import java.util.Iterator;
import java.util.NoSuchElementException;
/**
* The <tt>Queue</tt> class represents a first-in-first-out (FIFO)
* queue of generic items.
* It supports the usual <em>enqueue</em> and <em>dequeue</em>
* operations, along with methods for peeking at the top item,
* testing if the queue is empty, and iterating through
* the items in FIFO order.
* <p>
* All queue operations except iteration are constant time.
* <p>
* For additional documentation, see <a href="http://algs4.cs.princeton.edu/13stacks">Section 1.3</a> of
* <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
*/
public class Queue<Item> implements Iterable<Item> {
private int N; // number of elements on queue
private Node first; // beginning of queue
private Node last; // end of queue
// helper linked list class
private class Node {
private Item item;
private Node next;
}
/**
* Create an empty queue.
*/
public Queue() {
first = null;
last = null;
}
/**
* Is the queue empty?
*/
public boolean isEmpty() {
return first == null;
}
/**
* Return the number of items in the queue.
*/
public int size() {
return N;
}
/**
* Return the item least recently added to the queue.
* Throw an exception if the queue is empty.
*/
public Item peek() {
if (isEmpty()) throw new RuntimeException("Queue underflow");
return first.item;
}
/**
* Add the item to the queue.
*/
public void enqueue(Item item) {
Node x = new Node();
x.item = item;
if (isEmpty()) { first = x; last = x; }
else { last.next = x; last = x; }
N++;
}
/**
* Remove and return the item on the queue least recently added.
* Throw an exception if the queue is empty.
*/
public Item dequeue() {
if (isEmpty()) throw new RuntimeException("Queue underflow");
Item item = first.item;
first = first.next;
N--;
if (isEmpty()) last = null; // to avoid loitering
return item;
}
/**
* Return string representation.
*/
public String toString() {
StringBuilder s = new StringBuilder();
for (Item item : this)
s.append(item + " ");
return s.toString();
}
/**
* Return an iterator that iterates over the items on the queue in FIFO order.
*/
public Iterator<Item> iterator() {
return new FIFOIterator();
}
// an iterator, doesn't implement remove() since it's optional
private class FIFOIterator implements Iterator<Item> {
private Node current = first;
public boolean hasNext() { return current != null; }
public void remove() { throw new UnsupportedOperationException(); }
public Item next() {
if (!hasNext()) throw new NoSuchElementException();
Item item = current.item;
current = current.next;
return item;
}
}
/**
* A test client.
*/
public static void main(String[] args) {
/***********************************************
* A queue of strings
***********************************************/
Queue<String> q1 = new Queue<String>();
q1.enqueue("Vertigo");
q1.enqueue("Just Lose It");
q1.enqueue("Pieces of Me");
System.out.println(q1.dequeue());
q1.enqueue("Drop It Like It's Hot");
while (!q1.isEmpty())
System.out.println(q1.dequeue());
System.out.println();
/*********************************************************
* A queue of integers. Illustrates autoboxing and
* auto-unboxing.
*********************************************************/
Queue<Integer> q2 = new Queue<Integer>();
for (int i = 0; i < 10; i++)
q2.enqueue(i);
// test out iterator
for (int i : q2)
System.out.print(i + " ");
System.out.println();
// test out dequeue and enqueue
while (q2.size() >= 2) {
int a = q2.dequeue();
int b = q2.dequeue();
int c = a + b;
System.out.println(c);
q2.enqueue(a + b);
}
}
}
/*************************************************************************
* Execution: java Queue
*
* A generic queue, implemented using a linked list. Each queue
* element is of type Item.
*
*************************************************************************/
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