core/plugins/org.eclipse.dltk.core/utils/org/apache/commons/collections/map/AbstractReferenceMap.java

/*
 *  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.
 */
package org.apache.commons.collections.map;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.util.ArrayList;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;

import org.apache.commons.collections.MapIterator;
import org.apache.commons.collections.keyvalue.DefaultMapEntry;

/**
 * An abstract implementation of a hash-based map that allows the entries to be
 * removed by the garbage collector.
 * <p>
 * This class implements all the features necessary for a subclass reference
 * hash-based map. Key-value entries are stored in instances of the
 * <code>ReferenceEntry</code> class which can be overridden and replaced. The
 * iterators can similarly be replaced, without the need to replace the KeySet,
 * EntrySet and Values view classes.
 * <p>
 * Overridable methods are provided to change the default hashing behaviour, and
 * to change how entries are added to and removed from the map. Hopefully, all
 * you need for unusual subclasses is here.
 * <p>
 * When you construct an <code>AbstractReferenceMap</code>, you can specify
 * what kind of references are used to store the map's keys and values. If
 * non-hard references are used, then the garbage collector can remove mappings
 * if a key or value becomes unreachable, or if the JVM's memory is running low.
 * For information on how the different reference types behave, see
 * {@link Reference}.
 * <p>
 * Different types of references can be specified for keys and values. The keys
 * can be configured to be weak but the values hard, in which case this class
 * will behave like a <a
 * href="http://java.sun.com/j2se/1.4/docs/api/java/util/WeakHashMap.html">
 * <code>WeakHashMap</code></a>. However, you can also specify hard keys and
 * weak values, or any other combination. The default constructor uses hard keys
 * and soft values, providing a memory-sensitive cache.
 * <p>
 * This {@link Map} implementation does <i>not</i> allow null elements.
 * Attempting to add a null key or value to the map will raise a
 * <code>NullPointerException</code>.
 * <p>
 * All the available iterators can be reset back to the start by casting to
 * <code>ResettableIterator</code> and calling <code>reset()</code>.
 * <p>
 * This implementation is not synchronized. You can use
 * {@link java.util.Collections#synchronizedMap} to provide synchronized access
 * to a <code>ReferenceMap</code>.
 * 
 * @see java.lang.ref.Reference
 * @since Commons Collections 3.1 (extracted from ReferenceMap in 3.0)
 * @version $Revision: 1.2 $ $Date: 2009/01/20 23:39:19 $
 * 
 * @author Paul Jack
 * @author Stephen Colebourne
 */
public abstract class AbstractReferenceMap extends AbstractHashedMap {

	/** Constant indicating that hard references should be used */
	public static final int HARD = 0;

	/** Constant indicating that soft references should be used */
	public static final int SOFT = 1;

	/** Constant indicating that weak references should be used */
	public static final int WEAK = 2;

	/**
	 * The reference type for keys. Must be HARD, SOFT, WEAK.
	 * 
	 * @serial
	 */
	protected int keyType;

	/**
	 * The reference type for values. Must be HARD, SOFT, WEAK.
	 * 
	 * @serial
	 */
	protected int valueType;

	/**
	 * Should the value be automatically purged when the associated key has been
	 * collected?
	 */
	protected boolean purgeValues;

	/**
	 * ReferenceQueue used to eliminate stale mappings. See purge.
	 */
	private transient ReferenceQueue queue;

	// -----------------------------------------------------------------------
	/**
	 * Constructor used during deserialization.
	 */
	protected AbstractReferenceMap() {
		super();
	}

	/**
	 * Constructs a new empty map with the specified reference types, load
	 * factor and initial capacity.
	 * 
	 * @param keyType
	 *            the type of reference to use for keys; must be {@link #HARD},
	 *            {@link #SOFT}, {@link #WEAK}
	 * @param valueType
	 *            the type of reference to use for values; must be {@link #HARD},
	 *            {@link #SOFT}, {@link #WEAK}
	 * @param capacity
	 *            the initial capacity for the map
	 * @param loadFactor
	 *            the load factor for the map
	 * @param purgeValues
	 *            should the value be automatically purged when the key is
	 *            garbage collected
	 */
	protected AbstractReferenceMap(int keyType, int valueType, int capacity,
			float loadFactor, boolean purgeValues) {
		super(capacity, loadFactor);
		verify("keyType", keyType); //$NON-NLS-1$
		verify("valueType", valueType); //$NON-NLS-1$
		this.keyType = keyType;
		this.valueType = valueType;
		this.purgeValues = purgeValues;
	}

