BinaryNode should be a nested class within BinarySearchTree (BST), for the purpose of information hiding. (That is, no user of our BST class should need to know about BinaryNodes.)

Note that this may require a few changes in your code:

• If you have a NULL_NODE, you'll need to make it non-static. This is less efficient, but information hiding is worth it.
• You may need to move the NULL_NODE to the BST class. Just make it private if you do. (And keep it final, though non-static.)
• What I ended up with was:

  private final BinaryNode NULL_NODE = new BinaryNode();

  This required me to write a second constructor that took no parameters.
• If you have already started to add type parameters to your code, you may need to undo a few things. Primarily, since BinaryNode inherits the type parameter, it doesn't need its own. So just use BinaryNode, not BinaryNode<T>. (If you included the T, the inner T type would shadow the other and the compiler doesn't allow that.) BinaryNode parameters and methods will still be of type T.