A hashtable-based Map implementation with soft keys : Customized Map « Collections Data Structure « Java






A hashtable-based Map implementation with soft keys

   
/*
 * Copyright 2006 (C) TJDO.
 * All rights reserved.
 *
 * This software is distributed under the terms of the TJDO License version 1.0.
 * See the terms of the TJDO License in the documentation provided with this software.
 *
 * $Id: SoftHashMap.java,v 1.2 2007/10/03 01:23:43 jackknifebarber Exp $
 */


import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.util.AbstractCollection;
import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.ArrayList;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;


/**
 * A hashtable-based <tt>Map</tt> implementation with <em>soft keys</em>.
 * An entry in a <tt>SoftHashMap</tt> will automatically be removed when
 * its key is no longer in ordinary use.
 * More precisely, the presence of a mapping for a given key will not prevent
 * the key from being discarded by the garbage collector, that is, made
 * finalizable, finalized, and then reclaimed.
 * When a key has been discarded its entry is effectively removed from the map,
 * so this class behaves somewhat differently than other <tt>Map</tt>
 * implementations.
 * <p>
 * Both null values and the null key are supported. This class has performance
 * characteristics similar to those of the <tt>HashMap</tt> class, and has the
 * same efficiency parameters of <em>initial capacity</em> and <em>load
 * factor</em>.
 * <p>
 * Like most collection classes, this class is not synchronized.
 * A synchronized <tt>SoftHashMap</tt> may be constructed using the
 * <tt>Collections.synchronizedMap</tt> method.
 * <p>
 * This class is intended primarily for use with key objects whose
 * <tt>equals</tt> methods test for object identity using the <tt>==</tt>
 * operator.
 * Once such a key is discarded it can never be recreated, so it is impossible
 * to do a lookup of that key in a <tt>SoftHashMap</tt> at some later time and
 * be surprised that its entry has been removed.
 * This class will work perfectly well with key objects whose <tt>equals</tt>
 * methods are not based upon object identity, such as <tt>String</tt>
 * instances.
 * With such recreatable key objects, however, the automatic removal of
 * <tt>SoftHashMap</tt> entries whose keys have been discarded may prove to be
 * confusing.
 * <p>
 * The behavior of the <tt>SoftHashMap</tt> class depends in part upon the
 * actions of the garbage collector, so several familiar (though not required)
 * <tt>Map</tt> invariants do not hold for this class.
 * Because the garbage collector may discard keys at any time, a
 * <tt>SoftHashMap</tt> may behave as though an unknown thread is silently
 * removing entries.s
 * In particular, even if you synchronize on a <tt>SoftHashMap</tt> instance and
 * invoke none of its mutator methods, it is possible for the <tt>size</tt>
 * method to return smaller values over time, for the <tt>isEmpty</tt> method to
 * return <tt>false</tt> and then <tt>true</tt>, for the <tt>containsKey</tt>
 * method to return <tt>true</tt> and later <tt>false</tt> for a given key, for
 * the <tt>get</tt> method to return a value for a given key but later return
 * <tt>null</tt>, for the <tt>put</tt> method to return <tt>null</tt> and the
 * <tt>remove</tt> method to return <tt>false</tt> for a key that previously
 * appeared to be in the map, and for successive examinations of the key set,
 * the value set, and the entry set to yield successively smaller numbers of
 * elements.
 * <p>
 * Each key object in a <tt>SoftHashMap</tt> is stored indirectly as the
 * referent of a soft reference.
 * Therefore a key will automatically be removed only after the soft references
 * to it, both inside and outside of the map, have been cleared by the garbage
 * collector.
 * <p>
 * <strong>Implementation note:</strong> The value objects in a
 * <tt>SoftHashMap</tt> are held by ordinary strong references.
 * Thus care should be taken to ensure that value objects do not strongly refer
 * to their own keys, either directly or indirectly, since that will prevent the
 * keys from being discarded.
 * Note that a value object may refer indirectly to its key via the
 * <tt>SoftHashMap</tt> itself; that is, a value object may strongly refer to
 * some other key object whose associated value object, in turn, strongly refers
 * to the key of the first value object.
 * One way to deal with this is to wrap values themselves within
 * <tt>SoftReferences</tt> before inserting, as in: <tt>m.put(key, new
 * SoftReference(value))</tt>, and then unwrapping upon each <tt>get</tt>.
 * <p>
 * The iterators returned by all of this class's "collection view methods" are
 * <i>fail-fast</i>: if the map is structurally modified at any time after the
 * iterator is created, in any way except through the iterator's own
 * <tt>remove</tt> or <tt>add</tt> methods, the iterator will throw a
 * <tt>ConcurrentModificationException</tt>.
 * Thus, in the face of concurrent modification, the iterator fails quickly and
 * cleanly, rather than risking arbitrary, non-deterministic behavior at an
 * undetermined time in the future.
 * <p>
 * Note that the fail-fast behavior of an iterator cannot be guaranteed as it
 * is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification.
 * Fail-fast iterators throw <tt>ConcurrentModificationException</tt> on a
 * best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness:  <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i>
 *
 * @author  <a href="mailto:jackknifebarber@users.sourceforge.net">Mike Martin</a>
 *          (borrowing liberally from java.util.WeakHashMap)
 * @version $Revision: 1.2 $
 */