	/**
	 * Initialise this subclass during construction, cloning or deserialization.
	 */
	protected void init() {
		queue = new ReferenceQueue();
	}

	// -----------------------------------------------------------------------
	/**
	 * Checks the type int is a valid value.
	 * 
	 * @param name
	 *            the name for error messages
	 * @param type
	 *            the type value to check
	 * @throws IllegalArgumentException
	 *             if the value if invalid
	 */
	private static void verify(String name, int type) {
		if ((type < HARD) || (type > WEAK)) {
			throw new IllegalArgumentException(name
					+ " must be HARD, SOFT, WEAK."); //$NON-NLS-1$
		}
	}

	// -----------------------------------------------------------------------
	/**
	 * Gets the size of the map.
	 * 
	 * @return the size
	 */
	public int size() {
		purgeBeforeRead();
		return super.size();
	}

	/**
	 * Checks whether the map is currently empty.
	 * 
	 * @return true if the map is currently size zero
	 */
	public boolean isEmpty() {
		purgeBeforeRead();
		return super.isEmpty();
	}

	/**
	 * Checks whether the map contains the specified key.
	 * 
	 * @param key
	 *            the key to search for
	 * @return true if the map contains the key
	 */
	public boolean containsKey(Object key) {
		purgeBeforeRead();
		Entry entry = getEntry(key);
		if (entry == null) {
			return false;
		}
		return (entry.getValue() != null);
	}

	/**
	 * Checks whether the map contains the specified value.
	 * 
	 * @param value
	 *            the value to search for
	 * @return true if the map contains the value
	 */
	public boolean containsValue(Object value) {
		purgeBeforeRead();
		if (value == null) {
			return false;
		}
		return super.containsValue(value);
	}

	/**
	 * Gets the value mapped to the key specified.
	 * 
	 * @param key
	 *            the key
	 * @return the mapped value, null if no match
	 */
	public Object get(Object key) {
		purgeBeforeRead();
		Entry entry = getEntry(key);
		if (entry == null) {
			return null;
		}
		return entry.getValue();
	}

	/**
	 * Puts a key-value mapping into this map. Neither the key nor the value may
	 * be null.
	 * 
	 * @param key
	 *            the key to add, must not be null
	 * @param value
	 *            the value to add, must not be null
	 * @return the value previously mapped to this key, null if none
	 * @throws NullPointerException
	 *             if either the key or value is null
	 */
	public Object put(Object key, Object value) {
		if (key == null) {
			throw new NullPointerException("null keys not allowed"); //$NON-NLS-1$
		}
		if (value == null) {
			throw new NullPointerException("null values not allowed"); //$NON-NLS-1$
		}

		purgeBeforeWrite();
		return super.put(key, value);
	}

	/**
	 * Removes the specified mapping from this map.
	 * 
	 * @param key
	 *            the mapping to remove
	 * @return the value mapped to the removed key, null if key not in map
	 */
	public Object remove(Object key) {
		if (key == null) {
			return null;
		}
		purgeBeforeWrite();
		return super.remove(key);
	}

	/**
	 * Clears this map.
	 */
	public void clear() {
		super.clear();
		while (queue.poll() != null) {
		} // drain the queue
	}

	// -----------------------------------------------------------------------
	/**
	 * Gets a MapIterator over the reference map. The iterator only returns
	 * valid key/value pairs.
	 * 
	 * @return a map iterator
	 */
	public MapIterator mapIterator() {
		return new ReferenceMapIterator(this);
	}

	/**
	 * Returns a set view of this map's entries. An iterator returned entry is
	 * valid until <code>next()</code> is called again. The
	 * <code>setValue()</code> method on the <code>toArray</code> entries
	 * has no effect.
	 * 
	 * @return a set view of this map's entries
	 */
	public Set entrySet() {
		if (entrySet == null) {
			entrySet = new ReferenceEntrySet(this);
		}
		return entrySet;
	}

	/**
	 * Returns a set view of this map's keys.
	 * 
	 * @return a set view of this map's keys
	 */
	public Set keySet() {
		if (keySet == null) {
			keySet = new ReferenceKeySet(this);
		}
		return keySet;
	}

	/**
	 * Returns a collection view of this map's values.
	 * 
	 * @return a set view of this map's values
	 */
	public Collection values() {
		if (values == null) {
			values = new ReferenceValues(this);
		}
		return values;
	}

