Java Queue interface orders the element in FIFO(First In First Out) manner. In FIFO, first element is removed first and last element is removed at last.

Table of Contents

Queue Interface declaration

1. public interface Queue<E> extends Collection<E>

Methods of Java Queue Interface

Method	Description
boolean add(object)	It is used to insert the specified element into this queue and return true upon success.
boolean offer(object)	It is used to insert the specified element into this queue.
Object remove()	It is used to retrieves and removes the head of this queue.
Object poll()	It is used to retrieves and removes the head of this queue, or returns null if this queue is empty.
Object element()	It is used to retrieves, but does not remove, the head of this queue.
Object peek()	It is used to retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.

PriorityQueue class

The PriorityQueue class provides the facility of using queue. But it does not orders the elements in FIFO manner. It inherits AbstractQueue class.

PriorityQueue class declaration

Let’s see the declaration for java.util.PriorityQueue class.
1. public class PriorityQueue<E> extends AbstractQueue<E> implements Serializable

Java PriorityQueue Example

1. import java.util.*; 
2. class TestCollection12{ 
3. public static void main(String args[]){ 
4. PriorityQueue<String> queue=new PriorityQueue<String>(); 
5. queue.add("Amit"); 
6. queue.add("Vijay"); 
7. queue.add("Karan"); 
8. queue.add("Jai"); 
9. queue.add("Rahul"); 
10. System.out.println("head:"+queue.element()); 
11. System.out.println("head:"+queue.peek()); 
12. System.out.println("iterating the queue elements:"); 
13. Iterator itr=queue.iterator(); 
14. while(itr.hasNext()){ 
15. System.out.println(itr.next()); 
16. } 
17. queue.remove(); 
18. queue.poll(); 
19. System.out.println("after removing two elements:"); 
20. Iterator<String> itr2=queue.iterator(); 
21. while(itr2.hasNext()){ 
22. System.out.println(itr2.next()); 
23. } 
24. } 
25. }

Test it Now
Output:head:Amit
head:Amit
iterating the queue elements:
Amit
Jai
Karan
Vijay
Rahul
after removing two elements:
Karan
Rahul
Vijay

Java PriorityQueue Example: Book

Let’s see a PriorityQueue example where we are adding books to queue and printing all the books. The elements in PriorityQueue must be of Comparable type. String and Wrapper classes are Comparable by default. To add user-defined objects in PriorityQueue, you need to implement Comparable interface.

1. import java.util.*; 
2. class Book implements Comparable<Book>{ 
3. int id; 
4. String name,author,publisher; 
5. int quantity; 
6. public Book(int id, String name, String author, String publisher, int quantity) { 
7. this.id = id; 
8. this.name = name; 
9. this.author = author; 
10. this.publisher = publisher; 
11. this.quantity = quantity; 
12. } 
13. public int compareTo(Book b) { 
14. if(id>b.id){ 
15. return 1; 
16. }else if(id<b.id){ 
17. return -1; 
18. }else{ 
19. return 0; 
20. } 
21. } 
22. } 
23. public class LinkedListExample { 
24. public static void main(String[] args) { 
25. Queue<Book> queue=new PriorityQueue<Book>(); 
26. //Creating Books 
27. Book b1=new Book(121,"Let us C","Yashwant Kanetkar","BPB",8); 
28. Book b2=new Book(233,"Operating System","Galvin","Wiley",6); 
29. Book b3=new Book(101,"Data Communications & Networking","Forouzan","Mc Graw Hill",4); 
30. //Adding Books to the queue 
31. queue.add(b1); 
32. queue.add(b2); 
33. queue.add(b3); 
34. System.out.println("Traversing the queue elements:"); 
35. //Traversing queue elements 
36. for(Book b:queue){ 
37. System.out.println(b.id+" "+b.name+" "+b.author+" "+b.publisher+" "+b.quantity); 
38. } 
39. queue.remove(); 
40. System.out.println("After removing one book record:"); 
41. for(Book b:queue){ 
42. System.out.println(b.id+" "+b.name+" "+b.author+" "+b.publisher+" "+b.quantity); 
43. } 
44. } 
45. }

Output:
Traversing the queue elements:
101 Data Communications & Networking Forouzan Mc Graw Hill 4
233 Operating System Galvin Wiley 6
121 Let us C Yashwant Kanetkar BPB 8
After removing one book record:
121 Let us C Yashwant Kanetkar BPB 8
233 Operating System Galvin Wiley 6

Java Deque Interface

Java Deque Interface is a linear collection that supports element insertion and removal at both ends. Deque is an acronym for “double ended queue”.

