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 <div class="subTitle">org.apache.river.concurrent</div>
 <h2 title="Enum Ref" class="title">Enum Ref</h2>
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 <dt>All Implemented Interfaces:</dt>
 <dd><a href="http://docs.oracle.com/javase/6/docs/api/java/io/Serializable.html?is-external=true" title="class or interface in java.io">Serializable</a>, <a href="http://docs.oracle.com/javase/6/docs/api/java/lang/Comparable.html?is-external=true" title="class or interface in java.lang">Comparable</a>&lt;<a href="../../../../org/apache/river/concurrent/Ref.html" title="enum in org.apache.river.concurrent">Ref</a>&gt;</dd>
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 <pre>public enum <span class="typeNameLabel">Ref</span>
 extends <a href="http://docs.oracle.com/javase/6/docs/api/java/lang/Enum.html?is-external=true" title="class or interface in java.lang">Enum</a>&lt;<a href="../../../../org/apache/river/concurrent/Ref.html" title="enum in org.apache.river.concurrent">Ref</a>&gt;</pre>
 <div class="block"><p>
  Ref enum represents types of references available for use in java
  collection framework implementations.
  </p><p>
  Only use STRONG, WEAK_IDENTITY and TIME based references as keys
  in Maps, use of other reference types should be discouraged
  due to unpredictable results, when for example, an equal WEAK
  Reference Key disappears due to garbage collection, or SOFT reference keys
  aren't garbage collected as expected.
  </p><p>
  Map implementations delete their key -> value mapping when either the
  key or value References become unreachable. ConcurrentMap's will retry until
  putIfAbsent is successful when an existing Referrer key is cleared.
  </p><p>
  Only use STRONG and TIME based references in Queue's, other types break
  Queue's contract, null has special meaning, a cleared reference returns
  null.
  </p><p>
  Object.toString() is overridden in reference implementations to call
  toString() on the referent, if still reachable, otherwise the reference
  calls the superclass toString() method, where the superclass is a java
  Reference subclass.  Consideration is being given to returning a null string
  "" instead, if you feel strongly about this, please contact the author.
  </p><p>
  Phantom references are not used, they are designed to replace
  <a href="http://docs.oracle.com/javase/6/docs/api/java/lang/Object.html?is-external=true#finalize--" title="class or interface in java.lang"><code>Object.finalize()</code></a> and remain unreachable, but not garbage collected until
  the <a href="http://docs.oracle.com/javase/6/docs/api/java/lang/ref/PhantomReference.html?is-external=true" title="class or interface in java.lang.ref"><code>PhantomReference</code></a> also becomes unreachable, get() always returns
  null.
  </p><p>
  TIME and SOFT and SOFT_IDENTITY references update their access timestamp,
  when Referrer.get(), Referrer.equals() or Referrer.toString() is called.
  SOFT and SOFT_IDENTITY do so lazily and are not guaranteed to
  succeed in updating the access time.  SOFT references also update their
  access timestamp when either Referrer.hashCode() or Comparable.compareTo()
  is called.
  </p><p>
  For sets and map keys that require traversal using a Comparator or
  Comparable, access times will be updated during each traversal. In
  these circumstances, SOFT and SOFT_IDENTITY references are typically not enqueued and
  do not behave as expected.
  </p><p>
  SOFT references are only suited for use in lists or as values in maps and not
  suitable for keys in hash or tree maps.
  </p><p>
  TIME references only update their access timestamp during traversal when
  a Comparator or Comparable returns zero (a successful match), or when equals is true.
  TIME references are suitable for use as keys in tree and hash based maps
  as well as sets and tasks in Queues.  In fact TIME is the only referrer
  suitable for use in Queue's and their subtypes.  Tasks in a Queue, if
  timed out, are first cancelled, then removed, they are not cleared as
  doing so could cause a null poll() return when a queue is not empty,
  which would violate the contract for Queue.
  </p><p>
  SOFT_IDENTITY references are suitable for use as
  keys in hash tables, hash maps and hash sets, since hashCode() does not update
  the access timestamp, while equals() does.  SOFT_IDENTITY references are not
  recommended for use in tree maps, tree sets or queues.
