Question

java.util.PriorityQueue allows a Comparator to be passed at construction time. When inserting elements, they are ordered according to the priority specified by the comparator.

What happens when the priority of an element changes after it has been inserted? When does the PriorityQueue reorder elements? Is it possible to poll an element that does not actually have minimal priority?

Are there good implementations of a priority queue which allow efficient priority updates?

Answer 1

You should remove the element, change it, and re-insert, since ordering occurs when it is inserted. Although it involves several steps, it is efficient might be good enough. (I just noticed the comment about removal being O(n).)

One problem is that it will also re-order when you remove the element, which is redundant if you are just going to re-insert it a moment later. If you implement your own priority queue from scratch, you could have an update() that skips this step, but extending Java’s class won’t work because you are still limited to the remove() and add() provided by the base.