public class SoftHashMap extends AbstractMap implements Map
{
    /**
     * The default initial capacity -- MUST be a power of two.
     */
    private static final int DEFAULT_INITIAL_CAPACITY = 16;

    /**
     * The maximum capacity, used if a higher value is implicitly specified
     * by either of the constructors with arguments.
     * MUST be a power of two &lt;= 1&lt;&lt;30.
     */
    private static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The load fast used when none specified in constructor.
     */
    private static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * The table, resized as necessary. Length MUST Always be a power of two.
     */
    private Entry[] table;

    /**
     * The number of key-value mappings contained in this soft hash map.
     */
    private int size;

    /**
     * The next size value at which to resize (capacity * load factor).
     */
    private int threshold;

    /**
     * The load factor for the hash table.
     */
    private final float loadFactor;

    /**
     * Reference queue for cleared SoftEntries.
     */
    private final ReferenceQueue queue = new ReferenceQueue();

    /**
     * The number of times this map has been structurally modified.
     * Structural modifications are those that change the number of mappings or
     * otherwise modify its internal structure (e.g., rehash).
     * This field is used to make iterators on Collection-views of the map
     * fail-fast.
     * (See ConcurrentModificationException).
     */
    private volatile int modCount;

    /**
     * Constructs a new, empty <tt>SoftHashMap</tt> with the given initial
     * capacity and the given load factor.
     *
     * @param initialCapacity
     *      The initial capacity of the <tt>SoftHashMap</tt>
     * @param loadFactor
     *      The load factor of the <tt>SoftHashMap</tt>
     * @throws IllegalArgumentException
     *      If the initial capacity is negative, or if the load factor is
     *      nonpositive.
     */
    public SoftHashMap(int initialCapacity, float loadFactor)
    {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Initial Capacity: " + initialCapacity);
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal Load factor: " + loadFactor);

        int capacity = 1;
        while (capacity < initialCapacity)
            capacity <<= 1;
        table = new Entry[capacity];
        this.loadFactor = loadFactor;
        threshold = (int)(capacity * loadFactor);
    }

    /**
     * Constructs a new, empty <tt>SoftHashMap</tt> with the given initial
     * capacity and the default load factor, which is <tt>0.75</tt>.
     *
     * @param  initialCapacity
     *      The initial capacity of the <tt>SoftHashMap</tt>
     * @throws IllegalArgumentException
     *      If the initial capacity is negative.
     */
    public SoftHashMap(int initialCapacity)
    {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

