Queue Interface

The Queue interface is present in java.util package and extends the Collection interface is used to hold the elements about to be processed in FIFO(First In First Out) order. It is an ordered list of objects with its use limited to inserting elements at the end of the list and deleting elements from the start of the list, (i.e.), it follows the FIFO or the First-In-First-Out principle.

Declaration #

public interface Queue extends Collection  

Characteristics of Queue #

• The Queue is used to insert elements at the end of the queue and removes from the beginning of the queue. It follows FIFO concept.

• The Java Queue supports all methods of Collection interface including insertion, deletion, etc.

• LinkedList, ArrayBlockingQueue and PriorityQueue are the most frequently used implementations.

• If any null operation is performed on BlockingQueues, NullPointerException is thrown.

• The Queues which are available in java.util package are Unbounded Queues.

• The Queues which are available in java.util.concurrent package are the Bounded Queues.

• All Queues except the Deques supports insertion and removal at the tail and head of the queue respectively. The Deques support element insertion and removal at both ends.

Methods #

Operations on Queue #

Adding Elements #

In order to add an element in a queue, we can use the add() method. The insertion order is not retained in the PriorityQueue. The elements are stored based on the priority order which is ascending by default.

// Java program to add elements
// to a Queue

import java.util.*;

public class TFT {

	public static void main(String args[])
	{
		Queue<String> pq = new PriorityQueue<>();

		pq.add("Tit");
		pq.add("For");
		pq.add("Tat");

		System.out.println(pq);
	}
}

Output

[For, Tit, Tat]

Removing Elements #

In order to remove an element from a queue, we can use the remove() method. If there are multiple such objects, then the first occurrence of the object is removed. Apart from that, poll() method is also used to remove the head and return it.

// Java program to remove elements
// from a Queue

import java.util.*;

public class TFT {

	public static void main(String args[])
	{
		Queue<String> pq = new PriorityQueue<>();

		pq.add("Tit");
		pq.add("For");
		pq.add("Tat");

		System.out.println("Initial Queue " + pq);

		pq.remove("Tit");

		System.out.println("After Remove " + pq);

		System.out.println("Poll Method " + pq.poll());

		System.out.println("Final Queue " + pq);
	}
}

Output

Initial Queue [For, Tit, Tat]
After Remove [For, Tat]
Poll Method For
Final Queue [Tat]

Iterating the Queue #

There are multiple ways to iterate through the Queue. The most famous way is converting the queue to the array and traversing using the for loop. However, the queue also has an inbuilt iterator which can be used to iterate through the queue.

// Java program to iterate elements
// to a Queue

import java.util.*;

public class TFT {

	public static void main(String args[])
	{
		Queue<String> pq = new PriorityQueue<>();

		pq.add("Tit");
		pq.add("For");
		pq.add("Tat");

		Iterator iterator = pq.iterator();

		while (iterator.hasNext()) {
			System.out.print(iterator.next() + " ");
		}
	}
}

Output

For Tit Tat