Why Java Interfaces Cannot Have Constructor But Abstract Classes Can Have?

Prerequisite: Interface and Abstract class in Java.

A Constructor is a special member function used to initialize the newly created object. It is automatically called when an object of a class is created.

Why interfaces can not have the constructor?

• An Interface is a complete abstraction of class. All data members present in the interface are by default public, static, and final. All the static final fields should be assigned values at the time of declaration, otherwise it will throw compilation error saying “variable variable_Name not initialized in default constructor”.
• The methods inside the interface are by default public abstract which means the method implementation cannot be provided by the interface itself, it has to be provided by the implementing class. Therefore, no need of having a constructor inside the interface.
• A constructor is used to initializing non-static data members and as there are no non-static data members in the interface, there is no need of constructor
• Methods present in the interface are only declared not defined, As there is no implementation of methods, there is no need of making objects for calling methods in the interface and thus no point of having constructor in it.
• If we try to create a constructor inside the interface, the compiler will give a compile-time error.

// Java program that demonstrates why
// interface can not have a constructor

// Creating an interface
interface Subtraction {

    // Creating a method, by default
    // this is a abstract method
    int subtract(int a, int b);
}

// Creating a class that implements
// the Subtraction interface
class GFG implements Subtraction {

    // Defining subtract method
    public int subtract(int a, int b)
    {
        int k = a - b;
        return k;
    }

    // Driver Code
    public static void main(String[] args)
    {

        // Creating an instance of
        // GFG class
        GFG g = new GFG();
        System.out.println(g.subtract(20, 5));
    }
}

15

In the above program, we have created an interface Subtraction which defines a method subtract(), whose implementation is provided by the class GFG. In order to call a method, we need to create an object, since the methods inside the interface by default public abstract which means the method inside the interface doesn't have the body. Therefore, there is no need for calling the method in the interface. Since we cannot call the methods in the interface, there is no need for creating the object for interface and there is no need of having a constructor in it.

Why abstract classes have a constructor? 

• The main purpose of the constructor is to initialize the newly created object. In abstract class, we have an instance variable, abstract methods, and non-abstract methods. We need to initialize the non-abstract methods and instance variables, therefore abstract classes have a constructor.
• Also, even if we don't provide any constructor the compiler will add default constructor in an abstract class.
• An abstract class can be inherited by any number of sub-classes, thus functionality of constructor present in abstract class can be used by them.
• The constructor inside the abstract class can only be called during constructor chaining i.e. when we create an instance of sub-classes. This is also one of the reasons abstract class can have a constructor.

// A Java program to demonstrates
// an abstract class with constructors

// Creating an abstract class Car
abstract class Car {

  // Creating a constructor in
  // the abstract class
    Car() { 
      System.out.println("car is created"); 
    }
}

// A class that extends the 
// abstract class Car
class Maruti extends Car {

  // Method defining inside
  // the Maruti class
    void run() { 
      System.out.println("Maruti is running"); 
    }
}

class GFG {

    public static void main(String[] args)
    {
        Maruti c = new Maruti();
        c.run();
    }
}

car is created
Maruti is running