The Queue Interface

Question 1

The Queue Interface

Answer 1

A Queue is a collection for holding elements prior to processing. Besides basic Collection operations, queues provide additional insertion, removal, and inspection operations. The Queue interface follows.

public interface Queue extends Collection {
    E element();
    boolean offer(E e);
    E peek();
    E poll();
    E remove();
}

Question 2

How many forms each Queue method exists?

Answer 2

Each Queue method exists in two forms: (1) one throws an exception if the operation fails, and (2) the other returns a special value if the operation fails (either null or false, depending on the operation). The regular structure of the interface is illustrated in the following table.

Queue Interface Structure

Type of Operation supported
Insert
Remove
Examine

Throws exception
add(e)
remove()
element()

Returns special value,
offer(e)
poll()
peek()

Question 3

Queue Ordering

Answer 3

Queues typically, but not necessarily, order elements in a FIFO (first-in-first-out) manner. Among the exceptions are priority queues, which order elements according to their values — see the Object Ordering section for details). Whatever ordering is used, the head of the queue is the element that would be removed by a call to remove or poll. In a FIFO queue, all new elements are inserted at the tail of the queue. Other kinds of queues may use different placement rules. Every Queue implementation must specify its ordering properties.

Question 4

It is possible for a Queue implementation to restrict the number of elements that it holds?

Answer 4

such queues are known as bounded. Some Queue implementations in java.util.concurrent are bounded, but the implementations in java.util are not.

Question 5

Queue Diff between Add & offer

Answer 5

The add method, which Queue inherits from Collection, inserts an element unless it would violate the queue’s capacity restrictions, in which case it throws IllegalStateException. The offer method, which is intended solely for use on bounded queues, differs from add only in that it indicates failure to insert an element by returning false.

Question 6

Queue Diff between remove and poll

Answer 6

The remove and poll methods both remove and return the head of the queue. Exactly which element gets removed is a function of the queue’s ordering policy. The remove and poll methods differ in their behavior only when the queue is empty. Under these circumstances, remove throws NoSuchElementException, while poll returns null.

Question 7

Queue Diff between element and peek

Answer 7

The element and peek methods return, but do not remove, the head of the queue. They differ from one another in precisely the same fashion as remove and poll: If the queue is empty, element throws NoSuchElementException, while peek returns null.

Question 8

Does Queue Allow Insertion of null elements?

Answer 8

Queue implementations generally do not allow insertion of null elements. The LinkedList implementation, which was retrofitted to implement Queue, is an exception. For historical reasons, it permits null elements, but you should refrain from taking advantage of this, because null is used as a special return value by the poll and peek methods.

Question 9

Queue equal & hashcode method.

Answer 9

Queue implementations generally do not define element-based versions of the equals and hashCode methods but instead inherit the identity-based versions from Object.

Question 10

BlockingQueue

Answer 10

The Queue interface does not define the blocking queue methods, which are common in concurrent programming. These methods, which wait for elements to appear or for space to become available, are defined in the interface java.util.concurrent.BlockingQueue, which extends Queue.

Question 11

Queue example with LL

Answer 11

In the following example program, a queue is used to implement a countdown timer. The queue is preloaded with all the integer values from a number specified on the command line to zero, in descending order. Then, the values are removed from the queue and printed at one-second intervals. The program is artificial in that it would be more natural to do the same thing without using a queue, but it illustrates the use of a queue to store elements prior to subsequent processing.

import java.util.*;

public class Countdown {
    public static void main(String[] args) throws InterruptedException {
        int time = Integer.parseInt(args[0]);
        Queue queue = new LinkedList();

    for (int i = time; i >= 0; i--)
        queue.add(i);

        while (!queue.isEmpty()) {
            System.out.println(queue.remove());
            Thread.sleep(1000);
        }
    }
}

Question 12

Queue example with PriorityQueue

Answer 12

In the following example, a priority queue is used to sort a collection of elements. Again this program is artificial in that there is no reason to use it in favor of the sort method provided in Collections, but it illustrates the behavior of priority queues.

static  List heapSort(Collection c) {
    Queue queue = new PriorityQueue(c);
    List result = new ArrayList();

while (!queue.isEmpty())
    result.add(queue.remove());

return result; }