270 lines
8.0 KiB
Java
270 lines
8.0 KiB
Java
/*
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* Copyright 2015 dorkbox, llc
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package dorkbox.collections;
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import java.io.Serializable;
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import java.util.Collection;
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import java.util.Collections;
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import java.util.HashMap;
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import java.util.Iterator;
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import java.util.Map;
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import java.util.Set;
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import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
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/**
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* This class uses the "single-writer-principle" for lock-free publication.
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* <p>
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* Since there are only 2 methods to guarantee that modifications can only be called one-at-a-time (either it is only called by
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* one thread, or only one thread can access it at a time) -- we chose the 2nd option -- and use 'synchronized' to make sure that only
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* one thread can access this modification methods at a time. Getting or checking the presence of values can then happen in a lock-free
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* manner.
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* <p>
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* According to my benchmarks, this is approximately 25% faster than ConcurrentHashMap for (all types of) reads, and a lot slower for
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* contended writes.
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* <p>
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* This data structure is for many-read/few-write scenarios
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*/
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public final
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class LockFreeHashMap<K, V> implements Map<K, V>, Cloneable, Serializable {
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// Recommended for best performance while adhering to the "single writer principle". Must be static-final
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private static final AtomicReferenceFieldUpdater<LockFreeHashMap, HashMap> mapREF = AtomicReferenceFieldUpdater.newUpdater(
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LockFreeHashMap.class,
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HashMap.class,
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"hashMap");
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private volatile HashMap<K, V> hashMap;
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// synchronized is used here to ensure the "single writer principle", and make sure that ONLY one thread at a time can enter this
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// section. Because of this, we can have unlimited reader threads all going at the same time, without contention (which is our
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// use-case 99% of the time)
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/**
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* Constructs an empty <tt>HashMap</tt> with the default initial capacity
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* (16) and the default load factor (0.75).
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*/
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public
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LockFreeHashMap() {
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hashMap = new HashMap<K, V>();
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}
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/**
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* Constructs an empty <tt>HashMap</tt> with the specified initial
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* capacity and the default load factor (0.75).
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*
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* @param initialCapacity the initial capacity.
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*
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* @throws IllegalArgumentException if the initial capacity is negative.
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*/
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public
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LockFreeHashMap(int initialCapacity) {
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hashMap = new HashMap<K, V>(initialCapacity);
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}
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/**
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* Constructs a new <tt>HashMap</tt> with the same mappings as the
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* specified <tt>Map</tt>. The <tt>HashMap</tt> is created with
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* default load factor (0.75) and an initial capacity sufficient to
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* hold the mappings in the specified <tt>Map</tt>.
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*
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* @param map the map whose mappings are to be placed in this map
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*
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* @throws NullPointerException if the specified map is null
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*/
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public
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LockFreeHashMap(Map<K, V> map) {
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this.hashMap = new HashMap<K, V>(map);
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}
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/**
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* Constructs an empty <tt>HashMap</tt> with the specified initial
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* capacity and load factor.
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*
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* @param initialCapacity the initial capacity
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* @param loadFactor the load factor
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*
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* @throws IllegalArgumentException if the initial capacity is negative
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* or the load factor is nonpositive
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*/
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public
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LockFreeHashMap(int initialCapacity, float loadFactor) {
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this.hashMap = new HashMap<K, V>(initialCapacity, loadFactor);
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}
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@SuppressWarnings("unchecked")
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public
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Map<K, V> getMap() {
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// use the SWP to get a lock-free get of the map. It's values are only valid at the moment this method is called.
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return Collections.unmodifiableMap(mapREF.get(this));
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}
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@Override
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public
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int size() {
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// use the SWP to get a lock-free get of the value
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return mapREF.get(this)
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.size();
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}
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@Override
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public
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boolean isEmpty() {
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// use the SWP to get a lock-free get of the value
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return mapREF.get(this)
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.isEmpty();
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}
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@Override
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public
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boolean containsKey(final Object key) {
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// use the SWP to get a lock-free get of the value
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return mapREF.get(this)
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.containsKey(key);
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}
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@Override
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public
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boolean containsValue(final Object value) {
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// use the SWP to get a lock-free get of the value
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return mapREF.get(this)
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.containsValue(value);
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}
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@SuppressWarnings("unchecked")
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@Override
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public
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V get(final Object key) {
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// use the SWP to get a lock-free get of the value
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return (V) mapREF.get(this)
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.get(key);
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}
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@Override
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public synchronized
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V put(final K key, final V value) {
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return hashMap.put(key, value);
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}
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@Override
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public synchronized
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V remove(final Object key) {
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return hashMap.remove(key);
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}
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@SuppressWarnings("Java8CollectionRemoveIf")
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public synchronized
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void removeAllValues(final V value) {
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for (Iterator<Entry<K, V>> iterator = hashMap.entrySet().iterator(); iterator.hasNext(); ) {
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final Map.Entry<K, V> kvEntry = iterator.next();
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if (kvEntry.getValue().equals(value)) {
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iterator.remove();
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}
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}
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}
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@Override
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public synchronized
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void putAll(final Map<? extends K, ? extends V> map) {
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this.hashMap.putAll(map);
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}
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/**
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* This uses equals to update values. At first glance, this seems like a waste (since if it's equal, why update it?). This is because
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* the ONLY location this is used (in the Database, for updating all DeviceUser in the map), equals compares ONLY the DB ID. In only
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* this situation, this makes sense (since anything with the same DB ID, we should replace/update the value)
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*/
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public synchronized
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void updateAllWithValue(final V value) {
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for (Map.Entry<K, V> entry : hashMap.entrySet()) {
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if (value.equals(entry.getValue())) {
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// get's all device IDs that have this user assigned, and reassign the value
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entry.setValue(value);
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}
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}
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}
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public synchronized
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void replaceAll(Map<K,V> hashMap) {
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this.hashMap.clear();
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this.hashMap.putAll(hashMap);
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}
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@Override
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public synchronized
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void clear() {
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hashMap.clear();
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}
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@Override
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public
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Set<K> keySet() {
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return getMap().keySet();
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}
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@Override
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public
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Collection<V> values() {
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return getMap().values();
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}
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@Override
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public
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Set<Entry<K, V>> entrySet() {
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return getMap().entrySet();
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}
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@Override
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public
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boolean equals(final Object o) {
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return mapREF.get(this)
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.equals(o);
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}
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@Override
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public
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int hashCode() {
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return mapREF.get(this)
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.hashCode();
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}
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@Override
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public
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String toString() {
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return mapREF.get(this)
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.toString();
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}
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@SuppressWarnings("unchecked")
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public
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Collection<K> keys() {
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// use the SWP to get a lock-free get of the value
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return mapREF.get(this).keySet();
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}
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@SuppressWarnings("unchecked")
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public
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Map<K,V> elements() {
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// use the SWP to get a lock-free get of the value
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return mapREF.get(this);
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}
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@SuppressWarnings("unchecked")
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public
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HashMap<K, V> backingMap() {
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// use the SWP to get a lock-free get of the value
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return mapREF.get(this);
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}
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}
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