	// -----------------------------------------------------------------------
	/**
	 * Purges stale mappings from this map before read operations.
	 * <p>
	 * This implementation calls {@link #purge()} to maintain a consistent
	 * state.
	 */
	protected void purgeBeforeRead() {
		purge();
	}

	/**
	 * Purges stale mappings from this map before write operations.
	 * <p>
	 * This implementation calls {@link #purge()} to maintain a consistent
	 * state.
	 */
	protected void purgeBeforeWrite() {
		purge();
	}

	/**
	 * Purges stale mappings from this map.
	 * <p>
	 * Note that this method is not synchronized! Special care must be taken if,
	 * for instance, you want stale mappings to be removed on a periodic basis
	 * by some background thread.
	 */
	protected void purge() {
		Reference ref = queue.poll();
		while (ref != null) {
			purge(ref);
			ref = queue.poll();
		}
	}

	/**
	 * Purges the specified reference.
	 * 
	 * @param ref
	 *            the reference to purge
	 */
	protected void purge(Reference ref) {
		// The hashCode of the reference is the hashCode of the
		// mapping key, even if the reference refers to the
		// mapping value...
		int hash = ref.hashCode();
		int index = hashIndex(hash, data.length);
		HashEntry previous = null;
		HashEntry entry = data[index];
		while (entry != null) {
			if (((ReferenceEntry) entry).purge(ref)) {
				if (previous == null) {
					data[index] = entry.next;
				} else {
					previous.next = entry.next;
				}
				this.size--;
				return;
			}
			previous = entry;
			entry = entry.next;
		}

	}

	// -----------------------------------------------------------------------
	/**
	 * Gets the entry mapped to the key specified.
	 * 
	 * @param key
	 *            the key
	 * @return the entry, null if no match
	 */
	protected HashEntry getEntry(Object key) {
		if (key == null) {
			return null;
		} else {
			return super.getEntry(key);
		}
	}

	/**
	 * Gets the hash code for a MapEntry. Subclasses can override this, for
	 * example to use the identityHashCode.
	 * 
	 * @param key
	 *            the key to get a hash code for, may be null
	 * @param value
	 *            the value to get a hash code for, may be null
	 * @return the hash code, as per the MapEntry specification
	 */
	protected int hashEntry(Object key, Object value) {
		return (key == null ? 0 : key.hashCode())
				^ (value == null ? 0 : value.hashCode());
	}

	/**
	 * Compares two keys, in internal converted form, to see if they are equal.
	 * <p>
	 * This implementation converts the key from the entry to a real reference
	 * before comparison.
	 * 
	 * @param key1
	 *            the first key to compare passed in from outside
	 * @param key2
	 *            the second key extracted from the entry via
	 *            <code>entry.key</code>
	 * @return true if equal
	 */
	protected boolean isEqualKey(Object key1, Object key2) {
		key2 = (keyType > HARD ? ((Reference) key2).get() : key2);
		return (key1 == key2 || key1.equals(key2));
	}

	/**
	 * Creates a ReferenceEntry instead of a HashEntry.
	 * 
	 * @param next
	 *            the next entry in sequence
	 * @param hashCode
	 *            the hash code to use
	 * @param key
	 *            the key to store
	 * @param value
	 *            the value to store
	 * @return the newly created entry
	 */
	protected HashEntry createEntry(HashEntry next, int hashCode, Object key,
			Object value) {
		return new ReferenceEntry(this, next, hashCode, key, value);
	}

	/**
	 * Creates an entry set iterator.
	 * 
	 * @return the entrySet iterator
	 */
	protected Iterator createEntrySetIterator() {
		return new ReferenceEntrySetIterator(this);
	}

	/**
	 * Creates an key set iterator.
	 * 
	 * @return the keySet iterator
	 */
	protected Iterator createKeySetIterator() {
		return new ReferenceKeySetIterator(this);
	}

	/**
	 * Creates an values iterator.
	 * 
	 * @return the values iterator
	 */
	protected Iterator createValuesIterator() {
		return new ReferenceValuesIterator(this);
	}

	// -----------------------------------------------------------------------
	/**
	 * EntrySet implementation.
	 */
	static class ReferenceEntrySet extends EntrySet {

		protected ReferenceEntrySet(AbstractHashedMap parent) {
			super(parent);
		}

		public Object[] toArray() {
			return toArray(new Object[0]);
		}

		public Object[] toArray(Object[] arr) {
			// special implementation to handle disappearing entries
			ArrayList list = new ArrayList();
			Iterator iterator = iterator();
			while (iterator.hasNext()) {
				Entry e = (Entry) iterator.next();
				list.add(new DefaultMapEntry(e.getKey(), e.getValue()));
			}
			return list.toArray(arr);
		}
	}

