Abstract Classes in Java

Difficulty Level : Easy
Last Updated : 19 Feb, 2021

In C++, if a class has at least one pure virtual function, then the class becomes abstract. Unlike C++, in Java, a separate keyword abstract is used to make a class abstract.

Java

// An example abstract class in Java
abstract class Shape {
    int color;
  
    // An abstract function (like a pure virtual function in
    // C++)
    abstract void draw();
}

Following are some important observations about abstract classes in Java.

1) Like C++, in Java, an instance of an abstract class cannot be created, we can have references to abstract class type though.

Java

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();
    }
}

Output

Derived fun() called

2) Like C++, an abstract class can contain constructors in Java. And a constructor of abstract class is called when an instance of an inherited class is created. For example, the following is a valid Java program.

Java

// 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();
    }
}

Output

Base Constructor Called
Derived Constructor Called

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.

Java

// 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();
    }
}

Output

Base fun() called

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

Java

// 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();
    }
}

Output

Derived fun() called

5) For any abstract java class we are not allowed to create an object i.e., for abstract class instantiation is not possible.

Java

// An abstract class example
abstract class Test {
    public static void main(String args[])
    {
        // Try to create an object
        Test t = new Test();
    }
}

Output:

Compile time error. Test is abstract; 
cannot be instantiated Test t=new Test();

6) Similar to the interface we can define static methods in an abstract class that can be called independently without an object.

Java

abstract class Party {
    static void doParty()
    {
        System.out.println("Lets have some fun!!");
    }
}
  
public class Main extends Party {
    public static void main(String[] args)
    {
        Party.doParty();
    }
}

Output

Lets have some fun!!

Exercise:
1. Is it possible to create an abstract and final class in Java?
2. Is it possible to have an abstract method in a final class?
3. Is it possible to inherit from multiple abstract classes in Java?

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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