How can I use a custom class in a TreeSet?

Question

If I was using a Set similar to this:

Set<node> s=new TreeSet<node>();

class node {

  private int x;
  private int y;

}

Would this be acceptable, and since it's a TreeSet, would it also sort it?

Answer 1

It's not going to be able to sort it without you implementing Comparable<Node>, and it won't really be an appropriate for set operations until you override equals() and hashCode(). (You don't have to override equals and hashCode for TreeSet to work, but it would make sense to do so.)

Something like this:

final class Node implements Comparable<Node> {

  private final int x;
  private final int y;

  Node(int x, int y) {
    this.x = x;
    this.y = y;
  }

  @Override public boolean equals(Object other) {
    if (!(other instanceof Node)) {
      return false;
    }
    Node otherNode = (Node) other;
    return x == otherNode.x && y == otherNode.y;
  }

  @Override public int hashCode() {
    return x * 31 + y * 17; // For example...
  }

  @Override public int compareTo(Node other) {
    // As of Java 7, this can be replaced with
    // return x != other.x ? Integer.compare(x, other.x) 
    //     : Integer.compare(y, other.y);

    if (x < other.x || (x == other.x && y < other.y)) {
      return -1;
    }
    return x == other.x && y == other.y ? 0 : 1;
  }
}

(Note that by convention the class name would be Node, not node.)