    /**
     * Constructs a new, empty <tt>SoftHashMap</tt> with the default initial
     * capacity (16) and the default load factor (0.75).
     */
    public SoftHashMap()
    {
        loadFactor = DEFAULT_LOAD_FACTOR;
        threshold = DEFAULT_INITIAL_CAPACITY;
        table = new Entry[DEFAULT_INITIAL_CAPACITY];
    }

    /**
     * Constructs a new <tt>SoftHashMap</tt> with the same mappings as the
     * specified <tt>Map</tt>.
     * The <tt>SoftHashMap</tt> is created with default load factor, which is
     * <tt>0.75</tt> and an initial capacity sufficient to hold the mappings
     * in the specified <tt>Map</tt>.
     *
     * @param t
     *      the map whose mappings are to be placed in this map.
     * @throws NullPointerException
     *      if the specified map is null.
     */
    public SoftHashMap(Map t)
    {
        this(Math.max((int)(t.size() / DEFAULT_LOAD_FACTOR) + 1, 16), DEFAULT_LOAD_FACTOR);

        putAll(t);
    }

    // internal utilities

    /**
     * Value representing null keys inside tables.
     */
    private static final Object NULL_KEY = new Object();

    /**
     * Use NULL_KEY for key if it is null.
     */
    private static Object maskNull(Object key)
    {
        return key == null ? NULL_KEY : key;
    }

    /**
     * Return internal representation of null key back to caller as null
     */
    private static Object unmaskNull(Object key)
    {
        return key == NULL_KEY ? null : key;
    }

    /**
     * Returns a hash value for the specified object.
     * In addition to the object's own hashCode, this method applies a
     * "supplemental hash function," which defends against poor quality hash
     * functions.
     * This is critical because HashMap uses power-of two length hash tables.
     * <p>
     * The shift distances in this function were chosen as the result of an
     * automated search over the entire four-dimensional search space.
     */
    static int hash(Object x)
    {
        int h = x.hashCode();

        h += ~(h << 9);
        h ^=  (h >>> 14);
        h +=  (h << 4);
        h ^=  (h >>> 10);

        return h;
    }

    /**
     * Check for equality of non-null reference x and possibly-null y.
     * By default uses Object.equals.
     */
    static boolean eq(Object x, Object y)
    {
        return x == y || x.equals(y);
    }

    /**
     * Return index for hash code h.
     */
    static int indexFor(int h, int length)
    {
        return h & (length - 1);
    }

    /**
     * Expunge stale entries from the table.
     */
    private void expungeStaleEntries()
    {
        Object r;

        while ((r = queue.poll()) != null)
        {
            Entry e = (Entry)r;
            int h = e.hash;
            int i = indexFor(h, table.length);

            Entry prev = table[i];
            Entry p = prev;

            while (p != null)
            {
                Entry next = p.next;

                if (p == e)
                {
                    if (prev == e)
                        table[i] = next;
                    else
                        prev.next = next;

                    e.next = null;  // Help GC
                    e.value = null; //  "   "
                    size--;
                    break;
                }

                prev = p;
                p = next;
            }
        }
    }

    /**
     * Return the table after first expunging stale entries
     */
    private Entry[] getTable()
    {
        expungeStaleEntries();
        return table;
    }

    /**
     * Returns the number of key-value mappings in this map.
     * This result is a snapshot, and may not reflect unprocessed entries that
     * will be removed before next attempted access because they are no longer
     * referenced.
     */
    public int size()
    {
        if (size == 0)
            return 0;

        expungeStaleEntries();
        return size;
    }

    /**
     * Returns <tt>true</tt> if this map contains no key-value mappings.
     * This result is a snapshot, and may not reflect unprocessed entries that
     * will be removed before next attempted access because they are no longer
     * referenced.
     */
    public boolean isEmpty()
    {
        return size() == 0;
    }

