How do I use a PriorityQueue?

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

Also, is there a difference between the offer and add methods?

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 offer have 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 PriorityQueue which 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

Java 8 solution

We can use lambda expression or method reference introduced in Java 8. In case we have some String values stored in the Priority Queue (having capacity 5) we can provide inline comparator (based on length of String) :

Using lambda expression

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

Using Method reference

PriorityQueue<String> pq=
                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());
       // or pq = new PriorityQueue<String>(5, Comparator.comparing(String::length));
        pq.add("Apple");
        pq.add("PineApple");
        pq.add("Custard Apple");
        while (pq.size() != 0)
        {
            System.out.println(pq.remove());
        }
    }

This will print:

Apple
PineApple
Custard Apple

To reverse the order (to change it to max-priority queue) simply change the order in inline comparator or use reversed as:

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

We can also use Collections.reverseOrder:

PriorityQueue<Integer> pqInt = new PriorityQueue<>(10, Collections.reverseOrder());
PriorityQueue<String> pq = new PriorityQueue<String>(5, 
                Collections.reverseOrder(Comparator.comparing(String::length))

So we can see that Collections.reverseOrder is overloaded to take comparator which can be useful for custom objects. The reversed actually uses Collections.reverseOrder:

default Comparator<T> reversed() {
    return Collections.reverseOrder(this);
}

offer() vs add()

As per the doc

The offer method inserts an element if possible, otherwise returning
false. This differs from the Collection.add method, which can fail to
add an element only by throwing an unchecked exception. The offer
method is designed for use when failure is a normal, rather than
exceptional occurrence, for example, in fixed-capacity (or "bounded")
queues.

When using a capacity-restricted queue, offer() is generally preferable to add(), which can fail to insert an element only by throwing an exception. And PriorityQueue is an unbounded priority queue based on a priority heap.

Just pass appropriate Comparator to the constructor:

PriorityQueue(int initialCapacity, Comparator<? super E> comparator)

The only difference between offer and add is the interface they belong to. offer belongs to Queue<E>, whereas add is originally seen in Collection<E> interface. Apart from that both methods do exactly the same thing - insert the specified element into priority queue.

from Queue API:

The offer method inserts an element if possible, otherwise returning false. This differs from the Collection.add method, which can fail to add an element only by throwing an unchecked exception. The offer method is designed for use when failure is a normal, rather than exceptional occurrence, for example, in fixed-capacity (or "bounded") queues.