queue.java

java编程代码

public class Queue
{
    private class Node
    {
        private String item;
        private Node link;

        public Node( )
        {
             item = null;
             link = null;
        }

        public Node(String newItem, Node linkValue)
        {
            item = newItem;
            link = linkValue;
        }
     }//End of Node inner class

    private Node front;
    private Node back;

    public Queue( )
    {
        front = null;
        back = null;
    }

    /**
     Adds a String to the back of the queue.
    */
    public void addToBack(String itemName)
    {
        Node newEntry =
              new Node(itemName, null);
        if (front == null) //empty queue
        {
            back = newEntry;
            front = back;
        }
        else
        {
            back.link = newEntry;
            back = back.link;
        }
    }
    
    public boolean isEmpty( )
    {
        return (front == null);
    }

    public void clear( )
    {
        front = null;
        back = null;
    }

    /**
     Returns the String in the front of the queue.
     Returns null if queue is empty.
    */
    public String whoIsNext( )
    {
        if (front == null)
            return null;
        else
            return front.item;
    }

    /**
     Removes a String from the front of the queue.
      Returns false if the list is empty.
    */
    public boolean removeFront( )
    {
        if (front != null)
        {
            front = front.link;
            return true;
        }
        else
            return false;
    }
}