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From se...@apache.org
Subject [09/50] [abbrv] incubator-ignite git commit: ignite-sprint-6: revert changes in concurrent map
Date Fri, 12 Jun 2015 14:24:59 GMT
http://git-wip-us.apache.org/repos/asf/incubator-ignite/blob/8203caf7/modules/core/src/main/java/org/jsr166/ConcurrentHashMap8.java
----------------------------------------------------------------------
diff --git a/modules/core/src/main/java/org/jsr166/ConcurrentHashMap8.java b/modules/core/src/main/java/org/jsr166/ConcurrentHashMap8.java
index 727db4c..041130b 100644
--- a/modules/core/src/main/java/org/jsr166/ConcurrentHashMap8.java
+++ b/modules/core/src/main/java/org/jsr166/ConcurrentHashMap8.java
@@ -5,22 +5,20 @@
  */
 
 /*
- * The latest version of the file corresponds to the following CVS commit:
- * http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/jdk7/java/util/concurrent/ConcurrentHashMap.java?pathrev=1.43
+ * The latest version of the file was copied from the following CVS repository:
+ * http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/
  *
- * Note, that the repository above is JDK 7 based that is kept up-to-date too.
- * The main repository (JDK 8 based) uses JDK 8 features significantly that unavailable in JDK 7.
+ * Corresponding commit version in CVS repository is unknown (lost in our side).
+ * On the other hand we can't simply synch the latest version from CVS here, because Ignite uses functionality that
+ * is no longer supported.
  */
 
-
 package org.jsr166;
 
 import java.io.*;
 import java.util.*;
 import java.util.concurrent.*;
-import java.util.concurrent.atomic.*;
 import java.util.concurrent.locks.*;
-import java.lang.reflect.*;
 
 /**
  * A hash table supporting full concurrency of retrievals and
@@ -74,15 +72,21 @@ import java.lang.reflect.*;
  * expected {@code concurrencyLevel} as an additional hint for
  * internal sizing.  Note that using many keys with exactly the same
  * {@code hashCode()} is a sure way to slow down performance of any
- * hash table. To ameliorate impact, when keys are {@link Comparable},
- * this class may use comparison order among keys to help break ties.
+ * hash table.
  *
- * <p>A {@link Set} projection of a ConcurrentHashMap may be created
+ * <p>A {@link Set} projection of a ConcurrentHashMapV8 may be created
  * (using {@link #newKeySet()} or {@link #newKeySet(int)}), or viewed
  * (using {@link #keySet(Object)} when only keys are of interest, and the
  * mapped values are (perhaps transiently) not used or all take the
  * same mapping value.
  *
+ * <p>A ConcurrentHashMapV8 can be used as scalable frequency map (a
+ * form of histogram or multiset) by using {@link LongAdder8} values
+ * and initializing via {@link #computeIfAbsent}. For example, to add
+ * a count to a {@code ConcurrentHashMapV8<String,LongAdder8> freqs}, you
+ * can use {@code freqs.computeIfAbsent(k -> new
+ * LongAdder8()).increment();}
+ *
  * <p>This class and its views and iterators implement all of the
  * <em>optional</em> methods of the {@link Map} and {@link Iterator}
  * interfaces.
@@ -90,9 +94,90 @@ import java.lang.reflect.*;
  * <p>Like {@link Hashtable} but unlike {@link HashMap}, this class
  * does <em>not</em> allow {@code null} to be used as a key or value.
  *
- * <p>This class is a member of the
- * <a href="{@docRoot}/../technotes/guides/collections/index.html">
- * Java Collections Framework</a>.
+ * <ul>
+ * <li> forEach: Perform a given action on each element.
+ * A variant form applies a given transformation on each element
+ * before performing the action.</li>
+ *
+ * <li> search: Return the first available non-null result of
+ * applying a given function on each element; skipping further
+ * search when a result is found.</li>
+ *
+ * <li> reduce: Accumulate each element.  The supplied reduction
+ * function cannot rely on ordering (more formally, it should be
+ * both associative and commutative).  There are five variants:
+ *
+ * <ul>
+ *
+ * <li> Plain reductions. (There is not a form of this method for
+ * (key, value) function arguments since there is no corresponding
+ * return type.)</li>
+ *
+ * <li> Mapped reductions that accumulate the results of a given
+ * function applied to each element.</li>
+ *
+ * <li> Reductions to scalar doubles, longs, and ints, using a
+ * given basis value.</li>
+ *
+ * </li>
+ * </ul>
+ * </ul>
+ *
+ * <p>The concurrency properties of bulk operations follow
+ * from those of ConcurrentHashMapV8: Any non-null result returned
+ * from {@code get(key)} and related access methods bears a
+ * happens-before relation with the associated insertion or
+ * update.  The result of any bulk operation reflects the
+ * composition of these per-element relations (but is not
+ * necessarily atomic with respect to the map as a whole unless it
+ * is somehow known to be quiescent).  Conversely, because keys
+ * and values in the map are never null, null serves as a reliable
+ * atomic indicator of the current lack of any result.  To
+ * maintain this property, null serves as an implicit basis for
+ * all non-scalar reduction operations. For the double, long, and
+ * int versions, the basis should be one that, when combined with
+ * any other value, returns that other value (more formally, it
+ * should be the identity element for the reduction). Most common
+ * reductions have these properties; for example, computing a sum
+ * with basis 0 or a minimum with basis MAX_VALUE.
+ *
+ * <p>Search and transformation functions provided as arguments
+ * should similarly return null to indicate the lack of any result
+ * (in which case it is not used). In the case of mapped
+ * reductions, this also enables transformations to serve as
+ * filters, returning null (or, in the case of primitive
+ * specializations, the identity basis) if the element should not
+ * be combined. You can create compound transformations and
+ * filterings by composing them yourself under this "null means
+ * there is nothing there now" rule before using them in search or
+ * reduce operations.
+ *
+ * <p>Methods accepting and/or returning Entry arguments maintain
+ * key-value associations. They may be useful for example when
+ * finding the key for the greatest value. Note that "plain" Entry
+ * arguments can be supplied using {@code new
+ * AbstractMap.SimpleEntry(k,v)}.
+ *
+ * <p>Bulk operations may complete abruptly, throwing an
+ * exception encountered in the application of a supplied
+ * function. Bear in mind when handling such exceptions that other
+ * concurrently executing functions could also have thrown
+ * exceptions, or would have done so if the first exception had
+ * not occurred.
+ *
+ * <p>Parallel speedups for bulk operations compared to sequential
+ * processing are common but not guaranteed.  Operations involving
+ * brief functions on small maps may execute more slowly than
+ * sequential loops if the underlying work to parallelize the
+ * computation is more expensive than the computation itself.
+ * Similarly, parallelization may not lead to much actual parallelism
+ * if all processors are busy performing unrelated tasks.
+ *
+ * <p>All arguments to all task methods must be non-null.
+ *
+ * <p><em>jsr166e note: During transition, this class
+ * uses nested functional interfaces with different names but the
+ * same forms as those expected for JDK8.</em>
  *
  * @since 1.5
  * @author Doug Lea
@@ -100,9 +185,80 @@ import java.lang.reflect.*;
  * @param <V> the type of mapped values
  */
 @SuppressWarnings("ALL")
-public class ConcurrentHashMap8<K,V> implements ConcurrentMap<K,V>, Serializable {
+public class ConcurrentHashMap8<K, V>
+    implements ConcurrentMap<K, V>, Serializable {
     private static final long serialVersionUID = 7249069246763182397L;
 
