How Can I Implement a Custom Sort Order in Java's PriorityQueue?

Custom Sort Order in Java's PriorityQueue

Introduction

PriorityQueue, a fundamental data structure in Java, provides an efficient way to manage elements ordered according to their priority. However, the default sorting mechanism may not always align with the desired sort order.

Custom Sort Order

To specify a custom sorting order for a PriorityQueue, use the constructor overload that takes a Comparator comparator. Pass in a comparator that compares elements based on the desired sort order.

Comparator Implementation

The comparator should implement the compare method to specify the comparison logic. The method takes two elements and returns an integer indicating the relative order:

• -1 if the first element should come before the second
• 0 if the elements are equal
• 1 if the first element should come after the second

Example

Consider an example where we want to sort a PriorityQueue of strings by string length in ascending order. Here's a sample comparator and PriorityQueue usage:

import java.util.*;

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

...
Comparator<String> comparator = new StringLengthComparator();
PriorityQueue<String> queue = new PriorityQueue<>(10, comparator);
queue.add("short");
queue.add("very long indeed");
queue.add("medium");

add vs. offer

Both add and offer are method implementations in the PriorityQueue interface. While they can potentially behave differently in general, there is no practical difference in the context of PriorityQueue, since it is unbounded.

Output

The PriorityQueue will provide elements in the specified sorted order. In the above example, the output would be:

short
medium
very long indeed

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