in the abstract class, otherwise the class should be declared as abstract.

The main purpose of abstract class is to use the common code in the sub classes.

Example

abstract class Animal{
 String name;

 String getName(){
  return name;
 }

 void setName(String name){
  this.name = name;
 }

 abstract String run();
}

class Elephant extends Animal{
 String run(){
  return "I can run at the speed of 25 mph";
 }
}

   

class Lion extends Animal{
 String run(){
  return " I can run at the speed of 50 mph";
 }
}

     

class Tiger extends Animal{
 String run(){
  return "I can run at the speed of 60 mph";
 }
}

     

class AbstractTest{
 public static void main(String args[]){
  Animal anim1 = new Elephant();
  anim1.setName("Gaja");
  System.out.println(anim1.getName() +":" + anim1.run());

  anim1 = new Lion();
  anim1.setName("Aslam");
  System.out.println(anim1.getName() +":" + anim1.run());

  anim1 = new Tiger();
  anim1.setName("PTR");
  System.out.println(anim1.getName() +":" + anim1.run());
 }
}

Output

Gaja:I can run at the speed of 25 mph
Aslam: I can run at the speed of 50 mph
PTR:I can run at the speed of 60 mph

Every Animal has a name, so getName() and setName() are common to all the animals, So the implementation for these methods kept in the abstract class Animal. Where as different Animals runs at different speeds, so the run() method declared as abstract, so all the concrete classes that are extending the class Animal, must provide the implementation for the run method.

Some Points to Remember

1. Can I create constructor inside the abstract class ?

Yes, but you can't initialize an object for the abstract class.

Example

abstract class Animal{
 String name; 

 Animal(String name){
  this.name = name;
 }
}

 

2. Can I make abstract class as final ?

No, If you make abstract class as final, then no other class can able to extend it. So, compiler throws error. Abstract and final can't be used together.

Example

final abstract class Animal{
 String name; 

 Animal(String name){
  this.name = name;
 }
}

 

When you tries to compile the above program, compiler throws the below error.

 Animal.java:1: error: illegal combination of modifiers: abstract and final
    final abstract class Animal{
    ^
    1 error

3. Can I make abstract method as final ?

No, If you make abstract method as final, then no other class can able to override it. So, compiler throws error. Abstract and final can't be used together.

Example

abstract class Animal{
 String name; 

 final abstract void run();

 }
}

When you tries to compile the above program, compiler throws the below error.

Animal.java:4: error: illegal combination of modifiers: abstract and final
final abstract void run();
^
1 error

 

4. Can I define main method in abstract class ?

Yes, it is valid, but creation of object to the abstract is not possible.

Example

abstract class Animal{
 String name; 

 public static void main(String args[]){
  System.out.println("I am in main method");
 }
}

   

Output

I am in main method

5. What is wrong with the below program ?

    abstract class Animal{

        void run();

    }

Above program won't compile, since if you want to make a method as abstract, then it must be declared with the specifier abstract. When you tries to compile, compiler throws the below error.

Animal.java:2: error: missing method body, or declare abstract

void run();

^

1 error

To make the program run you have two options.

Option 1

    Make the method as abstract.

    abstract class Animal{

        abstract void run();

    }

Option 2

provide the implementation for the method run().

    abstract class Animal{

        void run(){

        }

    }

  

6. An abstract class allowed to have static methods, where as interfaces have instance methods only (From Java8 onwards, we can add static methods to interfaces).

7. The methods declared in interface are abstract by default.

8. Can an interface has constructor ?

No

9. Can an abstract class have a final method?

Yes, it can. But the final method cannot be abstract itself

Example

    abstract class Animal{

        final void print(){

            System.out.println(" I am final method in abstract class");

        }

    }

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