+    /**
+     * A partitionable iterator. A Spliterator can be traversed
+     * directly, but can also be partitioned (before traversal) by
+     * creating another Spliterator that covers a non-overlapping
+     * portion of the elements, and so may be amenable to parallel
+     * execution.
+     *
+     * <p>This interface exports a subset of expected JDK8
+     * functionality.
+     *
+     * <p>Sample usage: Here is one (of the several) ways to compute
+     * the sum of the values held in a map using the ForkJoin
+     * framework. As illustrated here, Spliterators are well suited to
+     * designs in which a task repeatedly splits off half its work
+     * into forked subtasks until small enough to process directly,
+     * and then joins these subtasks. Variants of this style can also
+     * be used in completion-based designs.
+     *
+     * <pre>
+     * {@code ConcurrentHashMapV8<String, Long> m = ...
+     * // split as if have 8 * parallelism, for load balance
+     * int n = m.size();
+     * int p = aForkJoinPool.getParallelism() * 8;
+     * int split = (n < p)? n : p;
+     * long sum = aForkJoinPool.invoke(new SumValues(m.valueSpliterator(), split, null));
+     * // ...
+     * static class SumValues extends RecursiveTask<Long> {
+     *   final Spliterator<Long> s;
+     *   final int split;             // split while > 1
+     *   final SumValues nextJoin;    // records forked subtasks to join
+     *   SumValues(Spliterator<Long> s, int depth, SumValues nextJoin) {
+     *     this.s = s; this.depth = depth; this.nextJoin = nextJoin;
+     *   }
+     *   public Long compute() {
+     *     long sum = 0;
+     *     SumValues subtasks = null; // fork subtasks
+     *     for (int s = split >>> 1; s > 0; s >>>= 1)
+     *       (subtasks = new SumValues(s.split(), s, subtasks)).fork();
+     *     while (s.hasNext())        // directly process remaining elements
+     *       sum += s.next();
+     *     for (SumValues t = subtasks; t != null; t = t.nextJoin)
+     *       sum += t.join();         // collect subtask results
+     *     return sum;
+     *   }
+     * }
+     * }</pre>
+     */
+    public static interface Spliterator<T> extends Iterator<T> {
+        /**
+         * Returns a Spliterator covering approximately half of the
+         * elements, guaranteed not to overlap with those subsequently
+         * returned by this Spliterator.  After invoking this method,
+         * the current Spliterator will <em>not</em> produce any of
+         * the elements of the returned Spliterator, but the two
+         * Spliterators together will produce all of the elements that
+         * would have been produced by this Spliterator had this
+         * method not been called. The exact number of elements
+         * produced by the returned Spliterator is not guaranteed, and
+         * may be zero (i.e., with {@code hasNext()} reporting {@code
+         * false}) if this Spliterator cannot be further split.
+         *
+         * @return a Spliterator covering approximately half of the
+         * elements
+         * @throws IllegalStateException if this Spliterator has
+         * already commenced traversing elements
+         */
+        Spliterator<T> split();
+    }
+
+
     /*
      * Overview:
      *
@@ -113,21 +269,18 @@ public class ConcurrentHashMap8<K,V> implements ConcurrentMap<K,V>, Serializable
      * the same or better than java.util.HashMap, and to support high
      * initial insertion rates on an empty table by many threads.
      *
-     * This map usually acts as a binned (bucketed) hash table.  Each
-     * key-value mapping is held in a Node.  Most nodes are instances
-     * of the basic Node class with hash, key, value, and next
-     * fields. However, various subclasses exist: TreeNodes are
-     * arranged in balanced trees, not lists.  TreeBins hold the roots
-     * of sets of TreeNodes. ForwardingNodes are placed at the heads
-     * of bins during resizing. ReservationNodes are used as
-     * placeholders while establishing values in computeIfAbsent and
-     * related methods.  The types TreeBin, ForwardingNode, and
-     * ReservationNode do not hold normal user keys, values, or
-     * hashes, and are readily distinguishable during search etc
-     * because they have negative hash fields and null key and value
-     * fields. (These special nodes are either uncommon or transient,
-     * so the impact of carrying around some unused fields is
-     * insignificant.)
+     * Each key-value mapping is held in a Node.  Because Node fields
+     * can contain special values, they are defined using plain Object
+     * types. Similarly in turn, all internal methods that use them
+     * work off Object types. And similarly, so do the internal
+     * methods of auxiliary iterator and view classes.  All public
+     * generic typed methods relay in/out of these internal methods,
+     * supplying null-checks and casts as needed. This also allows
+     * many of the public methods to be factored into a smaller number
+     * of internal methods (although sadly not so for the five
+     * variants of put-related operations). The validation-based
+     * approach explained below leads to a lot of code sprawl because
+     * retry-control precludes factoring into smaller methods.
      *
      * The table is lazily initialized to a power-of-two size upon the
      * first insertion.  Each bin in the table normally contains a
@@ -135,12 +288,24 @@ public class ConcurrentHashMap8<K,V> implements ConcurrentMap<K,V>, Serializable
      * Table accesses require volatile/atomic reads, writes, and
      * CASes.  Because there is no other way to arrange this without
      * adding further indirections, we use intrinsics
-     * (sun.misc.Unsafe) operations.
+     * (sun.misc.Unsafe) operations.  The lists of nodes within bins
+     * are always accurately traversable under volatile reads, so long
+     * as lookups check hash code and non-nullness of value before
+     * checking key equality.
      *
-     * We use the top (sign) bit of Node hash fields for control
-     * purposes -- it is available anyway because of addressing
-     * constraints.  Nodes with negative hash fields are specially
-     * handled or ignored in map methods.
+     * We use the top two bits of Node hash fields for control
+     * purposes -- they are available anyway because of addressing
+     * constraints.  As explained further below, these top bits are
+     * used as follows:
+     *  00 - Normal
+     *  01 - Locked
+     *  11 - Locked and may have a thread waiting for lock
+     *  10 - Node is a forwarding node
+     *
+     * The lower 30 bits of each Node's hash field contain a
+     * transformation of the key's hash code, except for forwarding
+     * nodes, for which the lower bits are zero (and so always have
+     * hash field == MOVED).
      *
      * Insertion (via put or its variants) of the first node in an
      * empty bin is performed by just CASing it to the bin.  This is
@@ -149,15 +314,22 @@ public class ConcurrentHashMap8<K,V> implements ConcurrentMap<K,V>, Serializable
      * delete, and replace) require locks.  We do not want to waste
      * the space required to associate a distinct lock object with
      * each bin, so instead use the first node of a bin list itself as
-     * a lock. Locking support for these locks relies on builtin
-     * "synchronized" monitors.
+     * a lock. Blocking support for these locks relies on the builtin
+     * "synchronized" monitors.  However, we also need a tryLock
+     * construction, so we overlay these by using bits of the Node
+     * hash field for lock control (see above), and so normally use
+     * builtin monitors only for blocking and signalling using
+     * wait/notifyAll constructions. See Node.tryAwaitLock.
      *
      * Using the first node of a list as a lock does not by itself
      * suffice though: When a node is locked, any update must first
      * validate that it is still the first node after locking it, and
      * retry if not. Because new nodes are always appended to lists,
      * once a node is first in a bin, it remains first until deleted
-     * or the bin becomes invalidated (upon resizing).
+     * or the bin becomes invalidated (upon resizing).  However,
+     * operations that only conditionally update may inspect nodes
+     * until the point of update. This is a converse of sorts to the
+     * lazy locking technique described by Herlihy & Shavit.
      *
      * The main disadvantage of per-bin locks is that other update
      * operations on other nodes in a bin list protected by the same
@@ -190,12 +362,15 @@ public class ConcurrentHashMap8<K,V> implements ConcurrentMap<K,V>, Serializable
      * sometimes deviate significantly from uniform randomness.  This
      * includes the case when N > (1<<30), so some keys MUST collide.
      * Similarly for dumb or hostile usages in which multiple keys are
-     * designed to have identical hash codes or ones that differs only
-     * in masked-out high bits. So we use a secondary strategy that
-     * applies when the number of nodes in a bin exceeds a
-     * threshold. These TreeBins use a balanced tree to hold nodes (a
-     * specialized form of red-black trees), bounding search time to
-     * O(log N).  Each search step in a TreeBin is at least twice as
+     * designed to have identical hash codes. Also, although we guard
+     * against the worst effects of this (see method spread), sets of
+     * hashes may differ only in bits that do not impact their bin
+     * index for a given power-of-two mask.  So we use a secondary
+     * strategy that applies when the number of nodes in a bin exceeds
+     * a threshold, and at least one of the keys implements
+     * Comparable.  These TreeBins use a balanced tree to hold nodes
+     * (a specialized form of red-black trees), bounding search time
+     * to O(log N).  Each search step in a TreeBin is around twice as
      * slow as in a regular list, but given that N cannot exceed
      * (1<<64) (before running out of addresses) this bounds search
      * steps, lock hold times, etc, to reasonable constants (roughly
@@ -206,50 +381,43 @@ public class ConcurrentHashMap8<K,V> implements ConcurrentMap<K,V>, Serializable
      * iterators in the same way.
      *
      * The table is resized when occupancy exceeds a percentage
-     * threshold (nominally, 0.75, but see below).  Any thread
-     * noticing an overfull bin may assist in resizing after the
-     * initiating thread allocates and sets up the replacement array.
-     * However, rather than stalling, these other threads may proceed
-     * with insertions etc.  The use of TreeBins shields us from the
-     * worst case effects of overfilling while resizes are in
-     * progress.  Resizing proceeds by transferring bins, one by one,
-     * from the table to the next table. However, threads claim small
-     * blocks of indices to transfer (via field transferIndex) before
-     * doing so, reducing contention.  A generation stamp in field
-     * sizeCtl ensures that resizings do not overlap. Because we are
-     * using power-of-two expansion, the elements from each bin must
-     * either stay at same index, or move with a power of two
-     * offset. We eliminate unnecessary node creation by catching
-     * cases where old nodes can be reused because their next fields
-     * won't change.  On average, only about one-sixth of them need
-     * cloning when a table doubles. The nodes they replace will be
-     * garbage collectable as soon as they are no longer referenced by
-     * any reader thread that may be in the midst of concurrently
-     * traversing table.  Upon transfer, the old table bin contains
-     * only a special forwarding node (with hash field "MOVED") that
-     * contains the next table as its key. On encountering a
-     * forwarding node, access and update operations restart, using
-     * the new table.
-     *
-     * Each bin transfer requires its bin lock, which can stall
-     * waiting for locks while resizing. However, because other
-     * threads can join in and help resize rather than contend for
-     * locks, average aggregate waits become shorter as resizing
-     * progresses.  The transfer operation must also ensure that all
-     * accessible bins in both the old and new table are usable by any
-     * traversal.  This is arranged in part by proceeding from the
-     * last bin (table.length - 1) up towards the first.  Upon seeing
-     * a forwarding node, traversals (see class Traverser) arrange to
-     * move to the new table without revisiting nodes.  To ensure that
-     * no intervening nodes are skipped even when moved out of order,
-     * a stack (see class TableStack) is created on first encounter of
-     * a forwarding node during a traversal, to maintain its place if
-     * later processing the current table. The need for these
-     * save/restore mechanics is relatively rare, but when one
-     * forwarding node is encountered, typically many more will be.
-     * So Traversers use a simple caching scheme to avoid creating so
-     * many new TableStack nodes. (Thanks to Peter Levart for
-     * suggesting use of a stack here.)
+     * threshold (nominally, 0.75, but see below).  Only a single
+     * thread performs the resize (using field "sizeCtl", to arrange
+     * exclusion), but the table otherwise remains usable for reads
+     * and updates. Resizing proceeds by transferring bins, one by
+     * one, from the table to the next table.  Because we are using
+     * power-of-two expansion, the elements from each bin must either
+     * stay at same index, or move with a power of two offset. We
+     * eliminate unnecessary node creation by catching cases where old
+     * nodes can be reused because their next fields won't change.  On
+     * average, only about one-sixth of them need cloning when a table
+     * doubles. The nodes they replace will be garbage collectable as
+     * soon as they are no longer referenced by any reader thread that
+     * may be in the midst of concurrently traversing table.  Upon
+     * transfer, the old table bin contains only a special forwarding
+     * node (with hash field "MOVED") that contains the next table as
+     * its key. On encountering a forwarding node, access and update
+     * operations restart, using the new table.
+     *
+     * Each bin transfer requires its bin lock. However, unlike other
+     * cases, a transfer can skip a bin if it fails to acquire its
+     * lock, and revisit it later (unless it is a TreeBin). Method
+     * rebuild maintains a buffer of TRANSFER_BUFFER_SIZE bins that
+     * have been skipped because of failure to acquire a lock, and
+     * blocks only if none are available (i.e., only very rarely).
+     * The transfer operation must also ensure that all accessible
+     * bins in both the old and new table are usable by any traversal.
+     * When there are no lock acquisition failures, this is arranged
+     * simply by proceeding from the last bin (table.length - 1) up
+     * towards the first.  Upon seeing a forwarding node, traversals
+     * (see class Iter) arrange to move to the new table
+     * without revisiting nodes.  However, when any node is skipped
+     * during a transfer, all earlier table bins may have become
+     * visible, so are initialized with a reverse-forwarding node back
+     * to the old table until the new ones are established. (This
+     * sometimes requires transiently locking a forwarding node, which
+     * is possible under the above encoding.) These more expensive
+     * mechanics trigger only when necessary.
      *
      * The traversal scheme also applies to partial traversals of
      * ranges of bins (via an alternate Traverser constructor)
@@ -264,54 +432,20 @@ public class ConcurrentHashMap8<K,V> implements ConcurrentMap<K,V>, Serializable
      * These cases attempt to override the initial capacity settings,
      * but harmlessly fail to take effect in cases of races.
      *
-     * The element count is maintained using a specialization of
-     * LongAdder. We need to incorporate a specialization rather than
-     * just use a LongAdder in order to access implicit
-     * contention-sensing that leads to creation of multiple
-     * CounterCells.  The counter mechanics avoid contention on
-     * updates but can encounter cache thrashing if read too
-     * frequently during concurrent access. To avoid reading so often,
-     * resizing under contention is attempted only upon adding to a
-     * bin already holding two or more nodes. Under uniform hash
-     * distributions, the probability of this occurring at threshold
-     * is around 13%, meaning that only about 1 in 8 puts check
-     * threshold (and after resizing, many fewer do so).
-     *
-     * TreeBins use a special form of comparison for search and
-     * related operations (which is the main reason we cannot use
-     * existing collections such as TreeMaps). TreeBins contain
-     * Comparable elements, but may contain others, as well as
-     * elements that are Comparable but not necessarily Comparable for
-     * the same T, so we cannot invoke compareTo among them. To handle
-     * this, the tree is ordered primarily by hash value, then by
-     * Comparable.compareTo order if applicable.  On lookup at a node,
-     * if elements are not comparable or compare as 0 then both left
-     * and right children may need to be searched in the case of tied
-     * hash values. (This corresponds to the full list search that
-     * would be necessary if all elements were non-Comparable and had
-     * tied hashes.) On insertion, to keep a total ordering (or as
-     * close as is required here) across rebalancings, we compare
-     * classes and identityHashCodes as tie-breakers. The red-black
-     * balancing code is updated from pre-jdk-collections
-     * (http://gee.cs.oswego.edu/dl/classes/collections/RBCell.java)
-     * based in turn on Cormen, Leiserson, and Rivest "Introduction to
-     * Algorithms" (CLR).
-     *
-     * TreeBins also require an additional locking mechanism.  While
-     * list traversal is always possible by readers even during
-     * updates, tree traversal is not, mainly because of tree-rotations
-     * that may change the root node and/or its linkages.  TreeBins
-     * include a simple read-write lock mechanism parasitic on the
-     * main bin-synchronization strategy: Structural adjustments
-     * associated with an insertion or removal are already bin-locked
-     * (and so cannot conflict with other writers) but must wait for
-     * ongoing readers to finish. Since there can be only one such
-     * waiter, we use a simple scheme using a single "waiter" field to
-     * block writers.  However, readers need never block.  If the root
-     * lock is held, they proceed along the slow traversal path (via
-     * next-pointers) until the lock becomes available or the list is
-     * exhausted, whichever comes first. These cases are not fast, but
-     * maximize aggregate expected throughput.
+     * The element count is maintained using a LongAdder8, which avoids
+     * contention on updates but can encounter cache thrashing if read
+     * too frequently during concurrent access. To avoid reading so
+     * often, resizing is attempted either when a bin lock is
+     * contended, or upon adding to a bin already holding two or more
+     * nodes (checked before adding in the xIfAbsent methods, after
+     * adding in others). Under uniform hash distributions, the
+     * probability of this occurring at threshold is around 13%,
+     * meaning that only about 1 in 8 puts check threshold (and after
+     * resizing, many fewer do so). But this approximation has high
+     * variance for small table sizes, so we check on any collision
+     * for sizes <= 64. The bulk putAll operation further reduces
+     * contention by only committing count updates upon these size
+     * checks.
      *
      * Maintaining API and serialization compatibility with previous
      * versions of this class introduces several oddities. Mainly: We
@@ -321,20 +455,8 @@ public class ConcurrentHashMap8<K,V> implements ConcurrentMap<K,V>, Serializable
      * time that we can guarantee to honor it.) We also declare an
      * unused "Segment" class that is instantiated in minimal form
      * only when serializing.
-     *
-     * Also, solely for compatibility with previous versions of this
-     * class, it extends AbstractMap, even though all of its methods
-     * are overridden, so it is just useless baggage.
-     *
-     * This file is organized to make things a little easier to follow
-     * while reading than they might otherwise: First the main static
-     * declarations and utilities, then fields, then main public
-     * methods (with a few factorings of multiple public methods into
-     * internal ones), then sizing methods, trees, traversers, and
-     * bulk operations.
      */
 