  </p></div>
 <dl>
 <dt><span class="simpleTagLabel">Author:</span></dt>
 <dd>Peter Firmstone.</dd>
 <dt><span class="seeLabel">See Also:</span></dt>
 <dd><a href="http://docs.oracle.com/javase/6/docs/api/java/lang/ref/Reference.html?is-external=true" title="class or interface in java.lang.ref"><code>Reference</code></a>,
 <a href="http://docs.oracle.com/javase/6/docs/api/java/lang/ref/WeakReference.html?is-external=true" title="class or interface in java.lang.ref"><code>WeakReference</code></a>,
 <a href="http://docs.oracle.com/javase/6/docs/api/java/lang/ref/SoftReference.html?is-external=true" title="class or interface in java.lang.ref"><code>SoftReference</code></a>,
 <a href="http://docs.oracle.com/javase/6/docs/api/java/lang/ref/PhantomReference.html?is-external=true" title="class or interface in java.lang.ref"><code>PhantomReference</code></a>,
 <a href="http://docs.oracle.com/javase/6/docs/api/java/lang/Comparable.html?is-external=true" title="class or interface in java.lang"><code>Comparable</code></a></dd>
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 <caption><span>Enum Constants</span><span class="tabEnd">&nbsp;</span></caption>
 <tr>
 <th class="colOne" scope="col">Enum Constant and Description</th>
 </tr>
 <tr class="altColor">
 <td class="colOne"><code><span class="memberNameLink"><a href="../../../../org/apache/river/concurrent/Ref.html#SOFT">SOFT</a></span></code>
 <div class="block">
  SOFT References implement equals based on equality of the referent
  objects, while the referent is still reachable.</div>
 </td>
 </tr>
 <tr class="rowColor">
 <td class="colOne"><code><span class="memberNameLink"><a href="../../../../org/apache/river/concurrent/Ref.html#SOFT_IDENTITY">SOFT_IDENTITY</a></span></code>
 <div class="block">
  SOFT_IDENTY References implement equals based on identity == of the
  referent objects.</div>
 </td>
 </tr>
 <tr class="altColor">
 <td class="colOne"><code><span class="memberNameLink"><a href="../../../../org/apache/river/concurrent/Ref.html#STRONG">STRONG</a></span></code>
 <div class="block">
  STRONG References implement equals and hashCode() based on the
  equality of the underlying Object.</div>
 </td>
 </tr>
 <tr class="rowColor">
 <td class="colOne"><code><span class="memberNameLink"><a href="../../../../org/apache/river/concurrent/Ref.html#TIME">TIME</a></span></code>
 <div class="block">
  TIME References implement equals based on equality of the referent
  objects.</div>
 </td>
 </tr>
 <tr class="altColor">
 <td class="colOne"><code><span class="memberNameLink"><a href="../../../../org/apache/river/concurrent/Ref.html#WEAK">WEAK</a></span></code>
 <div class="block">
  WEAK References implement equals based on equality of the referent
  objects, while the referent is still reachable.</div>
 </td>
 </tr>
 <tr class="rowColor">
 <td class="colOne"><code><span class="memberNameLink"><a href="../../../../org/apache/river/concurrent/Ref.html#WEAK_IDENTITY">WEAK_IDENTITY</a></span></code>
 <div class="block">
  WEAK_IDENTY References implement equals based on identity == of the
  referent objects.</div>
 </td>
 </tr>
 </table>
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 <td class="colFirst"><code>static <a href="../../../../org/apache/river/concurrent/Ref.html" title="enum in org.apache.river.concurrent">Ref</a></code></td>
 <td class="colLast"><code><span class="memberNameLink"><a href="../../../../org/apache/river/concurrent/Ref.html#valueOf-java.lang.String-">valueOf</a></span>(<a href="http://docs.oracle.com/javase/6/docs/api/java/lang/String.html?is-external=true" title="class or interface in java.lang">String</a>&nbsp;name)</code>
 <div class="block">Returns the enum constant of this type with the specified name.</div>
 </td>
 </tr>
 <tr id="i1" class="rowColor">
 <td class="colFirst"><code>static <a href="../../../../org/apache/river/concurrent/Ref.html" title="enum in org.apache.river.concurrent">Ref</a>[]</code></td>
 <td class="colLast"><code><span class="memberNameLink"><a href="../../../../org/apache/river/concurrent/Ref.html#values--">values</a></span>()</code>
 <div class="block">Returns an array containing the constants of this enum type, in
 the order they are declared.</div>
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 <h3>Enum Constant Detail</h3>
 <a name="TIME">
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 <h4>TIME</h4>
 <pre>public static final&nbsp;<a href="../../../../org/apache/river/concurrent/Ref.html" title="enum in org.apache.river.concurrent">Ref</a> TIME</pre>
 <div class="block"><P>
  TIME References implement equals based on equality of the referent
  objects.  Time references are STRONG references that are removed
  after a period of no access, even if they are strongly
  referenced outside the cache, they are removed.  TIME references don't
  rely on Garbage Collection algorithms.
