Java tutorial
/* * Licensed to the Apache Software Foundation (ASF) under one or more * contributor license agreements. See the NOTICE file distributed with * this work for additional information regarding copyright ownership. * The ASF licenses this file to You under the Apache License, Version 2.0 * (the "License"); you may not use this file except in compliance with * the License. You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ import java.util.Collection; import java.util.ConcurrentModificationException; import java.util.HashMap; import java.util.Iterator; import java.util.Map; import java.util.Set; /** * <p>A customized implementation of <code>java.util.HashMap</code> designed * to operate in a multithreaded environment where the large majority of * method calls are read-only, instead of structural changes. When operating * in "fast" mode, read calls are non-synchronized and write calls perform the * following steps:</p> * <ul> * <li>Clone the existing collection * <li>Perform the modification on the clone * <li>Replace the existing collection with the (modified) clone * </ul> * <p>When first created, objects of this class default to "slow" mode, where * all accesses of any type are synchronized but no cloning takes place. This * is appropriate for initially populating the collection, followed by a switch * to "fast" mode (by calling <code>setFast(true)</code>) after initialization * is complete.</p> * * <p><strong>NOTE</strong>: If you are creating and accessing a * <code>HashMap</code> only within a single thread, you should use * <code>java.util.HashMap</code> directly (with no synchronization), for * maximum performance.</p> * * <p><strong>NOTE</strong>: <i>This class is not cross-platform. * Using it may cause unexpected failures on some architectures.</i> * It suffers from the same problems as the double-checked locking idiom. * In particular, the instruction that clones the internal collection and the * instruction that sets the internal reference to the clone can be executed * or perceived out-of-order. This means that any read operation might fail * unexpectedly, as it may be reading the state of the internal collection * before the internal collection is fully formed. * For more information on the double-checked locking idiom, see the * <a href="http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html"> * Double-Checked Locking Idiom Is Broken Declaration</a>.</p> * * @since Commons Collections 1.0 * @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $ * * @author Craig R. McClanahan * @author Stephen Colebourne */ public class FastHashMap extends HashMap { /** * The underlying map we are managing. */ protected HashMap map = null; /** * Are we currently operating in "fast" mode? */ protected boolean fast = false; // Constructors // ---------------------------------------------------------------------- /** * Construct an empty map. */ public FastHashMap() { super(); this.map = new HashMap(); } /** * Construct an empty map with the specified capacity. * * @param capacity the initial capacity of the empty map */ public FastHashMap(int capacity) { super(); this.map = new HashMap(capacity); } /** * Construct an empty map with the specified capacity and load factor. * * @param capacity the initial capacity of the empty map * @param factor the load factor of the new map */ public FastHashMap(int capacity, float factor) { super(); this.map = new HashMap(capacity, factor); } /** * Construct a new map with the same mappings as the specified map. * * @param map the map whose mappings are to be copied */ public FastHashMap(Map map) { super(); this.map = new HashMap(map); } // Property access // ---------------------------------------------------------------------- /** * Returns true if this map is operating in fast mode. * * @return true if this map is operating in fast mode */ public boolean getFast() { return (this.fast); } /** * Sets whether this map is operating in fast mode. * * @param fast true if this map should operate in fast mode */ public void setFast(boolean fast) { this.fast = fast; } // Map access // ---------------------------------------------------------------------- // These methods can forward straight to the wrapped Map in 'fast' mode. // (because they are query methods) /** * Return the value to which this map maps the specified key. Returns * <code>null</code> if the map contains no mapping for this key, or if * there is a mapping with a value of <code>null</code>. Use the * <code>containsKey()</code> method to disambiguate these cases. * * @param key the key whose value is to be returned * @return the value mapped to that key, or null */ public Object get(Object key) { if (fast) { return (map.get(key)); } else { synchronized (map) { return (map.get(key)); } } } /** * Return the number of key-value mappings in this map. * * @return the current size of the map */ public int size() { if (fast) { return (map.size()); } else { synchronized (map) { return (map.size()); } } } /** * Return <code>true</code> if this map contains no mappings. * * @return is the map currently empty */ public boolean isEmpty() { if (fast) { return (map.isEmpty()); } else { synchronized (map) { return (map.isEmpty()); } } } /** * Return <code>true</code> if this map contains a mapping for the * specified key. * * @param key the key to be searched for * @return true if the map contains the key */ public boolean containsKey(Object key) { if (fast) { return (map.containsKey(key)); } else { synchronized (map) { return (map.containsKey(key)); } } } /** * Return <code>true</code> if this map contains one or more keys mapping * to the specified value. * * @param value the value to be searched for * @return true if the map contains the value */ public boolean containsValue(Object value) { if (fast) { return (map.containsValue(value)); } else { synchronized (map) { return (map.containsValue(value)); } } } // Map modification // ---------------------------------------------------------------------- // These methods perform special behaviour in 'fast' mode. // The map is cloned, updated and then assigned back. // See the comments at the top as to why this won't always work. /** * Associate the specified value with the specified key in this map. * If the map previously contained a mapping for this key, the old * value is replaced and returned. * * @param key the key with which the value is to be associated * @param value the value to be associated with this key * @return the value previously mapped to the key, or null */ public Object put(Object key, Object value) { if (fast) { synchronized (this) { HashMap temp = (HashMap) map.clone(); Object result = temp.put(key, value); map = temp; return (result); } } else { synchronized (map) { return (map.put(key, value)); } } } /** * Copy all of the mappings from the specified map to this one, replacing * any mappings with the same keys. * * @param in the map whose mappings are to be copied */ public void putAll(Map in) { if (fast) { synchronized (this) { HashMap temp = (HashMap) map.clone(); temp.putAll(in); map = temp; } } else { synchronized (map) { map.putAll(in); } } } /** * Remove any mapping for this key, and return any previously * mapped value. * * @param key the key whose mapping is to be removed * @return the value removed, or null */ public Object remove(Object key) { if (fast) { synchronized (this) { HashMap temp = (HashMap) map.clone(); Object result = temp.remove(key); map = temp; return (result); } } else { synchronized (map) { return (map.remove(key)); } } } /** * Remove all mappings from this map. */ public void clear() { if (fast) { synchronized (this) { map = new HashMap(); } } else { synchronized (map) { map.clear(); } } } // Basic object methods // ---------------------------------------------------------------------- /** * Compare the specified object with this list for equality. This * implementation uses exactly the code that is used to define the * list equals function in the documentation for the * <code>Map.equals</code> method. * * @param o the object to be compared to this list * @return true if the two maps are equal */ public boolean equals(Object o) { // Simple tests that require no synchronization if (o == this) { return (true); } else if (!(o instanceof Map)) { return (false); } Map mo = (Map) o; // Compare the two maps for equality if (fast) { if (mo.size() != map.size()) { return (false); } Iterator i = map.entrySet().iterator(); while (i.hasNext()) { Map.Entry e = (Map.Entry) i.next(); Object key = e.getKey(); Object value = e.getValue(); if (value == null) { if (!(mo.get(key) == null && mo.containsKey(key))) { return (false); } } else { if (!value.equals(mo.get(key))) { return (false); } } } return (true); } else { synchronized (map) { if (mo.size() != map.size()) { return (false); } Iterator i = map.entrySet().iterator(); while (i.hasNext()) { Map.Entry e = (Map.Entry) i.next(); Object key = e.getKey(); Object value = e.getValue(); if (value == null) { if (!(mo.get(key) == null && mo.containsKey(key))) { return (false); } } else { if (!value.equals(mo.get(key))) { return (false); } } } return (true); } } } /** * Return the hash code value for this map. This implementation uses * exactly the code that is used to define the list hash function in the * documentation for the <code>Map.hashCode</code> method. * * @return suitable integer hash code */ public int hashCode() { if (fast) { int h = 0; Iterator i = map.entrySet().iterator(); while (i.hasNext()) { h += i.next().hashCode(); } return (h); } else { synchronized (map) { int h = 0; Iterator i = map.entrySet().iterator(); while (i.hasNext()) { h += i.next().hashCode(); } return (h); } } } /** * Return a shallow copy of this <code>FastHashMap</code> instance. * The keys and values themselves are not copied. * * @return a clone of this map */ public Object clone() { FastHashMap