Generics in Java programming thinking implement a stack class share

Feel the author writes too well, have to collect.

Understanding of this example:

// type parameters cannot be of basic type. T and U are of the same type.

// each time new data is put, it becomes a new top. Press the old top down one level, and link through the pointer.

// terminal sentinel is a node created by the default constructor that returns true in accordance with end().

//: generics/LinkedStack.java
// A stack implemented with an internal linked structure.
package generics;
public class LinkedStack<T> {
  private static class Node<U> {
    U item;
    Node<U> next;
    Node() { item = null; next = null; }
    Node(U item, Node<U> next) {
      this.item = item;
      this.next = next;
    }
    boolean end() { return item == null && next == null; }
  }
  private Node<T> top = new Node<T>(); // End sentinel
  public void push(T item) {
    top = new Node<T>(item, top);
  }    
  public T pop() {
    T result = top.item;
    if(!top.end())
      top = top.next;
    return result;
  }
  public static void main(String[] args) {
    LinkedStack<String> lss = new LinkedStack<String>();
    for(String s : "Phasers on stun!".split(" "))
      lss.push(s);
    String ss;
    while((ss = lss.pop()) != null)
      System.out.println(ss);
      //----- if put integer into the LinkedList
      LinkedStack<Integer> lii = new LinkedStack<Integer>();
      for(Integer i = 0; i < 10; i++){
          lii.push(i);
      }
      Integer end;
      while((end = lii.pop()) != null)
          System.out.println(end);
      //----- integer test end!
  }

  
}