哪种Java设计模式适合以下情况？

Question

I am working on a chemistry package and I have a class which lists all the elements in the periodic table. Ideally the elements will be part of a Java enum. Unfortunately however I need one more element which is to serve as a wildcard : every other element should equal that element. Java does not allow to override the equals() method for enums otherwise I would have done that. Would anyone be able to suggest a reasonable design pattern for the situation that I have just described?

EDIT: Thank you for your contributions. I indeed failed to observe the transitive property required by equals() .

The elements of the periodic table will be assigned to different nodes on a graph structure(in mathematical sense). Given this graph structure I then look for all embeddings of a particular subgraph structure in the original graph (subgraph isomorphism problem). A desired property of the subgraph structure is to have certain nodes to which a wildcard is assigned so that I can map those nodes to any node in the original graph regardless of the element assigned to it. That is why I am looking for a non-transitive relation such that a wildcard may equal two different elements without implying that the elements themselves are equal. My current algorithm makes use of generics and calls equals() to check if elements in the two nodes are equal.

Answer 1

As mystarrocks pointed out, The big problem with your design is that it violates the equals contract . Specifically, as per the spec in class Object , the equals method should:

The equals method implements an equivalence relation on non-null object references:

• It is reflexive: for any non-null reference value x, x.equals(x) should return true.
• It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
• It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
• It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
• For any non-null reference value x, x.equals(null) should return false.

(Source)

Your design would violate the transitive property. If sodium equals wildcard, and wildcard equals potassium, then sodium must equal potassium.

A better way is to create a helper equals method for when you want to see if two elements can be considered equal (which is different than being equal). Wildcard is only truly equal to wildcard, but it can be considered equal to any element.

public enum Element {
  HYDROGEN,
  HELIUM,
  SODIUM,
  //.....
  URANIUM,
  WILD_CARD;

  public boolean consideredEqual(Object other) {
    if (other == null || ! (other instanceof Element)) return false;

    Element e = (Element) other;
    if (this.equals(Element.WILD_CARD) || e.equals(Element.WILD_CARD)) return true;
    return equals(other);
  }
}