/*

  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

  *

  * This code is free software; you can redistribute it and/or modify it

  * under the terms of the GNU General Public License version 2 only, as

  * published by the Free Software Foundation.  Oracle designates this

  * particular file as subject to the "Classpath" exception as provided

  * by Oracle in the LICENSE file that accompanied this code.

  *

  * This code is distributed in the hope that it will be useful, but WITHOUT

  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

  * version 2 for more details (a copy is included in the LICENSE file that

  * accompanied this code).

  *

  * You should have received a copy of the GNU General Public License version

  * 2 along with this work; if not, write to the Free Software Foundation,

  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

  *

  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

  * or visit www.oracle.com if you need additional information or have any

  * questions.

  */

 /*

  * This file is available under and governed by the GNU General Public

  * License version 2 only, as published by the Free Software Foundation.

  * However, the following notice accompanied the original version of this

  * file:

  *

  * Written by Doug Lea with assistance from members of JCP JSR-166

  * Expert Group and released to the public domain, as explained at

  * http://creativecommons.org/publicdomain/zero/1.0/

  */

 package java.util.concurrent;

 import java.util.*;

 import java.io.Serializable;

 import java.io.IOException;

 import java.io.ObjectInputStream;

 import java.io.ObjectOutputStream;

 /**

  * A hash table supporting full concurrency of retrievals and adjustable

  * expected concurrency for updates. This class obeys the same functional

  * specification as {@link java.util.Hashtable}, and includes versions of

  * methods corresponding to each method of

  * <tt>Hashtable</tt>. However, even though all operations are thread-safe,

  * retrieval operations do <em>not</em> entail locking, and there is

  * <em>not</em> any support for locking the entire table in a way that prevents

  * all access. This class is fully interoperable with <tt>Hashtable</tt> in

  * programs that rely on its thread safety but not on its synchronization

  * details.

  *

  * <p>

  * Retrieval operations (including <tt>get</tt>) generally do not block, so may

  * overlap with update operations (including

  * <tt>put</tt> and <tt>remove</tt>). Retrievals reflect the results of the most

  * recently <em>completed</em> update operations holding upon their onset. For

  * aggregate operations such as <tt>putAll</tt>

  * and <tt>clear</tt>, concurrent retrievals may reflect insertion or removal of

  * only some entries. Similarly, Iterators and Enumerations return elements

  * reflecting the state of the hash table at some point at or since the creation

  * of the iterator/enumeration. They do <em>not</em> throw

  * {@link ConcurrentModificationException}. However, iterators are designed to

  * be used by only one thread at a time.

  *

  * <p>

  * The allowed concurrency among update operations is guided by the optional

  * <tt>concurrencyLevel</tt> constructor argument (default <tt>16</tt>), which

  * is used as a hint for internal sizing. The table is internally partitioned to

  * try to permit the indicated number of concurrent updates without contention.

  * Because placement in hash tables is essentially random, the actual

  * concurrency will vary. Ideally, you should choose a value to accommodate as

  * many threads as will ever concurrently modify the table. Using a

  * significantly higher value than you need can waste space and time, and a

  * significantly lower value can lead to thread contention. But overestimates

  * and underestimates within an order of magnitude do not usually have much

  * noticeable impact. A value of one is appropriate when it is known that only

  * one thread will modify and all others will only read. Also, resizing this or

  * any other kind of hash table is a relatively slow operation, so, when

  * possible, it is a good idea to provide estimates of expected table sizes in

  * constructors.

  *

  * <p>

  * This class and its views and iterators implement all of the

  * <em>optional</em> methods of the {@link Map} and {@link Iterator} interfaces.

  *

  * <p>

  * Like {@link Hashtable} but unlike {@link HashMap}, this class does

  * <em>not</em> allow <tt>null</tt> 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>.

  *

  * @since 1.5

  * @author Doug Lea

  * @param <K> the type of keys maintained by this map

  * @param <V> the type of mapped values

  */

 public class ConcurrentHashMap<K, V> extends AbstractMap<K, V>

     implements ConcurrentMap<K, V>, Serializable {

     private static final long serialVersionUID = 7249069246763182397L;

     /**

      * The default initial capacity for this table,

      * used when not otherwise specified in a constructor.

      */

     static final int DEFAULT_INITIAL_CAPACITY = 16;

     /**

      * The default load factor for this table, used when not

      * otherwise specified in a constructor.

      */

     static final float DEFAULT_LOAD_FACTOR = 0.75f;

     /**

      * The default concurrency level for this table, used when not

      * otherwise specified in a constructor.

      */

     static final int DEFAULT_CONCURRENCY_LEVEL = 16;

     private final Map<K, V> delegate;

     /**

      * Creates a new, empty map with the specified initial

      * capacity, load factor and concurrency level.