	// -----------------------------------------------------------------------
	/**
	 * KeySet implementation.
	 */
	static class ReferenceKeySet extends KeySet {

		protected ReferenceKeySet(AbstractHashedMap parent) {
			super(parent);
		}

		public Object[] toArray() {
			return toArray(new Object[0]);
		}

		public Object[] toArray(Object[] arr) {
			// special implementation to handle disappearing keys
			List list = new ArrayList(parent.size());
			for (Iterator it = iterator(); it.hasNext();) {
				list.add(it.next());
			}
			return list.toArray(arr);
		}
	}

	// -----------------------------------------------------------------------
	/**
	 * Values implementation.
	 */
	static class ReferenceValues extends Values {

		protected ReferenceValues(AbstractHashedMap parent) {
			super(parent);
		}

		public Object[] toArray() {
			return toArray(new Object[0]);
		}

		public Object[] toArray(Object[] arr) {
			// special implementation to handle disappearing values
			List list = new ArrayList(parent.size());
			for (Iterator it = iterator(); it.hasNext();) {
				list.add(it.next());
			}
			return list.toArray(arr);
		}
	}

	// -----------------------------------------------------------------------
	/**
	 * A MapEntry implementation for the map.
	 * <p>
	 * If getKey() or getValue() returns null, it means the mapping is stale and
	 * should be removed.
	 * 
	 * @since Commons Collections 3.1
	 */
	protected static class ReferenceEntry extends HashEntry {
		/** The parent map */
		protected final AbstractReferenceMap parent;

		/**
		 * Creates a new entry object for the ReferenceMap.
		 * 
		 * @param parent
		 *            the parent map
		 * @param next
		 *            the next entry in the hash bucket
		 * @param hashCode
		 *            the hash code of the key
		 * @param key
		 *            the key
		 * @param value
		 *            the value
		 */
		public ReferenceEntry(AbstractReferenceMap parent, HashEntry next,
				int hashCode, Object key, Object value) {
			super(next, hashCode, null, null);
			this.parent = parent;
			this.key = toReference(parent.keyType, key, hashCode);
			this.value = toReference(parent.valueType, value, hashCode); // the
			// key
			// hashCode
			// is
			// passed
			// in
			// deliberately
		}

		/**
		 * Gets the key from the entry. This method dereferences weak and soft
		 * keys and thus may return null.
		 * 
		 * @return the key, which may be null if it was garbage collected
		 */
		public Object getKey() {
			return (parent.keyType > HARD) ? ((Reference) key).get() : key;
		}

		/**
		 * Gets the value from the entry. This method dereferences weak and soft
		 * value and thus may return null.
		 * 
		 * @return the value, which may be null if it was garbage collected
		 */
		public Object getValue() {
			return (parent.valueType > HARD) ? ((Reference) value).get()
					: value;
		}

		/**
		 * Sets the value of the entry.
		 * 
		 * @param obj
		 *            the object to store
		 * @return the previous value
		 */
		public Object setValue(Object obj) {
			Object old = getValue();
			if (parent.valueType > HARD) {
				((Reference) value).clear();
			}
			value = toReference(parent.valueType, obj, hashCode);
			return old;
		}

		/**
		 * Compares this map entry to another.
		 * <p>
		 * This implementation uses <code>isEqualKey</code> and
		 * <code>isEqualValue</code> on the main map for comparison.
		 * 
		 * @param obj
		 *            the other map entry to compare to
		 * @return true if equal, false if not
		 */
		public boolean equals(Object obj) {
			if (obj == this) {
				return true;
			}
			if (obj instanceof Map.Entry == false) {
				return false;
			}

			Map.Entry entry = (Map.Entry) obj;
			Object entryKey = entry.getKey(); // convert to hard reference
			Object entryValue = entry.getValue(); // convert to hard reference
			if ((entryKey == null) || (entryValue == null)) {
				return false;
			}
			// compare using map methods, aiding identity subclass
			// note that key is direct access and value is via method
			return parent.isEqualKey(entryKey, key)
					&& parent.isEqualValue(entryValue, getValue());
		}

		/**
		 * Gets the hashcode of the entry using temporary hard references.
		 * <p>
		 * This implementation uses <code>hashEntry</code> on the main map.
		 * 
		 * @return the hashcode of the entry
		 */
		public int hashCode() {
			return parent.hashEntry(getKey(), getValue());
		}

