Abstract Class & Abstract Methods in Java

Abstract Class –

A class that is declared with abstract keyword, is known as abstract class in java. It can have abstract and non-abstract methods (method with body). It needs to be extended and its method implemented. It cannot be instantiated.

abstract class A{
}

Abstract Methods –

A method that is declared as abstract and does not have implementation is known as abstract method.

abstract void printStatus();//no body and abstract

Understanding the real use of Abstract Class & Abstract Methods –

In this example, we have a Shape super class which is inherited by 3 sub classes – Rectangle, Triangle and Circle. The task here is to have 1 Area() method in the parent/super class and is inherited by all 3 child/sub classes. However, every child class has its own definition to calculate area as the area formula for each shape is different. Also, since our parent class shape is not known, we cannot have a valid method definition in the parent class. So this is where we can use Abstract class concept and make the Area() method in the parent class as abstract. Once we inherit from the Shape class, the subclasses can have their respective implementations done according to their needs.

Important Points about Abstract Classes & Abstract Methods in Java –

1. In Java, an instance of an abstract class cannot be created, we can have references of abstract class type though.

abstract class Base {
    abstract void fun();
}
class Derived extends Base {
    void fun() { System.out.println("Derived fun() called"); }
}
class Main {
    public static void main(String args[]) { 
     
        // Uncommenting the following line will cause compiler error as the 
        // line tries to create an instance of abstract class.
        // Base b = new Base();
 
        // We can have references of Base type.
        Base b = new Derived();
        b.fun(); 
    }
}

2. An abstract class can contain constructors in Java. And a constructor of abstract class is called when an instance of a inherited class is created. For example – 

// An abstract class with constructor
abstract class Base {
    Base() { System.out.println("Base Constructor Called"); }
    abstract void fun();
}
class Derived extends Base {
    Derived() { System.out.println("Derived Constructor Called"); }
    void fun() { System.out.println("Derived fun() called"); }
}
class Main {
    public static void main(String args[]) { 
       Derived d = new Derived();
    }
}

3. In Java, we can have an abstract class without any abstract method. This allows us to create classes that cannot be instantiated, but can only be inherited.

// An abstract class without any abstract method
abstract class Base {   
    void fun() { System.out.println("Base fun() called"); }
}
  
class Derived extends Base { }
  
class Main {
    public static void main(String args[]) { 
        Derived d = new Derived();
        d.fun();
    }
}

4. Abstract classes can also have final methods (methods that cannot be overridden). For example, the following program compiles and runs fine.

// An abstract class with a final method
abstract class Base {
    final void fun() { System.out.println("Derived fun() called"); }
}
  
class Derived extends Base {}
  
class Main {
    public static void main(String args[]) { 
       Base b = new Derived();
       b.fun();
    }
}

Some More important rules to be followed –

• If there is any abstract method in a class, that class must be abstract.
• If you are extending any abstract class that have abstract method, you must either provide the implementation of the method or make this class abstract.