-
     /* ---------------- Constants -------------- */
 
     /**
@@ -374,2362 +496,2737 @@ public class ConcurrentHashMap8<K,V> implements ConcurrentMap<K,V>, Serializable
     private static final float LOAD_FACTOR = 0.75f;
 
     /**
-     * The bin count threshold for using a tree rather than list for a
-     * bin.  Bins are converted to trees when adding an element to a
-     * bin with at least this many nodes. The value must be greater
-     * than 2, and should be at least 8 to mesh with assumptions in
-     * tree removal about conversion back to plain bins upon
-     * shrinkage.
+     * The buffer size for skipped bins during transfers. The
+     * value is arbitrary but should be large enough to avoid
+     * most locking stalls during resizes.
      */
-    static final int TREEIFY_THRESHOLD = 8;
+    private static final int TRANSFER_BUFFER_SIZE = 32;
 
     /**
-     * The bin count threshold for untreeifying a (split) bin during a
-     * resize operation. Should be less than TREEIFY_THRESHOLD, and at
-     * most 6 to mesh with shrinkage detection under removal.
+     * The bin count threshold for using a tree rather than list for a
+     * bin.  The value reflects the approximate break-even point for
+     * using tree-based operations.
      */
-    static final int UNTREEIFY_THRESHOLD = 6;
+    private static final int TREE_THRESHOLD = 8;
 
-    /**
-     * The smallest table capacity for which bins may be treeified.
-     * (Otherwise the table is resized if too many nodes in a bin.)
-     * The value should be at least 4 * TREEIFY_THRESHOLD to avoid
-     * conflicts between resizing and treeification thresholds.
+    /*
+     * Encodings for special uses of Node hash fields. See above for
+     * explanation.
      */
-    static final int MIN_TREEIFY_CAPACITY = 64;
+    static final int MOVED     = 0x80000000; // hash field for forwarding nodes
+    static final int LOCKED    = 0x40000000; // set/tested only as a bit
+    static final int WAITING   = 0xc0000000; // both bits set/tested together
+    static final int HASH_BITS = 0x3fffffff; // usable bits of normal node hash
 
-    /**
-     * Minimum number of rebinnings per transfer step. Ranges are
-     * subdivided to allow multiple resizer threads.  This value
-     * serves as a lower bound to avoid resizers encountering
-     * excessive memory contention.  The value should be at least
-     * DEFAULT_CAPACITY.
-     */
-    private static final int MIN_TRANSFER_STRIDE = 16;
+    /* ---------------- Fields -------------- */
 
     /**
-     * The number of bits used for generation stamp in sizeCtl.
-     * Must be at least 6 for 32bit arrays.
+     * The array of bins. Lazily initialized upon first insertion.
+     * Size is always a power of two. Accessed directly by iterators.
      */
-    private static int RESIZE_STAMP_BITS = 16;
+    transient volatile Node[] table;
 
     /**
-     * The maximum number of threads that can help resize.
-     * Must fit in 32 - RESIZE_STAMP_BITS bits.
+     * The counter maintaining number of elements.
      */
-    private static final int MAX_RESIZERS = (1 << (32 - RESIZE_STAMP_BITS)) - 1;
+    private transient final LongAdder8 counter;
 
