gistfile1.java

public static final class Ratio extends Number {
  /* .. */
}

public static final class Addition {

  public static Number add(Integer x, Integer y) {
    /* .. */
  }

  public static Number add(Ratio x, Ratio y) {
    /* .. */
  }

  public static Number add(Ratio x, Integer y) {
    /* .. */
  }

  public static Number add(Number x, Number y) {
    Class xClass = x.getClass();
    if (xClass == Integer.class)
      x = (Integer) x;
    else if (xClass == Ratio.class)
      x = (Ratio) x;

    Class yClass = y.getClass();
    if (yClass == Integer.class)
      y = (Integer) y;
    else if (yClass == Ratio.class)
      y = (Ratio) y;

    return (Number) add(x, y);
  } 

}

Addition.add(new Integer(..), new Integer(..)) // Is good
Addition.add(new Ratio(..), new Ratio(..)) // Is good
Addition.add(new Ratio(..), new Integer(..)) // Infinite recursion in the add(Number, Number) definition