    /**
     * Returns the value to which the specified key is mapped in this soft hash
     * map, or <tt>null</tt> if the map contains no mapping for this key.
     * A return value of <tt>null</tt> does not <i>necessarily</i> indicate that
     * the map contains no mapping for the key; it is also possible that the map
     * explicitly maps the key to <tt>null</tt>.
     * The <tt>containsKey</tt> method may be used to distinguish these two
     * cases.
     *
     * @param key
     *      the key whose associated value is to be returned.
     * @return
     *      the value to which this map maps the specified key, or <tt>null</tt>
     *      if the map contains no mapping for this key.
     *
     * @see #put(Object, Object)
     */
    public Object get(Object key)
    {
        Object k = maskNull(key);
        int h = hash(k);
        Entry[] tab = getTable();
        int index = indexFor(h, tab.length);
        Entry e = tab[index];

        while (e != null)
        {
            if (e.hash == h && eq(k, e.get()))
                return e.value;

            e = e.next;
        }

        return null;
    }

    /**
     * Returns <tt>true</tt> if this map contains a mapping for the specified
     * key.
     *
     * @param key
     *      The key whose presence in this map is to be tested
     * @return
     *      <tt>true</tt> if there is a mapping for <tt>key</tt>;
     *      <tt>false</tt> otherwise
     */
    public boolean containsKey(Object key)
    {
        return getEntry(key) != null;
    }

    /**
     * Returns the entry associated with the specified key in the map.
     * Returns null if the map contains no mapping for this key.
     */
    Entry getEntry(Object key)
    {
        Object k = maskNull(key);
        int h = hash(k);
        Entry[] tab = getTable();
        int index = indexFor(h, tab.length);
        Entry e = tab[index];

        while (e != null && !(e.hash == h && eq(k, e.get())))
            e = e.next;

        return e;
    }

    /**
     * Associates 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.
     *
     * @param key
     *      key with which the specified value is to be associated.
     * @param value
     *      value to be associated with the specified key.
     * @return
     *      previous value associated with specified key, or <tt>null</tt> if
     *      there was no mapping for key.  A <tt>null</tt> return can also
     *      indicate that the map previously associated <tt>null</tt> with the
     *      specified key.
     */
    public Object put(Object key, Object value)
    {
        Object k = maskNull(key);
        int h = hash(k);
        Entry[] tab = getTable();
        int i = indexFor(h, tab.length);

        for (Entry e = tab[i]; e != null; e = e.next)
        {
            if (h == e.hash && eq(k, e.get()))
            {
                Object oldValue = e.value;
                if (value != oldValue)
                    e.value = value;
                return oldValue;
            }
        }

        modCount++;
        tab[i] = new Entry(k, value, queue, h, tab[i]);
        if (++size >= threshold)
            resize(tab.length * 2);

        return null;
    }

    /**
     * Rehashes the contents of this map into a new array with a
     * larger capacity.
     * This method is called automatically when the number of keys in this map
     * reaches its threshold.
     * <p>
     * If current capacity is MAXIMUM_CAPACITY, this method does not resize the
     * map, but but sets threshold to Integer.MAX_VALUE.
     * This has the effect of preventing future calls.
     *
     * @param newCapacity
     *      the new capacity, MUST be a power of two; must be greater than
     *      current capacity unless current capacity is MAXIMUM_CAPACITY (in
     *      which case value is irrelevant).
     */
    void resize(int newCapacity)
    {
        Entry[] oldTable = getTable();
        int oldCapacity = oldTable.length;
        if (oldCapacity == MAXIMUM_CAPACITY)
        {
            threshold = Integer.MAX_VALUE;
            return;
        }

        Entry[] newTable = new Entry[newCapacity];
        transfer(oldTable, newTable);
        table = newTable;

        /*
         * If ignoring null elements and processing ref queue caused massive
         * shrinkage, then restore old table.  This should be rare, but avoids
         * unbounded expansion of garbage-filled tables.
         */
        if (size >= threshold / 2)
            threshold = (int)(newCapacity * loadFactor);
        else
        {
            expungeStaleEntries();
            transfer(newTable, oldTable);
            table = oldTable;
        }
    }