      *

      * @param initialCapacity the initial capacity. The implementation

      * performs internal sizing to accommodate this many elements.

      * @param loadFactor  the load factor threshold, used to control resizing.

      * Resizing may be performed when the average number of elements per

      * bin exceeds this threshold.

      * @param concurrencyLevel the estimated number of concurrently

      * updating threads. The implementation performs internal sizing

      * to try to accommodate this many threads.

      * @throws IllegalArgumentException if the initial capacity is

      * negative or the load factor or concurrencyLevel are

      * nonpositive.

      */

     @SuppressWarnings("unchecked")

     public ConcurrentHashMap(int initialCapacity,

                              float loadFactor, int concurrencyLevel) {

         if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0)

             throw new IllegalArgumentException();

         delegate = new HashMap<>(initialCapacity, loadFactor);

     }

     /**

      * Creates a new, empty map with the specified initial capacity

      * and load factor and with the default concurrencyLevel (16).

      *

      * @param initialCapacity The implementation performs internal

      * sizing to accommodate this many elements.

      * @param loadFactor  the load factor threshold, used to control resizing.

      * Resizing may be performed when the average number of elements per

      * bin exceeds this threshold.

      * @throws IllegalArgumentException if the initial capacity of

      * elements is negative or the load factor is nonpositive

      *

      * @since 1.6

      */

     public ConcurrentHashMap(int initialCapacity, float loadFactor) {

         this(initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL);

     }

     /**

      * Creates a new, empty map with the specified initial capacity,

      * and with default load factor (0.75) and concurrencyLevel (16).

      *

      * @param initialCapacity the initial capacity. The implementation

      * performs internal sizing to accommodate this many elements.

      * @throws IllegalArgumentException if the initial capacity of

      * elements is negative.

      */

     public ConcurrentHashMap(int initialCapacity) {

         this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);

     }

     /**

      * Creates a new, empty map with a default initial capacity (16),

      * load factor (0.75) and concurrencyLevel (16).

      */

     public ConcurrentHashMap() {

         this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);

     }

     /**

      * Creates a new map with the same mappings as the given map.

      * The map is created with a capacity of 1.5 times the number

      * of mappings in the given map or 16 (whichever is greater),

      * and a default load factor (0.75) and concurrencyLevel (16).

      *

      * @param m the map

      */

     public ConcurrentHashMap(Map<? extends K, ? extends V> m) {

         this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,

                       DEFAULT_INITIAL_CAPACITY),

              DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);

         putAll(m);

     }

     @Override

     public int size() {

         return delegate.size();

     }

     @Override

     public boolean isEmpty() {

         return delegate.isEmpty();

     }

     @Override

     public boolean containsKey(Object key) {

         return delegate.containsKey(key);

     }

     @Override

     public boolean containsValue(Object value) {

         return delegate.containsValue(value);

     }

     @Override

     public V get(Object key) {

         return delegate.get(key);

     }

     @Override

     public V put(K key, V value) {

         return delegate.put(key, value);

     }

     @Override

     public V remove(Object key) {

         return delegate.remove(key);

     }

     @Override

     public void putAll(Map<? extends K, ? extends V> m) {

         delegate.putAll(m);

     }

     @Override

     public void clear() {

         delegate.clear();

     }

     @Override

     public Set<K> keySet() {

         return delegate.keySet();

     }

     @Override

     public Collection<V> values() {

         return delegate.values();

     }

     @Override

     public Set<Entry<K, V>> entrySet() {

         return delegate.entrySet();

     }

     @Override

     public boolean equals(Object o) {

         return delegate.equals(o);

     }

     @Override

     public int hashCode() {

         return delegate.hashCode();

     }

     @Override

     public String toString() {

         return delegate.toString();

     }

     @Override

     public V putIfAbsent(K key, V value) {

         V old = delegate.get(key);

         if (old == null) {

             return delegate.put(key, value);

         }

         return old;

     }

     @Override

     public boolean remove(Object key, Object value) {

         if (equals(value, delegate.get(key))) {

             delegate.remove(key);

             return true;

         } else {

             return false;

         }

     }

     @Override

     public boolean replace(K key, V oldValue, V newValue) {

         if (equals(oldValue, delegate.get(key))) {

             delegate.put(key, newValue);

             return true;

         } else {

             return false;

         }

     }

     @Override

     public V replace(K key, V value) {

         if (delegate.containsKey(key)) {

             return delegate.put(key, value);

         } else {

             return null;

         }

     }

     private static boolean equals(Object a, Object b) {

         if (a == null) {

             return b == null;

         } else {

             return a.equals(b);

         }

     }

 }