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System.out.println(queue); System.out.println("queue.remove(): " + queue.remove()); System.out.println(queue); } }
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[GB, DE, FR, ES] queue.element(): GB queue.remove(): GB [DE, FR, ES] queue.remove(): DE [FR, ES] queue.add("IE"): [FR, ES, IE] queue.remove(): FR [ES, IE]
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The add, element, and remove operations are illustrated at lines 4, 9, and 10, respectively.
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By comparing the output in Example 6.1 with that of Example 5.1 on page 103, it is easy to see that the only operational difference between a queue and a stack is the access point. With a queue, it is at the front, where the oldest element the one that has been there the longest is found. With a stack, it is at the top, where the youngest element the one that arrived most recently is found. Notice that for the ArrayDeque class, the toString() method (invoked automatically by the println() method at line 8) displays the queue from front to back, and the stack from top to bottom. So in both cases, the access point is at the left end of the display. A SIMPLE Queue INTERFACE The operational requirements of a queue can be formalized by this simple Java interface: EXAMPLE 6.2 A Queue Interface
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public interface Queue<E> { public void add(E element); public E element(); public boolean isEmpty(); public E remove(); public int size(); }
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In addition to the three required queue operations, this interface also specifies an isEmpty() method and a size() method.
Compare the Queue interface shown in Example 6.2 with the JCF s Queue interface, shown in Figure 4.10 on page 89. It includes offer(), peek(), and poll() methods. In most situations, these are the same operations as the add(), element(), and remove() methods, respectively. If a limited-capacity queue is full, the add() method throws an IllegalStateException, while the offer() method merely returns false. If the queue is empty, the element() the remove() methods throw a NoSuchElementException, while the peek() and poll() methods merely returns null.
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QUEUES
AN INDEXED IMPLEMENTATION Like stacks and other linear data structures, queues can be implemented using an ordinary array. The ArrayQueue class shown in Example 6.3 is similar to the ArrayStack class shown in Example 5.3 on page 104. EXAMPLE 6.3 An ArrayQueue Implementation
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public class ArrayQueue<E> implements Queue<E> { private E[] elements; private int front; private int back; private static final int INITIAL_CAPACITY = 4; public ArrayQueue() { elements = (E[]) new Object[INITIAL_CAPACITY]; } public ArrayQueue(int capacity) { elements = (E[]) new Object[capacity]; } public void add(E element) { if (size() == elements.length - 1) { resize(); } elements[back] = element; if (back < elements.length - 1) { ++back; } else { back = 0; //wrap } } public E element() { if (size() == 0) { throw new java.util.NoSuchElementException(); } return elements[front]; } public boolean isEmpty() { return (size() == 0); } public E remove() { if (size() == 0) { throw new java.util.NoSuchElementException(); } E element = elements[front]; elements[front] = null; ++front; if (front == back) { // queue is empty front = back = 0; }
QUEUES
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if (front == elements.length) { front = 0; } return element; }
// wrap
public int size() { if (front <= back) { return back - front; } else { return back - front + elements.length; } } private void resize() { int size = size(); int len = elements.length; assert size == len; Object[] a = new Object[2*len]; System.arraycopy(elements, front, a, 0, len - front); System.arraycopy(elements, 0, a, len - front, back); elements = (E[])a; front = 0; back = size; } }
Instead of storing the size counter, this implementation stores front and back indexes into the array. The front element of the queue is always at elements[front], and the back element of the queue is always at elements[back-1] (except when back = 0). The front index is advanced each time an element is removed from the queue (at line 44), and the back index is advanced each time an element is added (at line 21). In both cases, when the index reaches the end of the array, it is advanced to 0. This wraps the queue around the end of the array, like a ring, allowing array elements to be reused.
AN INDEXED IMPLEMENTATION We can use a doubly linked list to implement the Queue interface the same way we implemented the Stack interface in Example 5.4 on page 106. EXAMPLE 6.4 A LinkedQueue Class
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public class LinkedQueue<E> implements Queue<E> { private Node<E> head = new Node<E>(); // dummy node private int size; public void add(E element) { head.prev = head.prev.next = new Node<E>(element, head.prev, head); ++size; } public E element() { if (size == 0) { throw new java.util.EmptyStackException(); } return head.next.element; // front of queue }
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