Queue.java Queue.java Below is the syntax highlighted version of Queue.java from §4.3 Stacks and Queues. /******************************************************************************
* Compilation: javac Queue.java
* Execution: java Queue < input.txt
* Data files: https://introcs.cs.princeton.edu/java/43stack/tobe.txt
*
* A generic queue, implemented using a linked list.
*
* % java Queue < tobe.txt
* to be or not to be (2 left on queue)
*
******************************************************************************/
import java.util.Iterator;
import java.util.NoSuchElementException;
/**
* The {@code Queue} class represents a first-in-first-out (FIFO) queue of
* generic items. It supports the usual
enqueue and
dequeue
* operations, along with methods for peeking at the top item, testing if the
* queue is empty, getting the number of items in the queue, and iterating
* over the items in FIFO order.
*
* This implementation uses a singly-linked list with a nested class for
* linked-list nodes. The enqueue, dequeue, peek,
* size, and is-empty operations all take constant time
* in the worst case.
*
* For additional documentation, see
* Section 4.3 of
* Computer Science: An Interdisciplinary Approach
* by Robert Sedgewick and Kevin Wayne.
*
* @param - the generic type of an item in this queue
* @author Robert Sedgewick
* @author Kevin Wayne
*/
public class Queue
- implements Iterable
- {
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; // the item in the node
private Node next; // reference to next item
}
/**
* Initializes an empty queue.
*/
public Queue() {
first = null;
last = null;
n = 0;
}
/**
* Returns true if this queue is empty.
*
* @return {@code true} if this queue is empty; {@code false} otherwise
*/
public boolean isEmpty() {
return first == null;
}
/**
* Returns the number of items in this queue.
*
* @return the number of items in this queue
*/
public int size() {
return n;
}
/**
* Returns the number of items in this queue.
*
* @return the number of items in this queue
*/
public int length() {
return n;
}
/**
* Returns the item least recently added to this queue.
*
* @return the item least recently added to this queue
* @throws NoSuchElementException if this queue is empty
*/
public Item peek() {
if (isEmpty()) throw new NoSuchElementException("Queue underflow");
return first.item;
}
/**
* Add the item to the queue.
*
* @param item the item to enqueue
*/
public void enqueue(Item item) {
Node oldlast = last;
last = new Node();
last.item = item;
last.next = null;
if (isEmpty()) first = last;
else oldlast.next = last;
n++;
}
/**
* Removes and returns the item on this queue that was least recently added.
*
* @return the item on this queue that was least recently added
* @throws NoSuchElementException if this queue is empty
*/
public Item dequeue() {
if (isEmpty()) throw new NoSuchElementException("Queue underflow");
Item item = first.item;
first = first.next;
n--;
if (isEmpty()) last = null; // to avoid loitering
return item;
}
/**
* Returns a string representation of this queue.
*
* @return the sequence of items in FIFO order, separated by spaces
*/
public String toString() {
StringBuilder s = new StringBuilder();
for (Item item : this)
s.append(item + " ");
return s.toString();
}
/**
* Returns an iterator that iterates over the items in this queue in FIFO order.
*
* @return an iterator that iterates over the items in this queue in FIFO order
*/
public Iterator
- iterator() {
return new ListIterator();
}
// an iterator, doesn't implement remove() since it's optional
private class ListIterator implements Iterator
- {
private Node current = first; // node containing current item
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;
}
}
/**
* Unit tests the {@code Queue} data type.
*
* @param args the command-line arguments
*/
public static void main(String[] args) {
Queue queue = new Queue();
while (!StdIn.isEmpty()) {
String item = StdIn.readString();
if (!item.equals("-")) queue.enqueue(item);
else if (!queue.isEmpty()) StdOut.print(queue.dequeue() + " ");
}
StdOut.println("(" + queue.size() + " left on queue)");
}
}
Copyright © 2000–2017, Robert Sedgewick and Kevin Wayne. Last updated: Wed Jul 31 15:08:53 EDT 2019.
