Interfaces

How to count the number of elements in the preceding organization(s) ?

11. To enforce subclasses to overload a set of operations, an operation can be declared as abstract (then have 'no code'):

public abstract class Element {  public abstract int count();  }
public abstract class NamedElement extends Element { ... }
public class Person extends NamedElement {
  public int count() { return 1; } // overloading/implementation
  ...
}
public class Group extends NamedElement {
  public int count() {
    int res = 1;
    for(int i=0; i<size;i++)
      res += elements[i].count();
    return res;
  }
  ...
}

Thus, sport.count(); will return 7 = 1 group + 2 subgroups + 2 persons/subgroup(x2).

UML/SysML class diagram