    /** Transfer all entries from src to dest tables. */
    private void transfer(Entry[] src, Entry[] dest)
    {
        for (int j = 0; j < src.length; ++j)
        {
            Entry e = src[j];
            src[j] = null;

            while (e != null)
            {
                Entry next = e.next;
                Object key = e.get();

                if (key == null)
                {
                    e.next = null;  // Help GC
                    e.value = null; //  "   "
                    size--;
                }
                else
                {
                    int i = indexFor(e.hash, dest.length);
                    e.next = dest[i];
                    dest[i] = e;
                }

                e = next;
            }
        }
    }

    /**
     * Copies all of the mappings from the specified map to this map.
     * These mappings will 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.
     * @throws NullPointerException
     *      if the specified map is null.
     */
    public void putAll(Map m)
    {
        int numKeysToBeAdded = m.size();
        if (numKeysToBeAdded == 0)
            return;

        /*
         * Expand the map if the map if the number of mappings to be added
         * is greater than or equal to threshold.  This is conservative; the
         * obvious condition is (m.size() + size) >= threshold, but this
         * condition could result in a map with twice the appropriate capacity,
         * if the keys to be added overlap with the keys already in this map.
         * By using the conservative calculation, we subject ourself
         * to at most one extra resize.
         */
        if (numKeysToBeAdded > threshold)
        {
            int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
            if (targetCapacity > MAXIMUM_CAPACITY)
                targetCapacity = MAXIMUM_CAPACITY;
            int newCapacity = table.length;
            while (newCapacity < targetCapacity)
                newCapacity <<= 1;
            if (newCapacity > table.length)
                resize(newCapacity);
        }

        for (Iterator i = m.entrySet().iterator(); i.hasNext();)
        {
            Map.Entry e = (Map.Entry)i.next();
            put(e.getKey(), e.getValue());
        }
    }

    /**
     * Removes the mapping for this key from this map if present.
     *
     * @param key
     *      key whose mapping is to be removed from the map.
     * @return
     *      previous value associated with specified key, or <tt>null</tt> if
     *      there was no mapping for key.  A <tt>null</tt> return can also
     *      indicate that the map previously associated <tt>null</tt> with the
     *      specified key.
     */
    public Object remove(Object key)
    {
        Object k = maskNull(key);
        int h = hash(k);
        Entry[] tab = getTable();
        int i = indexFor(h, tab.length);
        Entry prev = tab[i];
        Entry e = prev;

        while (e != null)
        {
            Entry next = e.next;

            if (h == e.hash && eq(k, e.get()))
            {
                modCount++;
                size--;

                if (prev == e)
                    tab[i] = next;
                else
                    prev.next = next;

                return e.value;
            }

            prev = e;
            e = next;
        }

        return null;
    }

    /** Special version of remove needed by Entry set. */
    Entry removeMapping(Object o)
    {
        if (!(o instanceof Map.Entry))
            return null;

        Entry[] tab = getTable();
        Map.Entry entry = (Map.Entry)o;
        Object k = maskNull(entry.getKey());
        int h = hash(k);
        int i = indexFor(h, tab.length);
        Entry prev = tab[i];
        Entry e = prev;

        while (e != null)
        {
            Entry next = e.next;

            if (h == e.hash && e.equals(entry))
            {
                modCount++;
                size--;

                if (prev == e)
                    tab[i] = next;
                else
                    prev.next = next;

                return e;
            }

            prev = e;
            e = next;
        }

        return null;
    }

    /**
     * Removes all mappings from this map.
     */
    public void clear()
    {
        /*
         * Clear out ref queue. We don't need to expunge entries since table
         * is getting cleared.
         */
        while (queue.poll() != null)
            ;

        modCount++;
        Entry tab[] = table;
        for (int i = 0; i < tab.length; ++i)
            tab[i] = null;
        size = 0;

        /*
         * Allocation of array may have caused GC, which may have caused
         * additional entries to go stale.  Removing these entries from the
         * reference queue will make them eligible for reclamation.
         */
        while (queue.poll() != null)
            ;
    }