  </P><P>
  TIME References support cancellation of tasks implementing Future, when
  the reference times out, if it contains a Future, it is cancelled.
  </P><P>
  A call to equals(), get() or toString(), will cause the timestamp on
  the reference to be updated, whereas hashCode() will not, in addition,
  Comparators and referents that implement Comparable, only update the
  timestamp if they return 0.  This allows referents to be inspected
  without update when they don't match.
  </P><P>
  TIME References require synchronisation for iteration, so during
  cleaning periods, a synchronised Collection or Map will be locked.
  A lock is still obtained for iterating over Concurrent Maps and
  Collections, however this does not normally synchronise access between threads,
  only other cleaning task threads.
  </P></div>
 <dl>
 <dt><span class="seeLabel">See Also:</span></dt>
 <dd><a href="http://docs.oracle.com/javase/6/docs/api/java/util/concurrent.Future.html?is-external=true" title="class or interface in java.util"><code>Future</code></a></dd>
 </dl>
 </li>
 </ul>
 <a name="SOFT">
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 <h4>SOFT</h4>
 <pre>public static final&nbsp;<a href="../../../../org/apache/river/concurrent/Ref.html" title="enum in org.apache.river.concurrent">Ref</a> SOFT</pre>
 <div class="block"><P>
  SOFT References implement equals based on equality of the referent
  objects, while the referent is still reachable.  The hashCode
  implementation is based on the referent implementation of hashCode,
  while the referent is reachable.
  </P><P>
  SOFT References implement Comparable allowing the referent Objects
  to be compared if they implement Comparable.  If the referent Object
  doesn't implement Comparable, the hashCode's of the Reference is
  compared instead.  If the referent Objects don't implement Comparable,
  then they shouldn't really be used in sorted collections.
  </P><P>
  Garbage collection must be the same as SoftReference.
  </P></div>
 <dl>
 <dt><span class="seeLabel">See Also:</span></dt>
 <dd><a href="http://docs.oracle.com/javase/6/docs/api/java/lang/ref/SoftReference.html?is-external=true" title="class or interface in java.lang.ref"><code>SoftReference</code></a>,
 <a href="http://docs.oracle.com/javase/6/docs/api/java/lang/ref/WeakReference.html?is-external=true" title="class or interface in java.lang.ref"><code>WeakReference</code></a>,
 <a href="http://docs.oracle.com/javase/6/docs/api/java/lang/Comparable.html?is-external=true" title="class or interface in java.lang"><code>Comparable</code></a></dd>
 </dl>
 </li>
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 <h4>SOFT_IDENTITY</h4>
 <pre>public static final&nbsp;<a href="../../../../org/apache/river/concurrent/Ref.html" title="enum in org.apache.river.concurrent">Ref</a> SOFT_IDENTITY</pre>
 <div class="block"><P>
  SOFT_IDENTY References implement equals based on identity == of the
  referent objects.
  </P><P>
  Garbage collection must be the same as SoftReference.
  </P></div>
 <dl>
 <dt><span class="seeLabel">See Also:</span></dt>
 <dd><a href="http://docs.oracle.com/javase/6/docs/api/java/lang/ref/SoftReference.html?is-external=true" title="class or interface in java.lang.ref"><code>SoftReference</code></a></dd>
 </dl>
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 <h4>WEAK</h4>
 <pre>public static final&nbsp;<a href="../../../../org/apache/river/concurrent/Ref.html" title="enum in org.apache.river.concurrent">Ref</a> WEAK</pre>
 <div class="block"><P>
  WEAK References implement equals based on equality of the referent
  objects, while the referent is still reachable.  The hashCode
  implementation is based on the referent implementation of hashCode,
  while the referent is reachable.
