Inheritance and the default superclass constructor.

In this short article I will write about the situation, when we extend some class but its default constructor is not accesible for us. For example it has a private modifier or a default one and we are in a different package. If this is a case, we have to remember to use another (accessible) superclass constructor in our subclass constructor. If we don't do it, a compiler will try to insert a call to the default constructor super() which obviously will not work. Below is an example of this:

 class U1 {
     private U1() { }
     public U1(int x) { }
}
class U2 extends U1 {
     U2() {
          System.out.println("U2 construction...");
     }
}

To mitigate the situation we should explicitly use the public but not default constructor as in the following code:

 class U1 {
     private U1() { }
     public U1(int x) { }
}
class U2 extends U1 {
     U2() {
          super(5);
          System.out.println("U2 construction...");
     }
}