Why would a PriorityQueue not act like a Queue?

Question

I am using the PriorityBlockingQueue with a priority field. In my test I use System#currentTime() for priorities—the same priorities are obtained by the computer being so quick that the milliseconds are the same (or more like the milliseconds on a PC has a margin of error).

When the priorities are the same the queue acts as if it’s a stack, which seems strange. Is there an alternative to make the queue act as if it’s a normal queue (that is, FIFO rather than LIFO behavior) when the priorities of the elements are the same?

Answer 1

Operations on this class make no guarantees about the ordering of elements with equal priority. If you need to enforce an ordering, you can define custom classes or comparators that use a secondary key to break ties in primary priority values.

The PriorityBlockingQueue docs themselves tell you this, and how to get around it if you need to.

Answer 2

I do not think the priority queue guarantees the order of getting equal elements. One option is to have the priority more complex - push the negative of the size of the queue when pushing the element along with its priority and compare these values for equal priority elements.