interface AdditiveGroup<T> {
   T negative();  // the inverse
   T plus(T x);   // how I would like a "+" in Java
   T minus(T x);  // as T plus(T x.negative());
   // can't do: static T zero(); // the neutral element
}

interface Group<T, op> {
   T op(T x);
   T negative();
   static T id();
}
 
// example: group of all integers  Z = {...,-2,-1,0,1,2,...}
public class Z implements Group<Z,+> { ... }

> interface AdditiveGroup<T> {
>    T negative();  // the inverse
>    T plus(T x);   // how I would like a "+" in Java
>    T minus(T x);  // as T plus(T x.negative());
>    // can't do: static T zero(); // the neutral element
> }
> 

> interface Group<T, op> {
>    T op(T x);
>    T negative();
>    static T id();
> }
> 
> // example: group of all integers  Z =
> {...,-2,-1,0,1,2,...}
> public class Z implements Group<Z,+> { ... }
> 

public interface Group<G> {
   public interface Element<G> {
      Element<G> inverse();
      Element<G> op();
      ...
   }
   public Element<G> id();
   ...
}

class AnnualStats {
    private final int begin;
    private final Stats[] stats;
 
    // end inclusive
    AnnualStats(final int begin, final int end)
    {
        this.begin = begin;
        stats = new Stats[(end - begin) + 1];
    }
 
    Stats getStats(int year)
    {
        return stats[year - begin];
    }
 
    void setStats(int year, Stats stats)
    {
        stats[year - begin] = stats;
    }
}