Friday, November 07, 2008

Priorities and signs

This was almost going to be titled 'signs are hard'. I hat a problem that a simple timer queue implementation did not quite work. Problem: Some timers seemed to vanish from the java.util.PriorityQueue, or were in the wrong order. I tried, for lack of other ideas to change the sign of the compare method, to see if I got that wrong. Letting the handler thread wait for and pick the latest timer is not a good idea. But alas, that wasn't it, and there goes the title.

Google ("java PriorityQueue remove"): Somewhat unexpectedly, the first result is not the documentation of the queue but rather exactly the report for the bug that is biting me: remove does remove the first element it finds that compares equal to its argument, not that exact object: Voila: wrong element removed; that timer never fires, I'm not getting #ticks.

Good. So now I know what's wrong and can finally implement a strategy to do fewer removes in the first place. Many timers here serve as a timeout and leave the queue not at the front but by remove. And the timeout is constant. So if a timer is set to a later time I can just leave it in the queue and reposition it when it finally appears up front.

Also there is a decision to be made whether I leave the workaround in. It's fixed somewhere in 1.6 (b51?), and I can't rely on the JVM always being that new.

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