Why default constructor is required in a parent class if it has an argument-ed constructor?

Question

Why default constructor is required(explicitly) in a parent class if it has an argumented constructor

class A {       A(int i){       } }  class B extends A { }  class Main {       public static void main(String a[]){     B b_obj = new B();   } } 

This will be an error.

Answer 1

There are two aspects at work here:

• If you do specify a constructor explicitly (as in A) the Java compiler will not create a parameterless constructor for you.

• If you don't specify a constructor explicitly (as in B) the Java compiler will create a parameterless constructor for you like this:

  B() {     super(); } 

(The accessibility depends on the accessibility of the class itself.)

That's trying to call the superclass parameterless constructor - so it has to exist. You have three options:

• Provide a parameterless constructor explicitly in A
• Provide a parameterless constructor explicitly in B which explicitly calls the base class constructor with an appropriate int argument.
• Provide a parameterized constructor in B which calls the base class constructor

Answer 2

Why default constructor is required(explicitly) in a parent class if it has an argumented constructor

I would say this statement is not always correct. As ideally its not required.

The Rule is : If you are explicitly providing an argument-ed constructer, then the default constructor (non-argumented) is not available to the class.

For Example :    class A {       A(int i){       } }  class B extends A { } 

So when you write

B obj_b = new B(); 

It actually calls the implicit constructor provided by java to B, which again calls the super(), which should be ideally A(). But since you have provided argument-ed constructor to A, the default constructor i:e A() is not available to B().

That's the reason you need A() to be specifically declared for B() to call super().