results = null; if (fast) { results = new FastHashMap(map); } else { synchronized (map) { results = new FastHashMap(map); } } results.setFast(getFast()); return (results); } // Map views // ---------------------------------------------------------------------- /** * Return a collection view of the mappings contained in this map. Each * element in the returned collection is a <code>Map.Entry</code>. */ public Set entrySet() { return new EntrySet(); } /** * Return a set view of the keys contained in this map. */ public Set keySet() { return new KeySet(); } /** * Return a collection view of the values contained in this map. */ public Collection values() { return new Values(); } // Map view inner classes // ---------------------------------------------------------------------- /** * Abstract collection implementation shared by keySet(), values() and entrySet(). */ private abstract class CollectionView implements Collection { public CollectionView() { } protected abstract Collection get(Map map); protected abstract Object iteratorNext(Map.Entry entry); public void clear() { if (fast) { synchronized (FastHashMap.this) { map = new HashMap(); } } else { synchronized (map) { get(map).clear(); } } } public boolean remove(Object o) { if (fast) { synchronized (FastHashMap.this) { HashMap temp = (HashMap) map.clone(); boolean r = get(temp).remove(o); map = temp; return r; } } else { synchronized (map) { return get(map).remove(o); } } } public boolean removeAll(Collection o) { if (fast) { synchronized (FastHashMap.this) { HashMap temp = (HashMap) map.clone(); boolean r = get(temp).removeAll(o); map = temp; return r; } } else { synchronized (map) { return get(map).removeAll(o); } } } public boolean retainAll(Collection o) { if (fast) { synchronized (FastHashMap.this) { HashMap temp = (HashMap) map.clone(); boolean r = get(temp).retainAll(o); map = temp; return r; } } else { synchronized (map) { return get(map).retainAll(o); } } } public int size() { if (fast) { return get(map).size(); } else { synchronized (map) { return get(map).size(); } } } public boolean isEmpty() { if (fast) { return get(map).isEmpty(); } else { synchronized (map) { return get(map).isEmpty(); } } } public boolean contains(Object o) { if (fast) { return get(map).contains(o); } else { synchronized (map) { return get(map).contains(o); } } } public boolean containsAll(Collection o) { if (fast) { return get(map).containsAll(o); } else { synchronized (map) { return get(map).containsAll(o); } } } public Object[] toArray(Object[] o) { if (fast) { return get(map).toArray(o); } else { synchronized (map) { return get(map).toArray(o); } } } public Object[] toArray() { if (fast) { return get(map).toArray(); } else { synchronized (map) { return get(map).toArray(); } } } public boolean equals(Object o) { if (o == this) return true; if (fast) { return get(map).equals(o); } else { synchronized (map) { return get(map).equals(o); } } } public int hashCode() { if (fast) { return get(map).hashCode(); } else { synchronized (map) { return get(map).hashCode(); } } } public boolean add(Object o) { throw new UnsupportedOperationException(); } public boolean addAll(Collection c) { throw new UnsupportedOperationException(); } public Iterator iterator() { return new CollectionViewIterator(); } private class CollectionViewIterator implements Iterator { private Map expected; private Map.Entry lastReturned = null; private Iterator iterator; public CollectionViewIterator() { this.expected = map; this.iterator = expected.entrySet().iterator(); } public boolean hasNext() { if (expected != map) { throw new ConcurrentModificationException(); } return iterator.hasNext(); } public Object next() { if (expected != map) { throw new ConcurrentModificationException(); } lastReturned = (Map.Entry) iterator.next(); return iteratorNext(lastReturned); } public void remove() { if (lastReturned == null) { throw new IllegalStateException(); } if (fast) { synchronized (FastHashMap.this) { if (expected != map) { throw new ConcurrentModificationException(); } FastHashMap.this.remove(lastReturned.getKey()); lastReturned = null; expected = map; } } else { iterator.remove(); lastReturned = null; } } } } /** * Set implementation over the keys of the FastHashMap */ private class KeySet extends CollectionView implements Set { protected Collection get(Map map) { return map.keySet(); } protected Object iteratorNext(Map.Entry entry) { return entry.getKey(); } } /** * Collection implementation over the values of the FastHashMap */ private class Values extends CollectionView { protected Collection get(Map map) { return map.values(); } protected Object iteratorNext(Map.Entry entry) { return entry.getValue(); } } /** * Set implementation over the entries of the FastHashMap */ private class EntrySet extends CollectionView implements Set { protected Collection get(Map map) { return map.entrySet(); } protected Object iteratorNext(Map.Entry entry) { return entry; } } }