import java.util.ArrayList;
 
  /**
     A first-in, first-out collection of messages. This
     implementation is not very efficient. We will consider
     a more efficient implementation in chapter 3.
  */
public class MessageQueue
{
    /**
       Constructs an empty message queue.
    */
  public MessageQueue()
    {
      queue = new ArrayList<Message>();
    }
 
    /**
       Remove message at head.
       @return message that has been removed from the queue
    */
  public Message removeFirst()
    {
      return queue.remove(0);
    }
 
    /**
       Append message at tail.
       @param newMessage the message to be appended
    */
  public void add(Message newMessage)
    {
      queue.add(newMessage);
    }
 
    /**
       Get the total number of messages in the queue.
       @return the total number of messages in the queue
    */
  public int size()
    {
      return queue.size();
    }
 
    /**
       Get message at head.
       @return message that is at the head of the queue
    */
  public Message getFirst()
    {
      return queue.get(0);
    }
 
  private ArrayList<Message> queue;
}