Deque Interface declaration

1. public interface Deque<E> extends Queue<E>

Methods of Java Deque Interface

Method	Description
boolean add(object)	It is used to insert the specified element into this deque and return true upon success.
boolean offer(object)	It is used to insert the specified element into this deque.
Object remove()	It is used to retrieves and removes the head of this deque.
Object poll()	It is used to retrieves and removes the head of this deque, or returns null if this deque is empty.
Object element()	It is used to retrieves, but does not remove, the head of this deque.
Object peek()	It is used to retrieves, but does not remove, the head of this deque, or returns null if this deque is empty.

ArrayDeque class

The ArrayDeque class provides the facility of using deque and resizable-array. It inherits AbstractCollection class and implements the Deque interface.
The important points about ArrayDeque class are:
• Unlike Queue, we can add or remove elements from both sides.
• Null elements are not allowed in the ArrayDeque.
• ArrayDeque is not thread safe, in the absence of external synchronization.
• ArrayDeque has no capacity restrictions.
• ArrayDeque is faster than LinkedList and Stack.

ArrayDeque Hierarchy

The hierarchy of ArrayDeque class is given in the figure displayed at the right side of the page.

ArrayDeque class declaration

Let’s see the declaration for java.util.ArrayDeque class.
1. public class ArrayDeque<E> extends AbstractCollection<E> implements Deque<E>, Cloneable, Serializable

Java ArrayDeque Example

1. import java.util.*; 
2. public class ArrayDequeExample { 
3. public static void main(String[] args) { 
4. //Creating Deque and adding elements 
5. Deque<String> deque = new ArrayDeque<String>(); 
6. deque.add("Ravi"); 
7. deque.add("Vijay"); 
8. deque.add("Ajay"); 
9. //Traversing elements 
10. for (String str : deque) { 
11. System.out.println(str); 
12. } 
13. } 
14. }

Output:
Ravi
Vijay
Ajay

Java ArrayDeque Example: offerFirst() and pollLast()

1. import java.util.*; 
2. public class DequeExample { 
3. public static void main(String[] args) { 
4. Deque<String> deque=new ArrayDeque<String>(); 
5. deque.offer("arvind"); 
6. deque.offer("vimal"); 
7. deque.add("mukul"); 
8. deque.offerFirst("jai"); 
9. System.out.println("After offerFirst Traversal..."); 
10. for(String s:deque){ 
11. System.out.println(s); 
12. } 
13. //deque.poll(); 
14. //deque.pollFirst();//it is same as poll() 
15. deque.pollLast(); 
16. System.out.println("After pollLast() Traversal..."); 
17. for(String s:deque){ 
18. System.out.println(s); 
19. } 
20. } 
21. }

Output:
After offerFirst Traversal…
jai
arvind
vimal
mukul
After pollLast() Traversal…
jai
arvind
vimal

Java ArrayDeque Example: Book

1. import java.util.*; 
2. class Book { 
3. int id; 
4. String name,author,publisher; 
5. int quantity; 
6. public Book(int id, String name, String author, String publisher, int quantity) { 
7. this.id = id; 
8. this.name = name; 
9. this.author = author; 
10. this.publisher = publisher; 
11. this.quantity = quantity; 
12. } 
13. } 
14. public class ArrayDequeExample { 
15. public static void main(String[] args) { 
16. Deque<Book> set=new ArrayDeque<Book>(); 
17. //Creating Books 
18. Book b1=new Book(101,"Let us C","Yashwant Kanetkar","BPB",8); 
19. Book b2=new Book(102,"Data Communications & Networking","Forouzan","Mc Graw Hill",4); 
20. Book b3=new Book(103,"Operating System","Galvin","Wiley",6); 
21. //Adding Books to Deque 
22. set.add(b1); 
23. set.add(b2); 
24. set.add(b3); 
25. //Traversing ArrayDeque 
26. for(Book b:set){ 
27. System.out.println(b.id+" "+b.name+" "+b.author+" "+b.publisher+" "+b.quantity); 
28. } 
29. } 
30. }

Output:
101 Let us C Yashwant Kanetkar BPB 8
102 Data Communications & Networking Forouzan Mc Graw Hill 4
103 Operating System Galvin Wiley 6

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