Java tutorial
/* * 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 <= 1<<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); } } }