     /**
-     * The bit shift for recording size stamp in sizeCtl.
+     * Table initialization and resizing control.  When negative, the
+     * table is being initialized or resized. Otherwise, when table is
+     * null, holds the initial table size to use upon creation, or 0
+     * for default. After initialization, holds the next element count
+     * value upon which to resize the table.
      */
-    private static final int RESIZE_STAMP_SHIFT = 32 - RESIZE_STAMP_BITS;
+    private transient volatile int sizeCtl;
+
+    // views
+    private transient KeySetView<K,V> keySet;
+    private transient ValuesView<K,V> values;
+    private transient EntrySetView<K,V> entrySet;
+
+    /** For serialization compatibility. Null unless serialized; see below */
+    private Segment<K,V>[] segments;
+
+    /* ---------------- Table element access -------------- */
 
     /*
-     * Encodings for Node hash fields. See above for explanation.
+     * Volatile access methods are used for table elements as well as
+     * elements of in-progress next table while resizing.  Uses are
+     * null checked by callers, and implicitly bounds-checked, relying
+     * on the invariants that tab arrays have non-zero size, and all
+     * indices are masked with (tab.length - 1) which is never
+     * negative and always less than length. Note that, to be correct
+     * wrt arbitrary concurrency errors by users, bounds checks must
+     * operate on local variables, which accounts for some odd-looking
+     * inline assignments below.
      */
-    static final int MOVED     = 0x8fffffff; // (-1) hash for forwarding nodes
-    static final int TREEBIN   = 0x80000000; // hash for roots of trees
-    static final int RESERVED  = 0x80000001; // hash for transient reservations
-    static final int HASH_BITS = 0x7fffffff; // usable bits of normal node hash
 
-    /** Number of CPUS, to place bounds on some sizings */
-    static final int NCPU = Runtime.getRuntime().availableProcessors();
+    static final Node tabAt(Node[] tab, int i) { // used by Iter
+        return (Node)UNSAFE.getObjectVolatile(tab, ((long)i<<ASHIFT)+ABASE);
+    }
+
+    private static final boolean casTabAt(Node[] tab, int i, Node c, Node v) {
+        return UNSAFE.compareAndSwapObject(tab, ((long)i<<ASHIFT)+ABASE, c, v);
+    }
 
-    /** For serialization compatibility. */
-    private static final ObjectStreamField[] serialPersistentFields = {
-        new ObjectStreamField("segments", Segment[].class),
-        new ObjectStreamField("segmentMask", Integer.TYPE),
-        new ObjectStreamField("segmentShift", Integer.TYPE)
-    };
+    private static final void setTabAt(Node[] tab, int i, Node v) {
+        UNSAFE.putObjectVolatile(tab, ((long)i<<ASHIFT)+ABASE, v);
+    }
 
     /* ---------------- Nodes -------------- */
 
     /**
-     * Key-value entry.  This class is never exported out as a
-     * user-mutable Map.Entry (i.e., one supporting setValue; see
-     * MapEntry below), but can be used for read-only traversals used
-     * in bulk tasks.  Subclasses of Node with a negative hash field
-     * are special, and contain null keys and values (but are never
-     * exported).  Otherwise, keys and vals are never null.
+     * Key-value entry. Note that this is never exported out as a
+     * user-visible Map.Entry (see MapEntry below). Nodes with a hash
+     * field of MOVED are special, and do not contain user keys or
+     * values.  Otherwise, keys are never null, and null val fields
+     * indicate that a node is in the process of being deleted or
+     * created. For purposes of read-only access, a key may be read
+     * before a val, but can only be used after checking val to be
+     * non-null.
      */
-    static class Node<K,V> implements Map.Entry<K,V> {
-        final int hash;
-        final K key;
-        volatile V val;
-        Node<K,V> next;
+    static class Node {
+        volatile int hash;
+        final Object key;
+        volatile Object val;
+        volatile Node next;
 
-        Node(int hash, K key, V val, Node<K,V> next) {
+        Node(int hash, Object key, Object val, Node next) {
             this.hash = hash;
             this.key = key;
             this.val = val;
             this.next = next;
         }
 
-        public final K getKey()       { return key; }
-        public final V getValue()     { return val; }
-        public final int hashCode()   { return key.hashCode() ^ val.hashCode(); }
-        public final String toString(){ return key + "=" + val; }
-        public final V setValue(V value) {
-            throw new UnsupportedOperationException();
+        /** CompareAndSet the hash field */
+        final boolean casHash(int cmp, int val) {
+            return UNSAFE.compareAndSwapInt(this, hashOffset, cmp, val);
         }
 
-        public final boolean equals(Object o) {
-            Object k, v, u; Map.Entry<?,?> e;
-            return ((o instanceof Map.Entry) &&
-                (k = (e = (Map.Entry<?,?>)o).getKey()) != null &&
-                (v = e.getValue()) != null &&
-                (k == key || k.equals(key)) &&
-                (v == (u = val) || v.equals(u)));
-        }
+        /** The number of spins before blocking for a lock */
+        static final int MAX_SPINS =
+            Runtime.getRuntime().availableProcessors() > 1 ? 64 : 1;
 
         /**
-         * Virtualized support for map.get(); overridden in subclasses.
+         * Spins a while if LOCKED bit set and this node is the first
+         * of its bin, and then sets WAITING bits on hash field and
+         * blocks (once) if they are still set.  It is OK for this
+         * method to return even if lock is not available upon exit,
+         * which enables these simple single-wait mechanics.
+         *
+         * The corresponding signalling operation is performed within
+         * callers: Upon detecting that WAITING has been set when
+         * unlocking lock (via a failed CAS from non-waiting LOCKED
+         * state), unlockers acquire the sync lock and perform a
+         * notifyAll.
+         *
+         * The initial sanity check on tab and bounds is not currently
+         * necessary in the only usages of this method, but enables
+         * use in other future contexts.
          */
-        Node<K,V> find(int h, Object k) {
-            Node<K,V> e = this;
-            if (k != null) {
-                do {
-                    K ek;
-                    if (e.hash == h &&
-                        ((ek = e.key) == k || (ek != null && k.equals(ek))))
-                        return e;
-                } while ((e = e.next) != null);
+        final void tryAwaitLock(Node[] tab, int i) {
+            if (tab != null && i >= 0 && i < tab.length) { // sanity check
+                int r = ThreadLocalRandom8.current().nextInt(); // randomize spins
+                int spins = MAX_SPINS, h;
+                while (tabAt(tab, i) == this && ((h = hash) & LOCKED) != 0) {
+                    if (spins >= 0) {
+                        r ^= r << 1; r ^= r >>> 3; r ^= r << 10; // xorshift
+                        if (r >= 0 && --spins == 0)
+                            Thread.yield();  // yield before block
+                    }
+                    else if (casHash(h, h | WAITING)) {
+                        synchronized (this) {
+                            if (tabAt(tab, i) == this &&
+                                (hash & WAITING) == WAITING) {
+                                try {
+                                    wait();
+                                } catch (InterruptedException ie) {
+                                    try {
+                                        Thread.currentThread().interrupt();
+                                    } catch (SecurityException ignore) {
+                                    }
+                                }
+                            }
+                            else
+                                notifyAll(); // possibly won race vs signaller
+                        }
+                        break;
+                    }
+                }
             }
-            return null;
         }
-    }
 
-    /* ---------------- Static utilities -------------- */
-
-    /**
-     * Spreads (XORs) higher bits of hash to lower and also forces top
-     * bit to 0. Because the table uses power-of-two masking, sets of
-     * hashes that vary only in bits above the current mask will
-     * always collide. (Among known examples are sets of Float keys
-     * holding consecutive whole numbers in small tables.)  So we
-     * apply a transform that spreads the impact of higher bits
-     * downward. There is a tradeoff between speed, utility, and
-     * quality of bit-spreading. Because many common sets of hashes
-     * are already reasonably distributed (so don't benefit from
-     * spreading), and because we use trees to handle large sets of
-     * collisions in bins, we just XOR some shifted bits in the
-     * cheapest possible way to reduce systematic lossage, as well as
-     * to incorporate impact of the highest bits that would otherwise
-     * never be used in index calculations because of table bounds.
-     */
-    static final int spread(int h) {
-        return (h ^ (h >>> 16)) & HASH_BITS;
-    }
+        // Unsafe mechanics for casHash
+        private static final sun.misc.Unsafe UNSAFE;
+        private static final long hashOffset;
 
