Java: "uses unchecked or unsafe operations. Recompile with..."

Question

Despite spending a significant amount of time googling for an answer to my predicament and re-reading the chapter on Generics in my Java textbook I cannot seem to fix the problems with the following code:

public class RedBlackTree<I extends Comparable>
{
    private int count = 0;
    private RedBlackNode root;

    private RedBlackNode current;
    private RedBlackNode[] stack;

    /**
     * Inner class used for representing the nodes of the red-black balanced binary search tree object.
     */
    private class RedBlackNode implements Comparable
    {
        private I id;
        private boolean is_btree;
        private RedBlackNode[] links;

        /**
         * Constructor for objects of the RedBlackNode class.
         * 
         * @param id The ID of the node.
         */
        private RedBlackNode(I id)
        {
            if (id == null)
            {
                throw new NullPointerException("ID cannot be null.");
            }

            this.id = id;
            this.is_btree = true;
        }

        /**
         * Function for comparing the RedBlackNode object to another object.
         * 
         * @param obj The object to be compared.
         * @return If invocant > passed, returns 1; if invocant < passed, returns -1; if invocant = passed, returns 0.
         */
        private int compareTo(Object obj)
        {
            if (obj instanceof RedBlackTree.RedBlackNode)
            {
                RedBlackNode node = (RedBlackNode)obj;

                int result = id.compareTo(node.id);

                return result > 0 ? 1 : result < 0 ? -1 : 0;
            }
            else
            {
                throw new ClassCastException("Expected a RedBlackNode object.");
            }
        }
    }
}

In particular, I'm receiving a popup with the following message:

Warnings from last compilation

C:\Users\...\RedBlackTree.java uses unchecked or unsafe operations.
Recompile with -Xlint:unchecked for details.

Nearly every combination of I here or Comparable there still leads to such a popup. I'm using the BlueJ environment for programming and this makes it impossible to incorporate the relevant compiler argument in order to see any details.

As far as I can discern from my research thus far, it's something to do with the fact that the inner class utilizes the I generic type and so the "RedBlackNode implements Comparable" and the compareTo method in the RedBlackNode inner class need to contend with that fact somehow.

I know that this question has been asked here on stackoverflow and elsewhere and answered many times, but I can't seem to apply what I've learned from those instances to my case. I'm pretty new to Generics, so any help I can get here would be very much appreciated!

Answer 1

Make the following changes

public class RedBlackTree<I extends Comparable<I>>

private class RedBlackNode implements Comparable<RedBlackNode>

    @Override
    public int compareTo(RedBlackNode node)
            int result = id.compareTo(node.id);
            return result > 0 ? 1 : result < 0 ? -1 : 0;

In the compareTo remove type checking. Mind the public, therefore always use @Override.