How Can I Achieve Custom Sorting in a Java PriorityQueue?

Custom Sort Order in PriorityQueue

PriorityQueue, a Java collection class, provides a default sorting mechanism that orders elements in ascending order based on their natural ordering. However, it is possible to customize this sorting behavior and sort on specific criteria.

Getting PriorityQueue to Sort as Desired

To achieve custom sorting in PriorityQueue, use the constructor overload that takes a Comparator comparator as an argument. The comparator compares elements and determines their ordering. For example, to sort Strings based on their length in ascending order:

Comparator<String> comparator = (x, y) -> x.length() - y.length();
PriorityQueue<String> queue = new PriorityQueue<>(comparator);

Offer vs. Add Methods

PriorityQueue offers two methods for adding elements: offer() and add(). While similar in functionality, they have a subtle difference. offer() returns true if the element is successfully added and false if the queue is full. In contrast, add() directly adds the element to the queue and throws an IllegalStateException if the queue is full.

In the case of PriorityQueue, which is unbounded, both methods are equivalent. However, in bounded priority queues, offer() can be used to check if the element can be added before attempting to enqueue it.

Example: String Length Sorting

Consider the following example where you want to create a PriorityQueue that sorts Strings based on their length:

public class StringLengthComparator implements Comparator<String> {
    @Override
    public int compare(String x, String y) {
        return x.length() - y.length();
    }
}

public class PriorityTest {
    public static void main(String[] args) {
        PriorityQueue<String> queue = new PriorityQueue<>(new StringLengthComparator());
        queue.offer("medium");
        queue.offer("short");
        queue.offer("very long indeed");

        while (!queue.isEmpty()) {
            System.out.println(queue.poll());
        }
    }
}

Output:

short
medium
very long indeed

This example demonstrates how to define a custom comparator and use the PriorityQueue constructor to achieve sorting based on String length.

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