		/**
		 * Constructs a reference of the given type to the given referent. The
		 * reference is registered with the queue for later purging.
		 * 
		 * @param type
		 *            HARD, SOFT or WEAK
		 * @param referent
		 *            the object to refer to
		 * @param hash
		 *            the hash code of the <i>key</i> of the mapping; this
		 *            number might be different from referent.hashCode() if the
		 *            referent represents a value and not a key
		 */
		protected Object toReference(int type, Object referent, int hash) {
			switch (type) {
			case HARD:
				return referent;
			case SOFT:
				return new SoftRef(hash, referent, parent.queue);
			case WEAK:
				return new WeakRef(hash, referent, parent.queue);
			default:
				throw new Error();
			}
		}

		/**
		 * Purges the specified reference
		 * 
		 * @param ref
		 *            the reference to purge
		 * @return true or false
		 */
		boolean purge(Reference ref) {
			boolean r = (parent.keyType > HARD) && (key == ref);
			r = r || ((parent.valueType > HARD) && (value == ref));
			if (r) {
				if (parent.keyType > HARD) {
					((Reference) key).clear();
				}
				if (parent.valueType > HARD) {
					((Reference) value).clear();
				} else if (parent.purgeValues) {
					value = null;
				}
			}
			return r;
		}

		/**
		 * Gets the next entry in the bucket.
		 * 
		 * @return the next entry in the bucket
		 */
		protected ReferenceEntry next() {
			return (ReferenceEntry) next;
		}
	}

	// -----------------------------------------------------------------------
	/**
	 * The EntrySet iterator.
	 */
	static class ReferenceEntrySetIterator implements Iterator {
		/** The parent map */
		final AbstractReferenceMap parent;

		// These fields keep track of where we are in the table.
		int index;
		ReferenceEntry entry;
		ReferenceEntry previous;

		// These Object fields provide hard references to the
		// current and next entry; this assures that if hasNext()
		// returns true, next() will actually return a valid element.
		Object nextKey, nextValue;
		Object currentKey, currentValue;

		int expectedModCount;

		public ReferenceEntrySetIterator(AbstractReferenceMap parent) {
			super();
			this.parent = parent;
			index = (parent.size() != 0 ? parent.data.length : 0);
			// have to do this here! size() invocation above
			// may have altered the modCount.
			expectedModCount = parent.modCount;
		}

		public boolean hasNext() {
			checkMod();
			while (nextNull()) {
				ReferenceEntry e = entry;
				int i = index;
				while ((e == null) && (i > 0)) {
					i--;
					e = (ReferenceEntry) parent.data[i];
				}
				entry = e;
				index = i;
				if (e == null) {
					currentKey = null;
					currentValue = null;
					return false;
				}
				nextKey = e.getKey();
				nextValue = e.getValue();
				if (nextNull()) {
					entry = entry.next();
				}
			}
			return true;
		}

		private void checkMod() {
			if (parent.modCount != expectedModCount) {
				throw new ConcurrentModificationException();
			}
		}

		private boolean nextNull() {
			return (nextKey == null) || (nextValue == null);
		}

		protected ReferenceEntry nextEntry() {
			checkMod();
			if (nextNull() && !hasNext()) {
				throw new NoSuchElementException();
			}
			previous = entry;
			entry = entry.next();
			currentKey = nextKey;
			currentValue = nextValue;
			nextKey = null;
			nextValue = null;
			return previous;
		}

		protected ReferenceEntry currentEntry() {
			checkMod();
			return previous;
		}

		public Object next() {
			return nextEntry();
		}

		public void remove() {
			checkMod();
			if (previous == null) {
				throw new IllegalStateException();
			}
			parent.remove(currentKey);
			previous = null;
			currentKey = null;
			currentValue = null;
			expectedModCount = parent.modCount;
		}
	}

	/**
	 * The keySet iterator.
	 */
	static class ReferenceKeySetIterator extends ReferenceEntrySetIterator {

		ReferenceKeySetIterator(AbstractReferenceMap parent) {
			super(parent);
		}

		public Object next() {
			return nextEntry().getKey();
		}
	}

	/**
	 * The values iterator.
	 */
	static class ReferenceValuesIterator extends ReferenceEntrySetIterator {

