public class SharedQueue
{ private class Link
  { public Object item;
    public Link next;
    public Link(Object i) { item=i; next=null; } }
  private Link head, tail;
  public SharedQueue() { head=null; tail=null; }
  public synchronized void add(Object i)
  { if (head==null)
    { head=new Link(i);
      tail=head; }
    else
    { Link temp=new Link(i);
      tail.next=temp;
      tail=temp; } }
  public synchronized boolean empty()
  { return (head==null); }
  public synchronized Object remove()
  { if (head==null) return (null);
    Object result=head.item;
    head=head.next;
    return (result); }
  public synchronized Object first()
  { if (head==null) return (null);
    return (head.item); } }