Using a Queue in Java

problem

You wish to use the Queue data structure in Java. It is a FIFO – First In First Out data structure meaning the first item that is added is firstly retrieved. Analogous to a queue of people standing to get a ticket at the cinema.

SOLUTION - LinkedList

Java comes with the Queue but it’s an abstract class, meaning we cannot instantiate it. In this post we will see how to use two of its implementations. The LinkedList in the example below:

				
					 Queue<String> codes = new LinkedList<>();

codes.add("abc");
codes.add("xyz");
codes.add("klm");

LinkedList is used to instantiate our queue. The queue is a Generic class and in this case we specified it’s holding Strings that represent some hypothetical codes.

Simply adding items by using the add() method. Note: duplicated values are allowed.

running

Let’s simply print the queue using the console output:

QueueExample.java

				
					package com.programmerabroad;

import java.util.LinkedList;
import java.util.Queue;

public class QueueExample {

    public static void main(String[] args) {

        Queue<String> codes = new LinkedList<>();

        codes.add("abc");
        codes.add("xyz");
        codes.add("klm");

        System.out.println(codes);
    }
}

output

				
					[abc, xyz, klm]

When we use an object as an argument to the println() method, automatically invokes the toString() method. In this case is quite helpful as it prints all the contents without us needing to use an iteration.

SOLUTION - PriorityQueue

Another one way of instantiating a Queue is by using a PriorityQueue. For example:

				
					Queue<String> ids = new PriorityQueue<>();

    ids.add("1");
    ids.add("2");
    ids.add("3");
    ids.add("3");

    System.out.println(ids);

output

				
					[1,2,3,3]

As you can see, duplicates are allowed in this implementation too.

retrieving items

To retrieve an item from the queue we can use the poll() method. This method is returning the head of the queue and removes it too. If the queue is empty, it will return null.

For example:

				
					System.out.println(ids.poll()); //retrieve head and remove it

Alternative

				
					String head = ids.peek(); // retrieve head but doesn't remove it

removing items

				
					String head = ids.remove(); //throws exception if empty

The method also can be used for retrieving items as it’s returning the head. It can be enclosed in a try-catch statement for error handling.

conclusion

In this post we saw how to use a Queue and instantiate it by using either a LinkedList or PriorityQueue. It is a First In First Out data structure meaning by design what we add first is then retrieved first. A helpful feature for problem solving. In contrast of the LIFO – Last In First Out data structures like Stack.

references

https://docs.oracle.com/javase/8/docs/api/java/util/Queue.html

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Ellion

Professional IT consultant, writer, programmer enthusiast interested in all sorts of coding.
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