Queue interface in collections framework

13 March 2021 - 5 mins read time
Tags: Core Java Collections

java 1.5 version enhancements(Queue interface)

It is the child interface of Collection.
If we want to represent a group of individual objects prior to processing, then we should go for queue.
Usually, Queue follows first in first out order but based on our requirement we can implement our own priority order also(priority queue).
From java 1.5 version onwards LinkedList class also implements Queue interface.
LinkedList based implementation of Queue always follows first in first out order.

Queue interface specific methods

boolean offer(Object o); // --> Same as enqueue
// For seeing the first element.
Object peek(); // to return head(first) element of the queue. If queue is empty, this method returns null.
Object element(); // to return head(first) element of the queue. If queue is empty, this method raises RE: NoSuchElementException.
// For dequeue purposes
Object poll(); // to remove and return head element of the queue. If queue is empty then this method returns null.
Object remove(); // to remove and return head element of the queue. If queue is empty then this method raises RE: NoSuchElementException.

Priority Queue:

If we want to represent a group of individual objects prior to processing according to some priority then, we should go for PriorityQueue.
The priority can be either default natural sorting order or customized sorting order defined by Comparator.
Insertion order is not preserved and it is based on some priority.
Duplicates are allowed.
If we are depending on default natural order, the objects should be homogeneous and comparable compulsorily otherwise we will get ClassCastException.
If we are defining our own sorting logic by using Comparator then, objects need not be homogenous and comparable.
null is not allowed even as the head element also.

Constructors

1.

 PriorityQueue q = new PriorityQueue();

The above initialization creates an empty PriorityQueue object with default initial capacity 11. and all objects will be inserted according to default natural sorting order.

2.

PriorityQueue q = new PriorityQueue(int initialCapacity);

The above initialization creates an empty PriorityQueue object with specified initial capacity.

3.

PriorityQueue q = new PriorityQueue(int initialCapacity, Comparator c);

4.

PriorityQueue q = new PriorityQueue(Collection c);

The above initialization creates an equivalent PriorityQueue object for the given Collection.

5.

PriorityQueue q = new PriorityQueue(SortedSet s);

Implementation demo 1

import java.util.PriorityQueue;
public class PriorityQueueDemo {
    public static void main(String[] args){
        PriorityQueue priorityQueue = new PriorityQueue();
//        System.out.println(priorityQueue.peek());
//        System.out.println(priorityQueue.element());
        for(int i = 0; i <= 10; i++)priorityQueue.offer(i);
        System.out.println(priorityQueue);
        System.out.println(priorityQueue.poll()); // 0
        priorityQueue.offer(1);
        System.out.println(priorityQueue); // [1, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
    }
}

Important Note: Some platforms won’t provide proper support for thread priorities and PriorityQueue classes. That is the reason we may get non-sorted outputs.

Implementation demo 2

import java.util.Comparator;
import java.util.PriorityQueue;
public class PriorityQueueDemo1 {
    public static void main(String[] args){
        PriorityQueue priorityQueue = new PriorityQueue(6, new MyComparator8());
        priorityQueue.offer("L");
        priorityQueue.offer("B");
        priorityQueue.offer("Z");
        priorityQueue.offer("A");
        System.out.println(priorityQueue); // [Z, L, B, A]

    }
}
class MyComparator8 implements Comparator{
    @Override
    public int compare(Object o1, Object o2) {
        String s1 = o1.toString();
        String s2 = o2.toString();
        return s2.compareTo(s1);
    }
}

There is a support for double ended queues in java which is provided by Dequeue interface. For further reading please visit this link.