Implementation of a basic generic tree, with labels of type T - Data Structure

// This implements a basic generic tree, with labels of type T,
// pointer to the parent node, and a singly linked list of children nodes.

class Tree<T>  {
    private T label;
    private Tree<T> parent;
    private Tree<T> nextSibling; // next node on the list of parents's 
                                 //  children
    private Tree<T> firstChild;  // first in the linked list of children

                                 
//  Getters and setters
    public T getLabel() { return label; }  
    public void setLabel(T v) { label = v; }
    public Tree<T> getParent() { return parent;}
    public Tree<T> getNextSibling() { return nextSibling;}
    public Tree<T> getFirstChild() { return firstChild;}

// Add C to the front of the children of this
    public void addChild(Tree<T> c) {
         c.parent = this;
         if (firstChild == null) 
           firstChild = c;
         else {
             c.nextSibling = firstChild;
             firstChild = c;
            }
    }

// Check if the node is a leaf
    public boolean Leaf() { return firstChild==null; }

// `Convert the tree into a string. The structure is indicated by
// square brackets. Uses a preorder listing.
    public String toString() {
       String S = "[ " + label.toString();
       Tree<T> N = firstChild;
       while (N != null) {
          S = S + " " + N.toString();
          N = N.nextSibling;
        }
       return S+" ]";
     }

     public void displayPreorder() {
         displayPreorder1(0); }

     public void displayPreorder1(int Indent) {
         for (int I = 0; I < Indent; I++) System.out.print(" ");
         System.out.println(label.toString());
         Tree<T> N = firstChild;
         while (N != null) { 
            N.displayPreorder1(Indent+3);
            N = N.nextSibling;
           }
         }

     public void displayPostorder() {
         displayPostorder1(0); }

     public void displayPostorder1(int Indent) {
         Tree<T> N = firstChild;
         while (N != null) { 
            N.displayPostorder1(Indent+3);
            N = N.nextSibling;
           }
         for (int I = 0; I < Indent; I++) System.out.print(" ");
         System.out.println(label.toString());
         }
}