-    /**
-     * Returns a power of two table size for the given desired capacity.
-     * See Hackers Delight, sec 3.2
-     */
-    private static final int tableSizeFor(int c) {
-        int n = c - 1;
-        n |= n >>> 1;
-        n |= n >>> 2;
-        n |= n >>> 4;
-        n |= n >>> 8;
-        n |= n >>> 16;
-        return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
-    }
-
-    /**
-     * Returns x's Class if it is of the form "class C implements
-     * Comparable<C>", else null.
-     */
-    static Class<?> comparableClassFor(Object x) {
-        if (x instanceof Comparable) {
-            Class<?> c; Type[] ts, as; Type t; ParameterizedType p;
-            if ((c = x.getClass()) == String.class) // bypass checks
-                return c;
-            if ((ts = c.getGenericInterfaces()) != null) {
-                for (int i = 0; i < ts.length; ++i) {
-                    if (((t = ts[i]) instanceof ParameterizedType) &&
-                        ((p = (ParameterizedType)t).getRawType() ==
-                            Comparable.class) &&
-                        (as = p.getActualTypeArguments()) != null &&
-                        as.length == 1 && as[0] == c) // type arg is c
-                        return c;
-                }
+        static {
+            try {
+                UNSAFE = getUnsafe();
+                Class<?> k = Node.class;
+                hashOffset = UNSAFE.objectFieldOffset
+                    (k.getDeclaredField("hash"));
+            } catch (Exception e) {
+                throw new Error(e);
             }
         }
-        return null;
-    }
-
-    /**
-     * Returns k.compareTo(x) if x matches kc (k's screened comparable
-     * class), else 0.
-     */
-    @SuppressWarnings({"rawtypes","unchecked"}) // for cast to Comparable
-    static int compareComparables(Class<?> kc, Object k, Object x) {
-        return (x == null || x.getClass() != kc ? 0 :
-            ((Comparable)k).compareTo(x));
-    }
-
-    /* ---------------- Table element access -------------- */
-
-    /*
-     * Volatile access methods are used for table elements as well as
-     * elements of in-progress next table while resizing.  All uses of
-     * the tab arguments must be null checked by callers.  All callers
-     * also paranoically precheck that tab's length is not zero (or an
-     * equivalent check), thus ensuring that any index argument taking
-     * the form of a hash value anded with (length - 1) is a valid
-     * index.  Note that, to be correct wrt arbitrary concurrency
-     * errors by users, these checks must operate on local variables,
-     * which accounts for some odd-looking inline assignments below.
-     * Note that calls to setTabAt always occur within locked regions,
-     * and so do not need full volatile semantics, but still require
-     * ordering to maintain concurrent readability.
-     */
-
-    @SuppressWarnings("unchecked")
-    static final <K,V> Node<K,V> tabAt(Node<K,V>[] tab, int i) {
-        return (Node<K,V>)U.getObjectVolatile(tab, ((long)i << ASHIFT) + ABASE);
-    }
-
-    static final <K,V> boolean casTabAt(Node<K,V>[] tab, int i,
-                                        Node<K,V> c, Node<K,V> v) {
-        return U.compareAndSwapObject(tab, ((long)i << ASHIFT) + ABASE, c, v);
     }
 
-    static final <K,V> void setTabAt(Node<K,V>[] tab, int i, Node<K,V> v) {
-        U.putOrderedObject(tab, ((long)i << ASHIFT) + ABASE, v);
-    }
-
-    /* ---------------- Fields -------------- */
-
-    /**
-     * The array of bins. Lazily initialized upon first insertion.
-     * Size is always a power of two. Accessed directly by iterators.
-     */
-    transient volatile Node<K,V>[] table;
+    /* ---------------- TreeBins -------------- */
 
     /**
-     * The next table to use; non-null only while resizing.
+     * Nodes for use in TreeBins
      */
-    private transient volatile Node<K,V>[] nextTable;
+    static final class TreeNode extends Node {
+        TreeNode parent;  // red-black tree links
+        TreeNode left;
+        TreeNode right;
+        TreeNode prev;    // needed to unlink next upon deletion
+        boolean red;
 
-    /**
-     * Base counter value, used mainly when there is no contention,
-     * but also as a fallback during table initialization
-     * races. Updated via CAS.
-     */
-    private transient volatile long baseCount;
+        TreeNode(int hash, Object key, Object val, Node next, TreeNode parent) {
+            super(hash, key, val, next);
+            this.parent = parent;
+        }
+    }
 
     /**
-     * Table initialization and resizing control.  When negative, the
-     * table is being initialized or resized: -1 for initialization,
-     * else -(1 + the number of active resizing threads).  Otherwise,
-     * when table is null, holds the initial table size to use upon
-     * creation, or 0 for default. After initialization, holds the
-     * next element count value upon which to resize the table.
-     */
-    private transient volatile int sizeCtl;
+     * A specialized form of red-black tree for use in bins
+     * whose size exceeds a threshold.
+     *
+     * TreeBins use a special form of comparison for search and
+     * related operations (which is the main reason we cannot use
+     * existing collections such as TreeMaps). TreeBins contain
+     * Comparable elements, but may contain others, as well as
+     * elements that are Comparable but not necessarily Comparable<T>
+     * for the same T, so we cannot invoke compareTo among them. To
+     * handle this, the tree is ordered primarily by hash value, then
+     * by getClass().getName() order, and then by Comparator order
+     * among elements of the same class.  On lookup at a node, if
+     * elements are not comparable or compare as 0, both left and
+     * right children may need to be searched in the case of tied hash
+     * values. (This corresponds to the full list search that would be
+     * necessary if all elements were non-Comparable and had tied
+     * hashes.)  The red-black balancing code is updated from
+     * pre-jdk-collections
+     * (http://gee.cs.oswego.edu/dl/classes/collections/RBCell.java)
+     * based in turn on Cormen, Leiserson, and Rivest "Introduction to
+     * Algorithms" (CLR).
+     *
+     * TreeBins also maintain a separate locking discipline than
+     * regular bins. Because they are forwarded via special MOVED
+     * nodes at bin heads (which can never change once established),
+     * we cannot use those nodes as locks. Instead, TreeBin
+     * extends AbstractQueuedSynchronizer to support a simple form of
+     * read-write lock. For update operations and table validation,
+     * the exclusive form of lock behaves in the same way as bin-head
+     * locks. However, lookups use shared read-lock mechanics to allow
+     * multiple readers in the absence of writers.  Additionally,
+     * these lookups do not ever block: While the lock is not
+     * available, they proceed along the slow traversal path (via
+     * next-pointers) until the lock becomes available or the list is
+     * exhausted, whichever comes first. (These cases are not fast,
+     * but maximize aggregate expected throughput.)  The AQS mechanics
+     * for doing this are straightforward.  The lock state is held as
+     * AQS getState().  Read counts are negative; the write count (1)
+     * is positive.  There are no signalling preferences among readers
+     * and writers. Since we don't need to export full Lock API, we
+     * just override the minimal AQS methods and use them directly.
+     */
+    static final class TreeBin extends AbstractQueuedSynchronizer {
+        private static final long serialVersionUID = 2249069246763182397L;
+        transient TreeNode root;  // root of tree
+        transient TreeNode first; // head of next-pointer list
+
+        /* AQS overrides */
+        public final boolean isHeldExclusively() { return getState() > 0; }
+        public final boolean tryAcquire(int ignore) {
+            if (compareAndSetState(0, 1)) {
+                setExclusiveOwnerThread(Thread.currentThread());
+                return true;
+            }
+            return false;
+        }
+        public final boolean tryRelease(int ignore) {
+            setExclusiveOwnerThread(null);
+            setState(0);
+            return true;
+        }
+        public final int tryAcquireShared(int ignore) {
+            for (int c;;) {
+                if ((c = getState()) > 0)
+                    return -1;
+                if (compareAndSetState(c, c -1))
+                    return 1;
+            }
+        }
+        public final boolean tryReleaseShared(int ignore) {
+            int c;
+            do {} while (!compareAndSetState(c = getState(), c + 1));
+            return c == -1;
+        }
 
-    /**
-     * The next table index (plus one) to split while resizing.
-     */
-    private transient volatile int transferIndex;
+        /** From CLR */
+        private void rotateLeft(TreeNode p) {
+            if (p != null) {
+                TreeNode r = p.right, pp, rl;
+                if ((rl = p.right = r.left) != null)
+                    rl.parent = p;
+                if ((pp = r.parent = p.parent) == null)
+                    root = r;
+                else if (pp.left == p)
+                    pp.left = r;
+                else
+                    pp.right = r;
+                r.left = p;
+                p.parent = r;
+            }
+        }
 
-    /**
-     * Spinlock (locked via CAS) used when resizing and/or creating CounterCells.
-     */
-    private transient volatile int cellsBusy;
+        /** From CLR */
+        private void rotateRight(TreeNode p) {
+            if (p != null) {
+                TreeNode l = p.left, pp, lr;
+                if ((lr = p.left = l.right) != null)
+                    lr.parent = p;
+                if ((pp = l.parent = p.parent) == null)
+                    root = l;
+                else if (pp.right == p)
+                    pp.right = l;
+                else
+                    pp.left = l;
+                l.right = p;
+                p.parent = l;
+            }
+        }
 
-    /**
-     * Table of counter cells. When non-null, size is a power of 2.
-     */
-    private transient volatile CounterCell[] counterCells;
+        /**
+         * Returns the TreeNode (or null if not found) for the given key
+         * starting at given root.
+         */
+        @SuppressWarnings("unchecked") final TreeNode getTreeNode
+        (int h, Object k, TreeNode p) {
+            Class<?> c = k.getClass();
+            while (p != null) {
+                int dir, ph;  Object pk; Class<?> pc;
+                if ((ph = p.hash) == h) {
+                    if ((pk = p.key) == k || k.equals(pk))
+                        return p;
+                    if (c != (pc = pk.getClass()) ||
+                        !(k instanceof Comparable) ||
+                        (dir = ((Comparable)k).compareTo((Comparable)pk)) == 0) {
+                        dir = (c == pc) ? 0 : c.getName().compareTo(pc.getName());
+                        TreeNode r = null, s = null, pl, pr;
+                        if (dir >= 0) {
+                            if ((pl = p.left) != null && h <= pl.hash)
+                                s = pl;
+                        }
+                        else if ((pr = p.right) != null && h >= pr.hash)
+                            s = pr;
+                        if (s != null && (r = getTreeNode(h, k, s)) != null)
+                            return r;
+                    }
+                }
+                else
+                    dir = (h < ph) ? -1 : 1;
+                p = (dir > 0) ? p.right : p.left;
+            }
+            return null;
+        }
 
