Abstract Classes in Java

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.

// 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 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(); 
    }
}

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 a inherited class is created. For example, the following is a valid Java program.

// 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.

// 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.

// 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

Exercise:
1. Is it possible to create 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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