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// $Id$
import java.util.Enumeration;
public class Queue
{
// Friendly?
Queue_Node head_ = null;
Queue_Node tail_ = null;
private int length_ = 0;
public Queue()
{
}
public Enumeration forward_iterator()
{
return new Queue_Iterator(this, Queue_Iterator.FORWARD);
}
public Enumeration reverse_iterator()
{
return new Queue_Iterator(this, Queue_Iterator.REVERSE);
}
public void enqueue_tail(Object new_data)
{
Queue_Node new_node = new Queue_Node(new_data);
if (tail_ == null)
{
tail_ = new_node;
head_ = new_node;
}
else
{
new_node.prev_ = tail_;
tail_.next_ = new_node;
tail_ = new_node;
}
length_++;
}
public void enqueue_head(Object new_data)
{
Queue_Node new_node = new Queue_Node(new_data);
if (head_ == null)
{
tail_ = new_node;
head_ = new_node;
}
else
{
new_node.next_ = head_;
head_.prev_ = new_node;
head_ = new_node;
}
length_++;
}
public Object dequeue_head()
{
Object return_value = null;
if (head_ == null)
return_value = null;
else if (tail_ == head_)
{
return_value = head_.data_;
tail_ = null;
head_ = null;
}
else
{
return_value = head_.data_;
head_ = head_.next_;
head_.prev_ = null;
}
length_--;
return return_value;
}
public Object dequeue_tail()
{
Object return_value = null;
if (tail_ == null)
return_value = null;
else if (tail_ == head_)
{
return_value = tail_.data_;
tail_ = null;
head_ = null;
}
else
{
return_value = tail_.data_;
tail_ = tail_.prev_;
tail_.next_ = null;
}
length_--;
return return_value;
}
public int length()
{
return length_;
}
public Object head()
{
if (head_ != null)
return head_.data_;
else
return null;
}
public Object tail()
{
if (tail_ != null)
return tail_.data_;
else
return null;
}
}
class Queue_Node
{
public Queue_Node prev_ = null;
public Queue_Node next_ = null;;
public Object data_;
public Queue_Node(Object data)
{
data_ = data;
}
}
class Queue_Iterator implements Enumeration
{
public static final boolean FORWARD = true;
public static final boolean REVERSE = false;
private Queue queue_;
private Queue_Node queue_ptr_;
private boolean direction_ = FORWARD;
public Queue_Iterator(Queue queue)
{
queue_ = queue;
queue_ptr_ = queue.head_;
}
public Queue_Iterator(Queue queue, boolean direction)
{
queue_ = queue;
direction_ = direction;
if (direction_)
queue_ptr_ = queue_.head_;
else
queue_ptr_ = queue_.tail_;
}
public Object nextElement()
{
Object data = queue_ptr_.data_;
if (direction_)
queue_ptr_ = queue_ptr_.next_;
else
queue_ptr_ = queue_ptr_.prev_;
return data;
}
public boolean hasMoreElements()
{
return queue_ptr_ != null;
}
}
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