RFR: JDK-8297605 DelayQueue javadoc is confusing

[Martin Buchholz]

On Thu, 23 Feb 2023 15:36:48 GMT, Viktor Klang <duke at openjdk.org> wrote:

> Clarifies the distinction between expiration of the head of DelayQueue and how it relates to `poll`, `take`, and `peek`. See discussion on https://bugs.openjdk.org/browse/JDK-8297605
> 
> @DougLea If possible, please weigh in on whether this is in line with your thoughts on the matter.

Here's my attempt:


 * An unbounded {@linkplain BlockingQueue blocking queue} of {@link Delayed}
 * elements, in which an element generally becomes eligible for removal when its
 * delay has expired.
 *
 * <p>An element is considered <em>expired</em> when its {@code
 * getDelay(TimeUnit.NANOSECONDS)} method would return a value less than or
 * equal to zero.
 *
 * <p>An element is considered the <em>head</em> of the queue if it is the
 * element with the earliest expiration time, whether in the past or the future,
 * if there is such an element.
 *
 * <p>An element is considered the <em>expired head</em> of the queue if it is
 * the <em>expired</em> element with the earliest expiration time in the
 * past, if there is such an element.
 * The <em>expired head</em>, when present, is also the <em>head</em>.
 *
 * <p>While this class implements the {@code BlockingQueue} interface, it
 * intentionally violates the general contract of {@code BlockingQueue}, in that
 * the following methods disregard the presence of unexpired elements and only
 * ever remove the <em>expired head</em>:
 *
 * <ul>
 * <li> {@link #poll()}
 * <li> {@link #poll(long,TimeUnit)}
 * <li> {@link #take()}
 * <li> {@link #remove()}
 * </ul>
 *
 * <p>All other methods operate on both expired and unexpired elements.  For
 * example, the {@code size} method returns the count of all elements.  Method
 * {@link peek()} may return the non-null <em>head</em> even when {@code
 * take()} would block waiting for an expired element.
 *
 * <p>This queue does not permit null elements.

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PR: https://git.openjdk.org/jdk/pull/12729