-    // views
-    private transient KeySetView<K,V> keySet;
-    private transient ValuesView<K,V> values;
-    private transient EntrySetView<K,V> entrySet;
+        /**
+         * Wrapper for getTreeNode used by CHM.get. Tries to obtain
+         * read-lock to call getTreeNode, but during failure to get
+         * lock, searches along next links.
+         */
+        final Object getValue(int h, Object k) {
+            Node r = null;
+            int c = getState(); // Must read lock state first
+            for (Node e = first; e != null; e = e.next) {
+                if (c <= 0 && compareAndSetState(c, c - 1)) {
+                    try {
+                        r = getTreeNode(h, k, root);
+                    } finally {
+                        releaseShared(0);
+                    }
+                    break;
+                }
+                else if ((e.hash & HASH_BITS) == h && k.equals(e.key)) {
+                    r = e;
+                    break;
+                }
+                else
+                    c = getState();
+            }
+            return r == null ? null : r.val;
+        }
 
+        /**
+         * Finds or adds a node.
+         * @return null if added
+         */
+        @SuppressWarnings("unchecked") final TreeNode putTreeNode
+        (int h, Object k, Object v) {
+            Class<?> c = k.getClass();
+            TreeNode pp = root, p = null;
+            int dir = 0;
+            while (pp != null) { // find existing node or leaf to insert at
+                int ph;  Object pk; Class<?> pc;
+                p = pp;
+                if ((ph = p.hash) == h) {
+                    if ((pk = p.key) == k || k.equals(pk))
+                        return p;
+                    if (c != (pc = pk.getClass()) ||
+                        !(k instanceof Comparable) ||
+                        (dir = ((Comparable)k).compareTo((Comparable)pk)) == 0) {
+                        dir = (c == pc) ? 0 : c.getName().compareTo(pc.getName());
+                        TreeNode r = null, s = null, pl, pr;
+                        if (dir >= 0) {
+                            if ((pl = p.left) != null && h <= pl.hash)
+                                s = pl;
+                        }
+                        else if ((pr = p.right) != null && h >= pr.hash)
+                            s = pr;
+                        if (s != null && (r = getTreeNode(h, k, s)) != null)
+                            return r;
+                    }
+                }
+                else
+                    dir = (h < ph) ? -1 : 1;
+                pp = (dir > 0) ? p.right : p.left;
+            }
 
-    /* ---------------- Public operations -------------- */
+            TreeNode f = first;
+            TreeNode x = first = new TreeNode(h, k, v, f, p);
+            if (p == null)
+                root = x;
+            else { // attach and rebalance; adapted from CLR
+                TreeNode xp, xpp;
+                if (f != null)
+                    f.prev = x;
+                if (dir <= 0)
+                    p.left = x;
+                else
+                    p.right = x;
+                x.red = true;
+                while (x != null && (xp = x.parent) != null && xp.red &&
+                    (xpp = xp.parent) != null) {
+                    TreeNode xppl = xpp.left;
+                    if (xp == xppl) {
+                        TreeNode y = xpp.right;
+                        if (y != null && y.red) {
+                            y.red = false;
+                            xp.red = false;
+                            xpp.red = true;
+                            x = xpp;
+                        }
+                        else {
+                            if (x == xp.right) {
+                                rotateLeft(x = xp);
+                                xpp = (xp = x.parent) == null ? null : xp.parent;
+                            }
+                            if (xp != null) {
+                                xp.red = false;
+                                if (xpp != null) {
+                                    xpp.red = true;
+                                    rotateRight(xpp);
+                                }
+                            }
+                        }
+                    }
+                    else {
+                        TreeNode y = xppl;
+                        if (y != null && y.red) {
+                            y.red = false;
+                            xp.red = false;
+                            xpp.red = true;
+                            x = xpp;
+                        }
+                        else {
+                            if (x == xp.left) {
+                                rotateRight(x = xp);
+                                xpp = (xp = x.parent) == null ? null : xp.parent;
+                            }
+                            if (xp != null) {
+                                xp.red = false;
+                                if (xpp != null) {
+                                    xpp.red = true;
+                                    rotateLeft(xpp);
+                                }
+                            }
+                        }
+                    }
+                }
+                TreeNode r = root;
+                if (r != null && r.red)
+                    r.red = false;
+            }
+            return null;
+        }
 
-    /**
-     * Creates a new, empty map with the default initial table size (16).
-     */
-    public ConcurrentHashMap8() {
-    }
-
-    /**
-     * Creates a new, empty map with an initial table size
-     * accommodating the specified number of elements without the need
-     * to dynamically resize.
-     *
-     * @param initialCapacity The implementation performs internal
-     * sizing to accommodate this many elements.
-     * @throws IllegalArgumentException if the initial capacity of
-     * elements is negative
-     */
-    public ConcurrentHashMap8(int initialCapacity) {
-        if (initialCapacity < 0)
-            throw new IllegalArgumentException();
-        int cap = ((initialCapacity >= (MAXIMUM_CAPACITY >>> 1)) ?
-            MAXIMUM_CAPACITY :
-            tableSizeFor(initialCapacity + (initialCapacity >>> 1) + 1));
-        this.sizeCtl = cap;
-    }
-
-    /**
-     * Creates a new map with the same mappings as the given map.
-     *
-     * @param m the map
-     */
-    public ConcurrentHashMap8(Map<? extends K, ? extends V> m) {
-        this.sizeCtl = DEFAULT_CAPACITY;
-        putAll(m);
+        /**
+         * Removes the given node, that must be present before this
+         * call.  This is messier than typical red-black deletion code
+         * because we cannot swap the contents of an interior node
+         * with a leaf successor that is pinned by "next" pointers
+         * that are accessible independently of lock. So instead we
+         * swap the tree linkages.
+         */
+        final void deleteTreeNode(TreeNode p) {
+            TreeNode next = (TreeNode)p.next; // unlink traversal pointers
+            TreeNode pred = p.prev;
+            if (pred == null)
+                first = next;
+            else
+                pred.next = next;
+            if (next != null)
+                next.prev = pred;
+            TreeNode replacement;
+            TreeNode pl = p.left;
+            TreeNode pr = p.right;
+            if (pl != null && pr != null) {
+                TreeNode s = pr, sl;
+                while ((sl = s.left) != null) // find successor
+                    s = sl;
+                boolean c = s.red; s.red = p.red; p.red = c; // swap colors
+                TreeNode sr = s.right;
+                TreeNode pp = p.parent;
+                if (s == pr) { // p was s's direct parent
+                    p.parent = s;
+                    s.right = p;
+                }
+                else {
+                    TreeNode sp = s.parent;
+                    if ((p.parent = sp) != null) {
+                        if (s == sp.left)
+                            sp.left = p;
+                        else
+                            sp.right = p;
+                    }
+                    if ((s.right = pr) != null)
+                        pr.parent = s;
+                }
+                p.left = null;
+                if ((p.right = sr) != null)
+                    sr.parent = p;
+                if ((s.left = pl) != null)
+                    pl.parent = s;
+                if ((s.parent = pp) == null)
+                    root = s;
+                else if (p == pp.left)
+                    pp.left = s;
+                else
+                    pp.right = s;
+                replacement = sr;
+            }
+            else
+                replacement = (pl != null) ? pl : pr;
+            TreeNode pp = p.parent;
+            if (replacement == null) {
+                if (pp == null) {
+                    root = null;
+                    return;
+                }
+                replacement = p;
+            }
+            else {
+                replacement.parent = pp;
+                if (pp == null)
+                    root = replacement;
+                else if (p == pp.left)
+                    pp.left = replacement;
+                else
+                    pp.right = replacement;
+                p.left = p.right = p.parent = null;
+            }
+            if (!p.red) { // rebalance, from CLR
+                TreeNode x = replacement;
+                while (x != null) {
+                    TreeNode xp, xpl;
+                    if (x.red || (xp = x.parent) == null) {
+                        x.red = false;
+                        break;
+                    }
+                    if (x == (xpl = xp.left)) {
+                        TreeNode sib = xp.right;
+                        if (sib != null && sib.red) {
+                            sib.red = false;
+                            xp.red = true;
+                            rotateLeft(xp);
+                            sib = (xp = x.parent) == null ? null : xp.right;
+                        }
+                        if (sib == null)
+                            x = xp;
+                        else {
+                            TreeNode sl = sib.left, sr = sib.right;
+                            if ((sr == null || !sr.red) &&
+                                (sl == null || !sl.red)) {
+                                sib.red = true;
+                                x = xp;
+                            }
+                            else {
+                                if (sr == null || !sr.red) {
+                                    if (sl != null)
+                                        sl.red = false;
+                                    sib.red = true;
+                                    rotateRight(sib);
+                                    sib = (xp = x.parent) == null ? null : xp.right;
+                                }
+                                if (sib != null) {
+                                    sib.red = (xp == null) ? false : xp.red;
+                                    if ((sr = sib.right) != null)
+                                        sr.red = false;
+                                }
+                                if (xp != null) {
+                                    xp.red = false;
+                                    rotateLeft(xp);
+                                }
+                                x = root;
+                            }
+                        }
+                    }
+                    else { // symmetric
+                        TreeNode sib = xpl;
+                        if (sib != null && sib.red) {
+                            sib.red = false;
+                            xp.red = true;
+                            rotateRight(xp);
+                            sib = (xp = x.parent) == null ? null : xp.left;
+                        }
+                        if (sib == null)
+                            x = xp;
+                        else {
+                            TreeNode sl = sib.left, sr = sib.right;
+                            if ((sl == null || !sl.red) &&
+                                (sr == null || !sr.red)) {
+                                sib.red = true;
+                                x = xp;
+                            }
+                            else {
+                                if (sl == null || !sl.red) {
+                                    if (sr != null)
+                                        sr.red = false;
+                                    sib.red = true;
+                                    rotateLeft(sib);
+                                    sib = (xp = x.parent) == null ? null : xp.left;
+                                }
+                                if (sib != null) {
+                                    sib.red = (xp == null) ? false : xp.red;
+                                    if ((sl = sib.left) != null)
+                                        sl.red = false;
+                                }
+                                if (xp != null) {
+                                    xp.red = false;
+                                    rotateRight(xp);
+                                }
+                                x = root;
+                            }
+                        }
+                    }
+                }
+            }
+            if (p == replacement && (pp = p.parent) != null) {
+                if (p == pp.left) // detach pointers
+                    pp.left = null;
+                else if (p == pp.right)
+                    pp.right = null;
+                p.parent = null;
+            }
+        }
     }
 
