Static Variable Within A Class?

I have a class Tree in which all the methods to build a tree are in place. But however I would want variable of by Tree which is pointing to the last node being added to the tree.

So basically every time you keep adding a node to the tree you tail pointer gets updated to point to the last node. I have this so far.

public class NonEmptyTree implements Tree {
private Tree left;
private int data;
private int leftleafCount;
private int rightleafCount;
private Tree right;
private Tree tail; // This variable must be shared by all the object. There needs to just one tail pointer to the tree.
public Tree insert( data ) {
tail = // gets updated every time when new node gets added.