Re: NPE in PriorityQueue.poll()

From:

"Daniel Pitts" <googlegroupie@coloraura.com>

Newsgroups:

comp.lang.java.programmer

Date:

16 Nov 2006 11:41:59 -0800

Message-ID:

<1163706119.474354.82190@f16g2000cwb.googlegroups.com>

Twisted wrote:

This is a strange one. App just recovered gracefully from:

java.lang.NullPointerException
    at java.util.PriorityQueue.siftDownComparable(PriorityQueue.java:627)
    at java.util.PriorityQueue.siftDown(PriorityQueue.java:614)
    at java.util.PriorityQueue.poll(PriorityQueue.java:523)
    at com.sourceforge.sphaera.SThread.run(SThread.java:158)

The line of my own code that's involved is basically

Foo bar = baz.poll();

with baz an instance of PriorityQueue<Foo> and definitely not itself
null (and besides, the stack trace would have consisted of only the
last line if it were).

Looks like a library bug. JDK 1.6.0 -server -incgc -Xmx256 under WinXP
in case it matters, with the 1.6.0 standard library (including
PriorityQueue implementation).

Is the poll in a synchronized block like your add is?
Foo bar;
synchronized (queue) {
bar = baz.poll()
}

You need to make sure all access to baz is syncronized if it is access
in more than one thread.

BTW, from the Javadoc (in 1.5.0):
* <p> <strong>Note that this implementation is not
synchronized.</strong>
* Multiple threads should not access a <tt>PriorityQueue</tt>
* instance concurrently if any of the threads modifies the list
* structurally. Instead, use the thread-safe {@link
* java.util.concurrent.PriorityBlockingQueue} class.