Constructor of a derived class (Java)

I'm very new in Java, and about to ask a fundamental question. Hope you guys could help me. Supposed I have a base classe Super and a derived class Sub, which inheritances from class Super as follows:

public class TestSuperSub {
  public static void main(String[] args) {
    Super ou = new Sub(5,10);
  }
}

class Super {
  Super() {
    System.out.println("Super()");
  }

  Super(int x, int y) {
    System.out.println("Super(int, int)");
  }
}

class Sub extends Super {
   public Sub(int x, int y) {
      System.out.println("Sub(int, int)");
   }
}

The output is

Super()
Sub(int, int)

I understand, that ou calls Sub::Sub(int,int) and therefore, Sub(int, int) is printed. But why is Super() printed, since Super::Super() hasn't never been called?

Could someone please explain it to me. Thanks a lot! Cheers

Jon Skeet
people
quotationmark

But why is Super() printed, since Super::Super() hasn't never been called?

It has, because your Sub constructor is implicitly calling it. It's as if you'd written:

public Sub(int x, int y) {
   super();
   System.out.println("Sub(int, int)");
}

From section 8.8.7 of the JLS:

The first statement of a constructor body may be an explicit invocation of another constructor of the same class or of the direct superclass (ยง8.8.7.1).

...

It is a compile-time error for a constructor to directly or indirectly invoke itself through a series of one or more explicit constructor invocations involving this.

If a constructor body does not begin with an explicit constructor invocation and the constructor being declared is not part of the primordial class Object, then the constructor body implicitly begins with a superclass constructor invocation "super();", an invocation of the constructor of its direct superclass that takes no arguments.

If you want to call a different superclass constructor, you need to do that implicitly. When you call a subclass constructor, that will always work its way up through the inheritance hierarchy, executing the body of the superclass constructor before the subclass constructor... indeed, even field initializers in the subclass are only run after the superclass constructor.

people

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