    /**
     * Returns <tt>true</tt> if this map maps one or more keys to the specified
     * value.
     *
     * @param value
     *      value whose presence in this map is to be tested.
     * @return
     *      <tt>true</tt> if this map maps one or more keys to the specified
     *      value.
     */
    public boolean containsValue(Object value)
    {
        if (value == null)
            return containsNullValue();

        Entry tab[] = getTable();
        for (int i = tab.length; i-- > 0;)
        {
            for (Entry e = tab[i]; e != null; e = e.next)
            {
                if (value.equals(e.value))
                    return true;
            }
        }

        return false;
    }

    /**
     * Special-case code for containsValue with null argument.
     */
    private boolean containsNullValue()
    {
        Entry tab[] = getTable();
        for (int i = tab.length; i-- > 0;)
        {
            for (Entry e = tab[i]; e != null; e = e.next)
            {
                if (e.value == null)
                    return true;
            }
        }

        return false;
    }

    /**
     * The entries in this hash table extend SoftReference, using its main ref
     * field as the key.
     */
    private static class Entry extends SoftReference implements Map.Entry
    {
        private Object value;
        private final int hash;
        private Entry next;

        /**
         * Create new entry.
         */
        Entry(Object key, Object value, ReferenceQueue queue, int hash, Entry next)
        {
            super(key, queue);

            this.value = value;
            this.hash  = hash;
            this.next  = next;
        }

        public Object getKey()
        {
            return unmaskNull(get());
        }

        public Object getValue()
        {
            return value;
        }

        public Object setValue(Object newValue)
        {
            Object oldValue = value;
            value = newValue;
            return oldValue;
        }

        public boolean equals(Object o)
        {
            if (!(o instanceof Map.Entry))
                return false;

            Map.Entry e = (Map.Entry)o;
            Object k1 = getKey();
            Object k2 = e.getKey();

            if (k1 == k2 || (k1 != null && k1.equals(k2)))
            {
                Object v1 = getValue();
                Object v2 = e.getValue();

                if (v1 == v2 || (v1 != null && v1.equals(v2)))
                    return true;
            }

            return false;
        }

        public int hashCode()
        {
            Object k = getKey();
            Object v = getValue();

            return ((k==null ? 0 : k.hashCode()) ^
                    (v==null ? 0 : v.hashCode()));
        }

        public String toString()
        {
            return getKey() + "=" + getValue();
        }
    }

    private abstract class HashIterator implements Iterator
    {
        int index;
        Entry entry = null;
        Entry lastReturned = null;
        int expectedModCount = modCount;

        /**
         * Strong reference needed to avoid disappearance of key
         * between hasNext and next
         */
        Object nextKey = null;

        /**
         * Strong reference needed to avoid disappearance of key
         * between nextEntry() and any use of the entry
         */
        Object currentKey = null;

        HashIterator()
        {
            index = (size() != 0 ? table.length : 0);
        }

        public boolean hasNext()
        {
            Entry[] t = table;

            while (nextKey == null)
            {
                Entry e = entry;
                int i = index;
                while (e == null && i > 0)
                    e = t[--i];
                entry = e;
                index = i;
                if (e == null)
                {
                    currentKey = null;
                    return false;
                }
                nextKey = e.get(); // hold on to key in strong ref
                if (nextKey == null)
                    entry = entry.next;
            }
            return true;
        }

        /** The common parts of next() across different types of iterators */
        protected Entry nextEntry()
        {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            if (nextKey == null && !hasNext())
                throw new NoSuchElementException();

            lastReturned = entry;
            entry = entry.next;
            currentKey = nextKey;
            nextKey = null;
            return lastReturned;
        }

        public void remove()
        {
            if (lastReturned == null)
                throw new IllegalStateException();
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();

