Question

According to my text book:

If the execution of any constructor in a subclass does not invoke a
superclass constructor, Java automatically invokes the no-parameter
constructor for the superclass.

Does that mean all the superclass data field(superclass variables) would be set to null(because the constructor is a no-parameter constructor)?

Answer 1

No, if the supers constructor with no arguments initializes some objects – this initialization will be done.

class A { 
  public int x;
  A () { 
     x = 1;
  }
}
class B extends A { 
  B() { 
  }
}

The B‘s constructor will still invoke super() as the first line (even though it is not explicitly written), and will initialize x=1 in the process.