Java中无向图的边缘

Question

Suppose I am writing a Java class to represent an edge of undirected graph. This class Edge contains two vertices to and from .

class Edge<Vertex> {

  private final Vertex to, from

  public Edge(Vertex to, Vertex from) {
    this.to = to;
    this.from = from;
  } 
  ... // getters, equals, hashCode ...
}

Obviously e1 = new Edge(v1, v2) and e2 = new Edge(v2, v1) are actually the same in an undirected graph. Does it make sense? How would you implement class Edge to meet that requirement?

Answer 1

Perform a sort on the vertices in the constructor based on some unique identifier. This way they are stored consistently regardless of order.

I find this preferable to noMAD's solution because all code interacting with these objects will treat them identically, not just your implementation of equals .

Also, calling your class members to and from is confusing because it sounds like a directed graph. I would rename these to something more generic like vertex1 and vertex2 .

  public Edge(Vertex x, Vertex y) {
      if (vertex2.getId() > vertex1.getId()) {
          this.vertex1 = x;
          this.vertex2 = y;
      } else {
          this.vertex1 = y;
          this.vertex2 = x;
      }
  } 

Answer 2

I actually wouldn't have this logic in my Edge class but rather some sort of over-seeing class such as a Graph class. The reason for this is because an Edge is just an object with 2 vertices. It doesn't know anything about the rest of the edges in the graph.

So, to expand on @noMad's answer, I would actually put his checkIfSameEdge method in my Graph class:

public class Graph {
    private List<Edge> edges;
    ....
    public void addEdge(Edge e) {
        for (Edge edge : edges) {
            if (isSameEdge(edge, e) {
                return; // Edge already in Graph, nothing to do
        }
        edges.add(e);
    }
    private boolean isSameEdge(Edge edge1, Edge edge2) {
        return ((edge1.to.equals(edge2.to) && edge1.from.equals(edge2.from))
             || (edge1.to.equals(edge2.from) && edge1.from.equals(edge2.to)))
    }
}

BTW: I would rename to and from to vertex1 and vertex2 because it is an undirected graph and to and from indicate direction, but that's just my opion.

Answer 3

Well, of the top of my head, the naivest method would be:

protected boolean checkIfSameEdge(Vertex to, Vertex from) {
  if(to.equals(this.from) && from.equals(this.to) || to.equals(this.to) && from.equals(this.from)) {
    return true;
  return false;
}

Obviously you would have to override equals and hashcode

Answer 4

假定节点包含某种标量值-根据这些值对参数进行排序（使用compareTo方法），并使用工厂创建新实例或返回现有实例。