Java tutorial
/* * JBoss, Home of Professional Open Source * Copyright 2005, JBoss Inc., and individual contributors as indicated * by the @authors tag. See the copyright.txt in the distribution for a * full listing of individual contributors. * * This is free software; you can redistribute it and/or modify it * under the terms of the GNU Lesser General Public License as * published by the Free Software Foundation; either version 2.1 of * the License, or (at your option) any later version. * * This software is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU * Lesser General Public License for more details. * * You should have received a copy of the GNU Lesser General Public * License along with this software; if not, write to the Free * Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA * 02110-1301 USA, or see the FSF site: http://www.fsf.org. */ import java.lang.ref.ReferenceQueue; import java.lang.ref.WeakReference; import java.util.AbstractCollection; 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>weak keys</em> and * using reference-equality in place of object-equality when comparing keys (and * values). In an <tt>WeakIdentityHashMap</tt>, two keys <tt>k1</tt> and * <tt>k2</tt> are considered equal if and only if <tt>(k1==k2)</tt>. An * entry in a <tt>WeakIdentityHashMap</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. * * <p> * Based on java.util.WeakHashMap * </p> * * @author Dawid Kurzyniec * @version <tt>$Revision: 2787 $</tt> * @author <a href="mailto:kabir.khan@jboss.org">Kabir Khan</a> * @see java.util.IdentityHashMap * @see java.util.WeakHashMap */ @SuppressWarnings("unchecked") public class WeakIdentityHashMap /* 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 weak 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 WeakEntries */ private final ReferenceQueue queue = new ReferenceQueue(); /** * The number of times this HashMap has been structurally modified Structural * modifications are those that change the number of mappings in the HashMap * or otherwise modify its internal structure (e.g., rehash). This field is * used to make iterators on Collection-views of the HashMap fail-fast. (See * ConcurrentModificationException). */ private volatile int modCount; /** * Each of these fields are initialized to contain an instance of the * appropriate view the first time this view is requested. The views are * stateless, so there's no reason to create more than one of each. */ transient volatile Set keySet = null; transient volatile Collection values = null; /** * Constructs a new, empty <tt>WeakIdentityHashMap</tt> with the given * initial capacity and the given load factor. * * @param initialCapacity * The initial capacity of the <tt>WeakIdentityHashMap</tt> * @param loadFactor * The load factor of the <tt>WeakIdentityHashMap</tt> * @throws IllegalArgumentException * If the initial capacity is negative, or if the load factor is * nonpositive. */ public WeakIdentityHashMap(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>WeakIdentityHashMap</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>WeakIdentityHashMap</tt> * @throws IllegalArgumentException * If the initial capacity is negative. */ public WeakIdentityHashMap(int initialCapacity) { this(initialCapacity, DEFAULT_LOAD_FACTOR); } /** * Constructs a new, empty <tt>WeakIdentityHashMap</tt> with the default * initial capacity (16) and the default load factor (0.75). */ public WeakIdentityHashMap() { this.loadFactor = DEFAULT_LOAD_FACTOR; threshold = DEFAULT_INITIAL_CAPACITY; table = new Entry[DEFAULT_INITIAL_CAPACITY]; } /** * Constructs a new <tt>WeakIdentityHashMap</tt> with the same mappings as * the specified <tt>Map</tt>. The <tt>WeakIdentityHashMap</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 WeakIdentityHashMap(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); } /** * Return a hash code for non-null Object x. */ int hash(Object x) { int h = System.identityHashCode(x); return h - (h << 7); // that is,, -127 * h } /** * 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 weak 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 && 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 HashMap. Returns * null if the HashMap 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 && 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 HashMap 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 && 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 <tt>HashMap</tt> instance * with a larger capacity. This method is called automatically when the number * of keys in this map exceeds its capacity and load factor. * * Note that this method is a no-op if it's called with newCapacity == * 2*MAXIMUM_CAPACITY (which is Integer.MIN_VALUE). * * @param newCapacity * the new capacity, MUST be a power of two. */ void resize(int newCapacity) { // assert (newCapacity & -newCapacity) == newCapacity; // power of 2 Entry[] oldTable = getTable(); int oldCapacity = oldTable.length; // check if needed if (size < threshold || oldCapacity > newCapacity) 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. * <p> * * @param t * mappings to be stored in this map. * @throws NullPointerException * if the specified map is null. */ public void putAll(Map t) { // Expand enough to hold t's elements without resizing. int n = t.size(); if (n == 0) return; if (n >= threshold) { n = (int) (n / loadFactor + 1); if (n > MAXIMUM_CAPACITY) n = MAXIMUM_CAPACITY; int capacity = table.length; while (capacity < n) capacity <<= 1; resize(capacity); } for (Iterator i = t.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 && 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 WeakReference, using its main ref * field as the key. */ private static class Entry extends WeakReference 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(this.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) { 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 : System.identityHashCode(k)) ^ (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; } 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(); WeakIdentityHashMap.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 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 WeakIdentityHashMap.this.size(); } public boolean contains(Object o) { return containsKey(o); } public boolean remove(Object o) { if (containsKey(o)) { WeakIdentityHashMap.this.remove(o); return true; } else return false; } public void clear() { WeakIdentityHashMap.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 WeakIdentityHashMap.this.size(); } public boolean contains(Object o) { return containsValue(o); } public void clear() { WeakIdentityHashMap.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. * @see java.util.Map.Entry */ 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 WeakIdentityHashMap.this.size(); } public void clear() { WeakIdentityHashMap.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 { Object key; Object value; public SimpleEntry(Object key, Object value) { this.key = key; this.value = 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)); } } }