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

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// 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());          } }