            SoftHashMap.this.remove(currentKey);
            expectedModCount = modCount;
            lastReturned = null;
            currentKey = null;
        }
    }

    private class ValueIterator extends HashIterator
    {
        public Object next()
        {
            return nextEntry().value;
        }
    }

    private class KeyIterator extends HashIterator
    {
        public Object next()
        {
            return nextEntry().getKey();
        }
    }

    private class EntryIterator extends HashIterator
    {
        public Object next()
        {
            return nextEntry();
        }
    }

    // Views

    private transient Set keySet = null;
    private transient Collection values = null;
    private transient Set entrySet = null;

    /**
     * Returns a set view of the keys contained in this map.
     * The set is backed by the map, so changes to the map are reflected in the
     * set, and vice-versa.
     * The set supports element removal, which removes the corresponding mapping
     * from this map, via the <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
     * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> operations.
     * It does not support the <tt>add</tt> or <tt>addAll</tt> operations.
     *
     * @return
     *      a set view of the keys contained in this map.
     */
    public Set keySet()
    {
        Set ks = keySet;
        return (ks != null ? ks : (keySet = new KeySet()));
    }

    private class KeySet extends AbstractSet
    {
        public Iterator iterator()
        {
            return new KeyIterator();
        }

        public int size()
        {
            return SoftHashMap.this.size();
        }

        public boolean contains(Object o)
        {
            return containsKey(o);
        }

        public boolean remove(Object o)
        {
            if (containsKey(o))
            {
                SoftHashMap.this.remove(o);
                return true;
            }
            else
                return false;
        }

        public void clear()
        {
            SoftHashMap.this.clear();
        }

        public Object[] toArray()
        {
            Collection c = new ArrayList(size());
            for (Iterator i = iterator(); i.hasNext(); )
                c.add(i.next());
            return c.toArray();
        }

        public Object[] toArray(Object a[])
        {
            Collection c = new ArrayList(size());
            for (Iterator i = iterator(); i.hasNext(); )
                c.add(i.next());
            return c.toArray(a);
        }
    }

    /**
     * Returns a collection view of the values contained in this map.
     * The collection is backed by the map, so changes to the map are reflected
     * in the collection, and vice-versa.
     * The collection supports element removal, which removes the corresponding
     * mapping from this map, via the <tt>Iterator.remove</tt>,
     * <tt>Collection.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt>, and
     * <tt>clear</tt> operations.
     * It does not support the <tt>add</tt> or <tt>addAll</tt> operations.
     *
     * @return
     *      a collection view of the values contained in this map.
     */
    public Collection values()
    {
        Collection vs = values;
        return (vs != null ?  vs : (values = new Values()));
    }

    private class Values extends AbstractCollection
    {
        public Iterator iterator()
        {
            return new ValueIterator();
        }

        public int size()
        {
            return SoftHashMap.this.size();
        }

        public boolean contains(Object o)
        {
            return containsValue(o);
        }

        public void clear()
        {
            SoftHashMap.this.clear();
        }

        public Object[] toArray()
        {
            Collection c = new ArrayList(size());
            for (Iterator i = iterator(); i.hasNext(); )
                c.add(i.next());
            return c.toArray();
        }

        public Object[] toArray(Object a[])
        {
            Collection c = new ArrayList(size());
            for (Iterator i = iterator(); i.hasNext(); )
                c.add(i.next());
            return c.toArray(a);
        }
    }

    /**
     * Returns a collection view of the mappings contained in this map.
     * Each element in the returned collection is a <tt>Map.Entry</tt>.
     * The collection is backed by the map, so changes to the map are reflected
     * in the collection, and vice-versa.
     * The collection supports element removal, which removes the corresponding
     * mapping from the map, via the <tt>Iterator.remove</tt>,
     * <tt>Collection.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt>, and
     * <tt>clear</tt> operations.
     * It does not support the <tt>add</tt> or <tt>addAll</tt> operations.
     *
     * @return
     *      a collection view of the mappings contained in this map.
     */
    public Set entrySet()
    {
        Set es = entrySet;
        return (es != null ? es : (entrySet = new EntrySet()));
    }