-    /**
-     * Creates a new, empty map with an initial table size based on
-     * the given number of elements ({@code initialCapacity}) and
-     * initial table density ({@code loadFactor}).
-     *
-     * @param initialCapacity the initial capacity. The implementation
-     * performs internal sizing to accommodate this many elements,
-     * given the specified load factor.
-     * @param loadFactor the load factor (table density) for
-     * establishing the initial table size
-     * @throws IllegalArgumentException if the initial capacity of
-     * elements is negative or the load factor is nonpositive
-     *
-     * @since 1.6
-     */
-    public ConcurrentHashMap8(int initialCapacity, float loadFactor) {
-        this(initialCapacity, loadFactor, 1);
-    }
+    /* ---------------- Collision reduction methods -------------- */
 
     /**
-     * Creates a new, empty map with an initial table size based on
-     * the given number of elements ({@code initialCapacity}), table
-     * density ({@code loadFactor}), and number of concurrently
-     * updating threads ({@code concurrencyLevel}).
-     *
-     * @param initialCapacity the initial capacity. The implementation
-     * performs internal sizing to accommodate this many elements,
-     * given the specified load factor.
-     * @param loadFactor the load factor (table density) for
-     * establishing the initial table size
-     * @param concurrencyLevel the estimated number of concurrently
-     * updating threads. The implementation may use this value as
-     * a sizing hint.
-     * @throws IllegalArgumentException if the initial capacity is
-     * negative or the load factor or concurrencyLevel are
-     * nonpositive
+     * Spreads higher bits to lower, and also forces top 2 bits to 0.
+     * Because the table uses power-of-two masking, sets of hashes
+     * that vary only in bits above the current mask will always
+     * collide. (Among known examples are sets of Float keys holding
+     * consecutive whole numbers in small tables.)  To counter this,
+     * we apply a transform that spreads the impact of higher bits
+     * downward. There is a tradeoff between speed, utility, and
+     * quality of bit-spreading. Because many common sets of hashes
+     * are already reasonably distributed across bits (so don't benefit
+     * from spreading), and because we use trees to handle large sets
+     * of collisions in bins, we don't need excessively high quality.
      */
-    public ConcurrentHashMap8(int initialCapacity,
-                             float loadFactor, int concurrencyLevel) {
-        if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0)
-            throw new IllegalArgumentException();
-        if (initialCapacity < concurrencyLevel)   // Use at least as many bins
-            initialCapacity = concurrencyLevel;   // as estimated threads
-        long size = (long)(1.0 + (long)initialCapacity / loadFactor);
-        int cap = (size >= (long)MAXIMUM_CAPACITY) ?
-            MAXIMUM_CAPACITY : tableSizeFor((int)size);
-        this.sizeCtl = cap;
+    private static final int spread(int h) {
+        h ^= (h >>> 18) ^ (h >>> 12);
+        return (h ^ (h >>> 10)) & HASH_BITS;
     }
 
-    // Original (since JDK1.2) Map methods
-
     /**
-     * {@inheritDoc}
+     * Replaces a list bin with a tree bin. Call only when locked.
+     * Fails to replace if the given key is non-comparable or table
+     * is, or needs, resizing.
      */
-    public int size() {
-        long n = sumCount();
-        return ((n < 0L) ? 0 :
-            (n > (long)Integer.MAX_VALUE) ? Integer.MAX_VALUE :
-                (int)n);
+    private final void replaceWithTreeBin(Node[] tab, int index, Object key) {
+        if ((key instanceof Comparable) &&
+            (tab.length >= MAXIMUM_CAPACITY || counter.sum() < (long)sizeCtl)) {
+            TreeBin t = new TreeBin();
+            for (Node e = tabAt(tab, index); e != null; e = e.next)
+                t.putTreeNode(e.hash & HASH_BITS, e.key, e.val);
+            setTabAt(tab, index, new Node(MOVED, t, null, null));
+        }
     }
 
-    /**
-     * {@inheritDoc}
-     */
-    public boolean isEmpty() {
-        return sumCount() <= 0L; // ignore transient negative values
-    }
+    /* ---------------- Internal access and update methods -------------- */
 
-    /**
-     * Returns the value to which the specified key is mapped,
-     * or {@code null} if this map contains no mapping for the key.
-     *
-     * <p>More formally, if this map contains a mapping from a key
-     * {@code k} to a value {@code v} such that {@code key.equals(k)},
-     * then this method returns {@code v}; otherwise it returns
-     * {@code null}.  (There can be at most one such mapping.)
-     *
-     * @throws NullPointerException if the specified key is null
-     */
-    public V get(Object key) {
-        Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
-        int h = spread(key.hashCode());
-        if ((tab = table) != null && (n = tab.length) > 0 &&
-            (e = tabAt(tab, (n - 1) & h)) != null) {
-            if ((eh = e.hash) == h) {
-                if ((ek = e.key) == key || (ek != null && key.equals(ek)))
-                    return e.val;
-            }
-            else if (eh < 0)
-                return (p = e.find(h, key)) != null ? p.val : null;
-            while ((e = e.next) != null) {
-                if (e.hash == h &&
-                    ((ek = e.key) == key || (ek != null && key.equals(ek))))
-                    return e.val;
+    /** Implementation for get and containsKey */
+    private final Object internalGet(Object k) {
+        int h = spread(k.hashCode());
+        retry: for (Node[] tab = table; tab != null;) {
+            Node e, p; Object ek, ev; int eh;      // locals to read fields once
+            for (e = tabAt(tab, (tab.length - 1) & h); e != null; e = e.next) {
+                if ((eh = e.hash) == MOVED) {
+                    if ((ek = e.key) instanceof TreeBin)  // search TreeBin
+                        return ((TreeBin)ek).getValue(h, k);
+                    else {                        // restart with new table
+                        tab = (Node[])ek;
+                        continue retry;
+                    }
+                }
+                else if ((eh & HASH_BITS) == h && (ev = e.val) != null &&
+                    ((ek = e.key) == k || k.equals(ek)))
+                    return ev;
             }
+            break;
         }
         return null;
     }
 
     /**
-     * Tests if the specified object is a key in this table.
-     *
-     * @param  key possible key
-     * @return {@code true} if and only if the specified object
-     *         is a key in this table, as determined by the
-     *         {@code equals} method; {@code false} otherwise
-     * @throws NullPointerException if the specified key is null
-     */
-    public boolean containsKey(Object key) {
-        return get(key) != null;
-    }
-
-    /**
-     * Returns {@code true} if this map maps one or more keys to the
-     * specified value. Note: This method may require a full traversal
-     * of the map, and is much slower than method {@code containsKey}.
-     *
-     * @param value value whose presence in this map is to be tested
-     * @return {@code true} if this map maps one or more keys to the
-     *         specified value
-     * @throws NullPointerException if the specified value is null
+     * Implementation for the four public remove/replace methods:
+     * Replaces node value with v, conditional upon match of cv if
+     * non-null.  If resulting value is null, delete.
      */
-    public boolean containsValue(Object value) {
-        if (value == null)
-            throw new NullPointerException();
-        Node<K,V>[] t;
-        if ((t = table) != null) {
-            Traverser<K,V> it = new Traverser<K,V>(t, t.length, 0, t.length);
-            for (Node<K,V> p; (p = it.advance()) != null; ) {
-                V v;
-                if ((v = p.val) == value || (v != null && value.equals(v)))
-                    return true;
+    private final Object internalReplace(Object k, Object v, Object cv) {
+        int h = spread(k.hashCode());
+        Object oldVal = null;
+        for (Node[] tab = table;;) {
+            Node f; int i, fh; Object fk;
+            if (tab == null ||
+                (f = tabAt(tab, i = (tab.length - 1) & h)) == null)
+                break;
+            else if ((fh = f.hash) == MOVED) {
+                if ((fk = f.key) instanceof TreeBin) {
+                    TreeBin t = (TreeBin)fk;
+                    boolean validated = false;
+                    boolean deleted = false;
+                    t.acquire(0);
+                    try {
+                        if (tabAt(tab, i) == f) {
+                            validated = true;
+                            TreeNode p = t.getTreeNode(h, k, t.root);
+                            if (p != null) {
+                                Object pv = p.val;
+                                if (cv == null || cv == pv || cv.equals(pv)) {
+                                    oldVal = pv;
+                                    if ((p.val = v) == null) {
+                                        deleted = true;
+                                        t.deleteTreeNode(p);
+                                    }
+                                }
+                            }
+                        }
+                    } finally {
+                        t.release(0);
+                    }
+                    if (validated) {
+                        if (deleted)
+                            counter.add(-1L);
+                        break;
+                    }
+                }
+                else
+                    tab = (Node[])fk;
+            }
+            else if ((fh & HASH_BITS) != h && f.next == null) // precheck
+                break;                          // rules out possible existence
+            else if ((fh & LOCKED) != 0) {
+                checkForResize();               // try resizing if can't get lock
+                f.tryAwaitLock(tab, i);
+            }
+            else if (f.casHash(fh, fh | LOCKED)) {
+                boolean validated = false;
+                boolean deleted = false;
+                try {
+                    if (tabAt(tab, i) == f) {
+                        validated = true;
+                        for (Node e = f, pred = null;;) {
+                            Object ek, ev;
+                            if ((e.hash & HASH_BITS) == h &&
+                                ((ev = e.val) != null) &&
+                                ((ek = e.key) == k || k.equals(ek))) {
+                                if (cv == null || cv == ev || cv.equals(ev)) {
+                                    oldVal = ev;
+                                    if ((e.val = v) == null) {
+                                        deleted = true;
+                                        Node en = e.next;
+                                        if (pred != null)
+                                            pred.next = en;
+                                        else
+                                            setTabAt(tab, i, en);
+                                    }
+                                }
+                                break;
+                            }
+                            pred = e;
+                            if ((e = e.next) == null)
+                                break;
+                        }
+                    }
+                } finally {
+                    if (!f.casHash(fh | LOCKED, fh)) {
+                        f.hash = fh;
+                        synchronized (f) { f.notifyAll(); };
+                    }
+                }
+                if (validated) {
+                    if (deleted)
+                        counter.add(-1L);
+                    break;
+                }
             }
         }
-        return false;
+        return oldVal;
     }
 
