public class Child  extends Father {

  private int age = 0;

  public Child() {
    
    System.out.println("initializing child ("+age+") ...");
  }

  public Child(int age) {
    
    // super(age);
    this.age = age;
    System.out.println("initializing a child "+age+" years old ...");
  }

  public void tell() {
    
    if (age > 0) {
      System.out.println("this is a child "+age+" years old.");
    } else {
      System.out.println("this is a child of unknown age");
    }
  }
}