How do I use a Priority Queue

Question

How do I get a PriorityQueue to sort on what I want it to sort on?

Answer 1

Using lambda expression

PriorityQueue<String> priorityQueue=
                    new PriorityQueue<String>(5,(a,b) -> a.length() - b.length());

Using Method reference

PriorityQueue<String> priorityQueue =
                new PriorityQueue<String>(5, Comparator.comparing(String::length));

Then we can use any of them as:

public static void main(String[] args) {
        PriorityQueue<String> pq=
                new PriorityQueue<String>(5, (a,b) -> a.length() - b.length());
        

priorityQueue = new PriorityQueue<String>(5, Comparator.comparing(String::length));
       

priorityQueue.add("Apple");

priorityQueue.add("PineApple");

priorityQueue.add("Custard Apple");
        while (priorityQueue.size() != 0)
        {
            System.out.println(priorityQueue.remove());
        }
    }

Answer 2

Use the constructor overload which takes a Comparator<? super E> comparator and pass in a comparator which compares in the appropriate way for your sort order. If you give an example of how you want to sort, we can provide some sample code to implement the comparator if you're not sure. (It's pretty straightforward though.)

As has been said elsewhere: offer and add are just different interface method implementations. In the JDK source I've got, add calls offer. Although add and offerhave potentially different behaviour in general due to the ability for offer to indicate that the value can't be added due to size limitations, this difference is irrelevant in PriorityQueuewhich is unbounded.

Here's an example of a priority queue sorting by string length:

// Test.java
import java.util.Comparator;
import java.util.PriorityQueue;

public class Test
{
    public static void main(String[] args)
    {
        Comparator<String> comparator = new StringLengthComparator();
        PriorityQueue<String> queue = 
            new PriorityQueue<String>(10, comparator);
        queue.add("short");
        queue.add("very long indeed");
        queue.add("medium");
        while (queue.size() != 0)
        {
            System.out.println(queue.remove());
        }
    }
}

// StringLengthComparator.java
import java.util.Comparator;

public class StringLengthComparator implements Comparator<String>
{
    @Override
    public int compare(String x, String y)
    {
        // Assume neither string is null. Real code should
        // probably be more robust
        // You could also just return x.length() - y.length(),
        // which would be more efficient.
        if (x.length() < y.length())
        {
            return -1;
        }
        if (x.length() > y.length())
        {
            return 1;
        }
        return 0;
    }
}

Here is the output:

short

medium

very long indeed