-    /**
-     * Maps the specified key to the specified value in this table.
-     * Neither the key nor the value can be null.
+    /*
+     * Internal versions of the six insertion methods, each a
+     * little more complicated than the last. All have
+     * the same basic structure as the first (internalPut):
+     *  1. If table uninitialized, create
+     *  2. If bin empty, try to CAS new node
+     *  3. If bin stale, use new table
+     *  4. if bin converted to TreeBin, validate and relay to TreeBin methods
+     *  5. Lock and validate; if valid, scan and add or update
      *
-     * <p>The value can be retrieved by calling the {@code get} method
-     * with a key that is equal to the original key.
+     * The others interweave other checks and/or alternative actions:
+     *  * Plain put checks for and performs resize after insertion.
+     *  * putIfAbsent prescans for mapping without lock (and fails to add
+     *    if present), which also makes pre-emptive resize checks worthwhile.
+     *  * computeIfAbsent extends form used in putIfAbsent with additional
+     *    mechanics to deal with, calls, potential exceptions and null
+     *    returns from function call.
+     *  * compute uses the same function-call mechanics, but without
+     *    the prescans
+     *  * merge acts as putIfAbsent in the absent case, but invokes the
+     *    update function if present
+     *  * putAll attempts to pre-allocate enough table space
+     *    and more lazily performs count updates and checks.
      *
-     * @param key key with which the specified value is to be associated
-     * @param value value to be associated with the specified key
-     * @return the previous value associated with {@code key}, or
-     *         {@code null} if there was no mapping for {@code key}
-     * @throws NullPointerException if the specified key or value is null
+     * Someday when details settle down a bit more, it might be worth
+     * some factoring to reduce sprawl.
      */
-    public V put(K key, V value) {
-        return putVal(key, value, false);
-    }
 
-    /** Implementation for put and putIfAbsent */
-    final V putVal(K key, V value, boolean onlyIfAbsent) {
-        if (key == null || value == null) throw new NullPointerException();
-        int hash = spread(key.hashCode());
-        int binCount = 0;
-        for (Node<K,V>[] tab = table;;) {
-            Node<K,V> f; int n, i, fh;
-            if (tab == null || (n = tab.length) == 0)
+    /** Implementation for put */
+    private final Object internalPut(Object k, Object v) {
+        int h = spread(k.hashCode());
+        int count = 0;
+        for (Node[] tab = table;;) {
+            int i; Node f; int fh; Object fk;
+            if (tab == null)
                 tab = initTable();
-            else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
-                if (casTabAt(tab, i, null,
-                             new Node<K,V>(hash, key, value, null)))
+            else if ((f = tabAt(tab, i = (tab.length - 1) & h)) == null) {
+                if (casTabAt(tab, i, null, new Node(h, k, v, null)))
                     break;                   // no lock when adding to empty bin
             }
-            else if ((fh = f.hash) == MOVED)
-                tab = helpTransfer(tab, f);
-            else {
-                V oldVal = null;
-                synchronized (f) {
-                    if (tabAt(tab, i) == f) {
-                        if (fh >= 0) {
-                            binCount = 1;
-                            for (Node<K,V> e = f;; ++binCount) {
-                                K ek;
-                                if (e.hash == hash &&
-                                    ((ek = e.key) == key ||
-                                        (ek != null && key.equals(ek)))) {
-                                    oldVal = e.val;
-                                    if (!onlyIfAbsent)
-                                        e.val = value;
-                                    break;
-                                }
-                                Node<K,V> pred = e;
-                                if ((e = e.next) == null) {
-                                    pred.next = new Node<K,V>(hash, key,
-                                                              value, null);
-                                    break;
-                                }
+            else if ((fh = f.hash) == MOVED) {
+                if ((fk = f.key) instanceof TreeBin) {
+                    TreeBin t = (TreeBin)fk;
+                    Object oldVal = null;
+                    t.acquire(0);
+                    try {
+                        if (tabAt(tab, i) == f) {
+                            count = 2;
+                            TreeNode p = t.putTreeNode(h, k, v);
+                            if (p != null) {
+                                oldVal = p.val;
+                                p.val = v;
                             }
                         }
-                        else if (f instanceof TreeBin) {
-                            Node<K,V> p;
-                            binCount = 2;
-                            if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,
-                                                                  value)) != null) {
-                                oldVal = p.val;
-                                if (!onlyIfAbsent)
-                                    p.val = value;
+                    } finally {
+                        t.release(0);
+                    }
+                    if (count != 0) {
+                        if (oldVal != null)
+                            return oldVal;
+                        break;
+                    }
+                }
+                else
+                    tab = (Node[])fk;
+            }
+            else if ((fh & LOCKED) != 0) {
+                checkForResize();
+                f.tryAwaitLock(tab, i);
+            }
+            else if (f.casHash(fh, fh | LOCKED)) {
+                Object oldVal = null;
+                try {                        // needed in case equals() throws
+                    if (tabAt(tab, i) == f) {
+                        count = 1;
+                        for (Node e = f;; ++count) {
+                            Object ek, ev;
+                            if ((e.hash & HASH_BITS) == h &&
+                                (ev = e.val) != null &&
+                                ((ek = e.key) == k || k.equals(ek))) {
+                                oldVal = ev;
+                                e.val = v;
+                                break;
+                            }
+                            Node last = e;
+                            if ((e = e.next) == null) {
+                                last.next = new Node(h, k, v, null);
+                                if (count >= TREE_THRESHOLD)
+                                    replaceWithTreeBin(tab, i, k);
+                                break;
                             }
                         }
                     }
+                } finally {                  // unlock and signal if needed
+                    if (!f.casHash(fh | LOCKED, fh)) {
+                        f.hash = fh;
+                        synchronized (f) { f.notifyAll(); };
+                    }
                 }
-                if (binCount != 0) {
-                    if (binCount >= TREEIFY_THRESHOLD)
-                        treeifyBin(tab, i);
+                if (count != 0) {
                     if (oldVal != null)
                         return oldVal;
+                    if (tab.length <= 64)
+                        count = 2;
                     break;
                 }
             }
         }
-        addCount(1L, binCount);
+        counter.add(1L);
+        if (count > 1)
+            checkForResize();
         return null;
     }
 
-    /**
-     * Copies all of the mappings from the specified map to this one.
-     * These mappings replace any mappings that this map had for any of the
-     * keys currently in the specified map.
-     *
-     * @param m mappings to be stored in this map
-     */
-    public void putAll(Map<? extends K, ? extends V> m) {
-        tryPresize(m.size());
-        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
-            putVal(e.getKey(), e.getValue(), false);
-    }
-
-    /**
-     * Removes the key (and its corresponding value) from this map.
-     * This method does nothing if the key is not in the map.
-     *
-     * @param  key the key that needs to be removed
-     * @return the previous value associated with {@code key}, or
-     *         {@code null} if there was no mapping for {@code key}
-     * @throws NullPointerException if the specified key is null
-     */
-    public V remove(Object key) {
-        return replaceNode(key, null, null);
+    /** Implementation for putIfAbsent */
+    private final Object internalPutIfAbsent(Object k, Object v) {
+        int h = spread(k.hashCode());
+        int count = 0;
+        for (Node[] tab = table;;) {
+            int i; Node f; int fh; Object fk, fv;
+            if (tab == null)
+                tab = initTable();
+            else if ((f = tabAt(tab, i = (tab.length - 1) & h)) == null) {
+                if (casTabAt(tab, i, null, new Node(h, k, v, null)))
+                    break;
+            }
+            else if ((fh = f.hash) == MOVED) {
+                if ((fk = f.key) instanceof TreeBin) {
+                    TreeBin t = (TreeBin)fk;
+                    Object oldVal = null;
+                    t.acquire(0);
+                    try {
+                        if (tabAt(tab, i) == f) {
+                            count = 2;
+                            TreeNode p = t.putTreeNode(h, k, v);
+                            if (p != null)
+                                oldVal = p.val;
+                        }
+                    } finally {
+                        t.release(0);
+                    }
+                    if (count != 0)

<TRUNCATED>

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