  </P><P>
  WEAK References implement comparable allowing the referent Objects
  to be compared if they implement Comparable.  If the referent Object
  doesn't implement Comparable, the hashCode's of the Reference is
  compared instead.  If the referent Object's don't implement Comparable,
  then they shouldn't really be used in sorted collections.
  </P><P>
  Garbage collection must be the same as WeakReference.
  </P></div>
 <dl>
 <dt><span class="seeLabel">See Also:</span></dt>
 <dd><a href="http://docs.oracle.com/javase/6/docs/api/java/lang/ref/WeakReference.html?is-external=true" title="class or interface in java.lang.ref"><code>WeakReference</code></a>,
 <a href="http://docs.oracle.com/javase/6/docs/api/java/lang/Comparable.html?is-external=true" title="class or interface in java.lang"><code>Comparable</code></a></dd>
 </dl>
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 <h4>WEAK_IDENTITY</h4>
 <pre>public static final&nbsp;<a href="../../../../org/apache/river/concurrent/Ref.html" title="enum in org.apache.river.concurrent">Ref</a> WEAK_IDENTITY</pre>
 <div class="block"><P>
  WEAK_IDENTY References implement equals based on identity == of the
  referent objects.
  </P><P>
  Garbage collection must be the same as WeakReference.
  </P></div>
 <dl>
 <dt><span class="seeLabel">See Also:</span></dt>
 <dd><a href="http://docs.oracle.com/javase/6/docs/api/java/lang/ref/WeakReference.html?is-external=true" title="class or interface in java.lang.ref"><code>WeakReference</code></a></dd>
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 <h4>STRONG</h4>
 <pre>public static final&nbsp;<a href="../../../../org/apache/river/concurrent/Ref.html" title="enum in org.apache.river.concurrent">Ref</a> STRONG</pre>
 <div class="block"><P>
  STRONG References implement equals and hashCode() based on the
  equality of the underlying Object.
  </P><P>
  STRONG References implement Comparable allowing the referent Objects
  to be compared if they implement Comparable.  If the referent Object
  doesn't implement Comparable, the hashCode's of the Reference is
  compared instead.  If the referent Object's don't implement Comparable,
  then they shouldn't really be used in sorted collections.
  </P><P>
  Garbage collection doesn't occur unless the Reference is cleared.
  </P></div>
 <dl>
 <dt><span class="seeLabel">See Also:</span></dt>
 <dd><a href="http://docs.oracle.com/javase/6/docs/api/java/lang/Comparable.html?is-external=true" title="class or interface in java.lang"><code>Comparable</code></a></dd>
 </dl>
 </li>
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 <pre>public static&nbsp;<a href="../../../../org/apache/river/concurrent/Ref.html" title="enum in org.apache.river.concurrent">Ref</a>[]&nbsp;values()</pre>
 <div class="block">Returns an array containing the constants of this enum type, in
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 <pre>
 for (Ref c : Ref.values())
 &nbsp;   System.out.println(c);
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 <pre>public static&nbsp;<a href="../../../../org/apache/river/concurrent/Ref.html" title="enum in org.apache.river.concurrent">Ref</a>&nbsp;valueOf(<a href="http://docs.oracle.com/javase/6/docs/api/java/lang/String.html?is-external=true" title="class or interface in java.lang">String</a>&nbsp;name)</pre>
 <div class="block">Returns the enum constant of this type with the specified name.
 The string must match <i>exactly</i> an identifier used to declare an
 enum constant in this type.  (Extraneous whitespace characters are
 not permitted.)</div>
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 <dd><code>name</code> - the name of the enum constant to be returned.</dd>
 <dt><span class="returnLabel">Returns:</span></dt>
 <dd>the enum constant with the specified name</dd>
 <dt><span class="throwsLabel">Throws:</span></dt>
 <dd><code><a href="http://docs.oracle.com/javase/6/docs/api/java/lang/IllegalArgumentException.html?is-external=true" title="class or interface in java.lang">IllegalArgumentException</a></code> - if this enum type has no constant with the specified name</dd>
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