class node {
  protected Object content;
  private node   next;

  /* Konstruktor */
  node (Object o) {
    this.content = o;
    this.next    = null;
  }
  
  public  void setNext (node succ) {
    next = succ;
  }
  public node getNext () {
    return next;
  }
}

public class list {
  private node    head;
  private node    tail;
  private boolean empty;

  list () {      // Konstruktor
    head = null;
    tail = null;
    empty = true;
  }

  /* gebe den Inhalt des ersten Elementes aus */
  public Object first () {
    if (!empty) {
      return head.content;
    }
    else { return null; }
  }
  
  /* gebe den Inhalt des letzten Elementes aus */
  public Object last () {
    if (!empty) {
      return tail.content;
    }
    else { return null; }
  }

  /* teste, ob die Liste leer ist */
  public boolean isEmpty () {
    return empty;
  }
  
  /* fuege am Ende der Liste ein neues Element an */
  public void append (Object o) {
    node newNode = new node (o);
    if (empty) {
      tail = newNode;
      head = newNode;
      empty = false;
    }
    else {
      tail.setNext(newNode);
      tail = newNode;
    }
  }
  
  /* fuege am Anfang der Liste ein neues Element an */
  public void insert (Object newObject) {
    node newNode = new node(newObject);
    if (empty) {
      tail = newNode;
      head = newNode;
      empty = false;
    }
    else {
      newNode.setNext(head);
      head = newNode;
    }
  }
  
  /* entferne das erste Element und gebe es zurueck */
  public Object removeFirst () {
    if (!empty) {
      node first = head;
      if (head == tail) {
	head = tail = null;
	empty = true;
      }
      else {  head = head.getNext (); }
      return first.content;      
    }
    else { return null; }
  }
}