Suppose we wish to use a collection that always has only zero or one element and we wish to dispatch based on this.

interface OneOrNone<T> {
  <A> A visit(OneOrNoneVisitor<A> v);
}

abstract class One<T> implements OneOrNone<T> {
  public <A> A visit(OneOrNoneVisitor<A> v) { return v.visit(this); }
  abstract T one();
}

class None<T> implements OneOrNone<T> {
  public <A> A visit(OneOrNoneVisitor<A> v) { return v.visit(this); }
}

interface OneOrNoneVisitor<A> {
  <T> A visit(One<T> o);
  <T> A visit(None<T> n);
}