comparing two TreeNode (or DefaultMutableTreeNode) objects in Java Comparator

Question

Hello everyone!

My goal is very simple today, I am trying to work out the proper way to implement compareTo (or the Comparable) interface for my class which extends DefaultMutableTreeNode.

The problem is this: Say I have a perfectly good class that represents times. I've already written a perfectly good compareTo method (which works as I desire) that I've tested out with Arrays.sort() with marvelous results.

Now lets say I have a JTree with bunches of different objects, like this:

    new SpecialNode("Zomg a string!"); // add this group of nodes right here
    new SpecialNode(new Time("8:55 PM"));
    new SpecialNode(new SomeTypeYouveNeverHeardOf());

So, being a professional programmer, I immediately start coding without any forethought whatsoever. Here is my SpecialNode class:

class SpecialNode extends DefaultMutableTreeNode implements Comparator<SpecialNode>
{
    public int compareTo(SpecialNode sn)
    {
        // Not only does this not work correctly (read: at all)
        // But, it is sub-par, how do I get the type information out of the userObject
        // So I can cast it correctly and call the correct compareTo method!!
        return this.getUserObject().toString().compareTo(sn.getUserObject().toString());
    }
}

Ok, so if you didn't read the comments (which, admit, you didn't); My problem is that within the compareTo method of SpecialNode, I only have access to the userObject. Sadly, I do not know what the userObject used to be, and as such, I cannot properly cast it to call the correct compareTo method!

This is really a pain, since I've already written several perfectly good compareTo methods in all of the classes which will be added to my tree. So can someone help a guy out and drop me a hint?

tl;dr - How do I get type information out of an generic object that DefaultMutableTreeNode stores? If that is not possible, how should I go about comparing two instances of SpecialNode when I don't even know what they may contain!

Thanks in advance.

Answer 1

I'm assuming you can't have a TreeNode for each type of data. Does it make sense in your use case to do comparisons if the type is different?

Ideas:

1. Can SimpleNode know all of the possible types and do instanceof and casts to the correct type for Comparable? This is the way I would have handled it a few years ago.

2. How do you feel about unchecked warnings? I had a similar problem before using a JList and couldn't quite make the compiler happy (I gave up the swing default model to make my life easier). Maybe someone else could improve on this answer?

   class SpecialNode<T extends Comparable<T>> extends DefaultMutableTreeNode 
                                             implements Comparable<SpecialNode>
   {
     T typedUserObject;
     SpecialNode(T t)
     {
        this.typedUserObject = t;
        setUserObject(t);
     }
   
   
     public int compareTo(SpecialNode node)
     {
         if(typedUserObject.getClass().isInstance(node.typedUserObject))
         {
             T otherObj = (T) node.typedUserObject;
             return typedUserObject.compareTo(otherObj);
         }
         else
         {
             //What are you going to do if they're not the same type?
             return -1;
         }
     }
   
   
   }
   

Edit: If you know they should be the same type - eliminates check

class SpecialNode<T extends Comparable<T>> extends DefaultMutableTreeNode
                                          implements Comparable<SpecialNode<T>>
{
  T typedUserObject;
  SpecialNode(T t)
  {
     this.typedUserObject = t;
     setUserObject(t);
  }


  public int compareTo(SpecialNode<T> node)
  {
     return typedUserObject.compareTo(node.typedUserObject);
  }    
}

If you don't want the code in the node itself (I don't think I would), you might create a separate class that implements Comparator<SpecialNode<T>>