		ReferenceValuesIterator(AbstractReferenceMap parent) {
			super(parent);
		}

		public Object next() {
			return nextEntry().getValue();
		}
	}

	/**
	 * The MapIterator implementation.
	 */
	static class ReferenceMapIterator extends ReferenceEntrySetIterator
			implements MapIterator {

		protected ReferenceMapIterator(AbstractReferenceMap parent) {
			super(parent);
		}

		public Object next() {
			return nextEntry().getKey();
		}

		public Object getKey() {
			HashEntry current = currentEntry();
			if (current == null) {
				throw new IllegalStateException(
						AbstractHashedMap.GETKEY_INVALID);
			}
			return current.getKey();
		}

		public Object getValue() {
			HashEntry current = currentEntry();
			if (current == null) {
				throw new IllegalStateException(
						AbstractHashedMap.GETVALUE_INVALID);
			}
			return current.getValue();
		}

		public Object setValue(Object value) {
			HashEntry current = currentEntry();
			if (current == null) {
				throw new IllegalStateException(
						AbstractHashedMap.SETVALUE_INVALID);
			}
			return current.setValue(value);
		}
	}

	// -----------------------------------------------------------------------
	// These two classes store the hashCode of the key of
	// of the mapping, so that after they're dequeued a quick
	// lookup of the bucket in the table can occur.

	/**
	 * A soft reference holder.
	 */
	static class SoftRef extends SoftReference {
		/** the hashCode of the key (even if the reference points to a value) */
		private int hash;

		public SoftRef(int hash, Object r, ReferenceQueue q) {
			super(r, q);
			this.hash = hash;
		}

		public int hashCode() {
			return hash;
		}
	}

	/**
	 * A weak reference holder.
	 */
	static class WeakRef extends WeakReference {
		/** the hashCode of the key (even if the reference points to a value) */
		private int hash;

		public WeakRef(int hash, Object r, ReferenceQueue q) {
			super(r, q);
			this.hash = hash;
		}

		public int hashCode() {
			return hash;
		}
	}

	// -----------------------------------------------------------------------
	/**
	 * Replaces the superclass method to store the state of this class.
	 * <p>
	 * Serialization is not one of the JDK's nicest topics. Normal serialization
	 * will initialise the superclass before the subclass. Sometimes however,
	 * this isn't what you want, as in this case the <code>put()</code> method
	 * on read can be affected by subclass state.
	 * <p>
	 * The solution adopted here is to serialize the state data of this class in
	 * this protected method. This method must be called by the
	 * <code>writeObject()</code> of the first serializable subclass.
	 * <p>
	 * Subclasses may override if they have a specific field that must be
	 * present on read before this implementation will work. Generally, the read
	 * determines what must be serialized here, if anything.
	 * 
	 * @param out
	 *            the output stream
	 */
	protected void doWriteObject(ObjectOutputStream out) throws IOException {
		out.writeInt(keyType);
		out.writeInt(valueType);
		out.writeBoolean(purgeValues);
		out.writeFloat(loadFactor);
		out.writeInt(data.length);
		for (MapIterator it = mapIterator(); it.hasNext();) {
			out.writeObject(it.next());
			out.writeObject(it.getValue());
		}
		out.writeObject(null); // null terminate map
		// do not call super.doWriteObject() as code there doesn't work for
		// reference map
	}

	/**
	 * Replaces the superclassm method to read the state of this class.
	 * <p>
	 * Serialization is not one of the JDK's nicest topics. Normal serialization
	 * will initialise the superclass before the subclass. Sometimes however,
	 * this isn't what you want, as in this case the <code>put()</code> method
	 * on read can be affected by subclass state.
	 * <p>
	 * The solution adopted here is to deserialize the state data of this class
	 * in this protected method. This method must be called by the
	 * <code>readObject()</code> of the first serializable subclass.
	 * <p>
	 * Subclasses may override if the subclass has a specific field that must be
	 * present before <code>put()</code> or <code>calculateThreshold()</code>
	 * will work correctly.
	 * 
	 * @param in
	 *            the input stream
	 */
	protected void doReadObject(ObjectInputStream in) throws IOException,
			ClassNotFoundException {
		this.keyType = in.readInt();
		this.valueType = in.readInt();
		this.purgeValues = in.readBoolean();
		this.loadFactor = in.readFloat();
		int capacity = in.readInt();
		init();
		data = new HashEntry[capacity];
		while (true) {
			Object key = in.readObject();
			if (key == null) {
				break;
			}
			Object value = in.readObject();
			put(key, value);
		}
		threshold = calculateThreshold(data.length, loadFactor);
		// do not call super.doReadObject() as code there doesn't work for
		// reference map
	}

}