    private class EntrySet extends AbstractSet
    {
        public Iterator iterator()
        {
            return new EntryIterator();
        }

        public boolean contains(Object o)
        {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry e = (Map.Entry)o;
            Entry candidate = getEntry(e.getKey());
            return candidate != null && candidate.equals(e);
        }

        public boolean remove(Object o)
        {
            return removeMapping(o) != null;
        }

        public int size()
        {
            return SoftHashMap.this.size();
        }

        public void clear()
        {
            SoftHashMap.this.clear();
        }

        public Object[] toArray()
        {
            Collection c = new ArrayList(size());
            for (Iterator i = iterator(); i.hasNext();)
                c.add(new SimpleEntry((Map.Entry)i.next()));
            return c.toArray();
        }

        public Object[] toArray(Object a[])
        {
            Collection c = new ArrayList(size());
            for (Iterator i = iterator(); i.hasNext();)
                c.add(new SimpleEntry((Map.Entry)i.next()));
            return c.toArray(a);
        }
    }

    static class SimpleEntry implements Map.Entry
    {
        private Object key;
        private Object value;

        public SimpleEntry(Map.Entry e)
        {
            this.key   = e.getKey();
            this.value = e.getValue();
        }

        public Object getKey()
        {
            return key;
        }

        public Object getValue()
        {
            return value;
        }

        public Object setValue(Object value)
        {
            Object oldValue = this.value;
            this.value = value;
            return oldValue;
        }

        public boolean equals(Object o)
        {
            if (!(o instanceof Map.Entry))
                return false;

            Map.Entry e = (Map.Entry)o;
            return eq(key, e.getKey()) && eq(value, e.getValue());
        }

        public int hashCode()
        {
            return ((key   == null)   ? 0 :   key.hashCode()) ^
                   ((value == null)   ? 0 : value.hashCode());
        }

        public String toString()
        {
            return key + "=" + value;
        }

        private static boolean eq(Object o1, Object o2)
        {
            return o1 == null ? o2 == null : o1.equals(o2);
        }
    }
}

   
    
    
  








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2.Case Insensitive Map
3.A Map collection with real-time behavior
4.Cache Map
5.Map implementation Optimized for Strings keys
6.An integer hashmap
7.An IdentityMap that uses reference-equality instead of object-equality
8.Int Object HashMap
9.Concurrent Skip List Map
10.A hash map that uses primitive ints for the key rather than objects.
11.Integer Map
12.Copy On Write Map
13.Expiring Map
14.Array Map
15.Int Object HashMap (from CERN)
16.Int HashMap from jodd.org
17.String Map
18.List Map
19.Map using Locale objects as keys
20.Map with keys iterated in insertion order
21.Most Recently Used Map
22.Multi Map
23.MultiMap is a Java version of the C++ STL class std::multimap
24.Object Int Map
25.Sequenced HashMap
26.Int Int Map
27.Int Object Map
28.Identity HashMap
29.A java.util.Map interface which can only hold a single object
30.A multi valued Map
31.A simple hashmap from keys to integers
32.A memory-efficient hash map.
33.An implementation of the java.util.Map interface which can only hold a single object.
34.Utility methods for operating on memory-efficient maps.
35.CaseBlindHashMap - a HashMap extension, using Strings as key values.
36.A fixed size map implementation.
37.Int HashMap
38.IntMap provides a simple hashmap from keys to integers
39.Complex Key HashMap
40.A Map with multiple values for a key
41.A Map that accepts int or Integer keys only
42.A Map where keys are compared by object identity, rather than equals()
43.Type-safe Map, from char array to String value
44.List ordered map
45.Hash map using String values as keys mapped to primitive int values.
46.Lookup table that stores a list of strings
47.HashNMap stores multiple values by a single key value. Values can be retrieved using a direct query or by creating an enumeration over the stored elements.
48.Combines multiple values to form a single composite key. MultiKey can often be used as an alternative to nested maps.