/** * Retrieve object hash code and applies a supplemental hash function to the result hash, which * defends against poor quality hash functions. This is critical because HashMap uses power-of-two * length hash tables, that otherwise encounter collisions for hashCodes that do not differ in * lower bits. Note: Null keys always map to hash 0, thus index 0. */ final int hash(Object k) { int h = 0; if (useAltHashing) { if (k instanceof String) { return sun.misc.Hashing.stringHash32((String) k); } h = hashSeed; } h ^= k.hashCode(); // This function ensures that hashCodes that differ only by // constant multiples at each bit position have a bounded // number of collisions (approximately 8 at default load factor). h ^= (h >>> 20) ^ (h >>> 12); return h ^ (h >>> 7) ^ (h >>> 4); }
/** Reconstitute the {@code HashMap} instance from a stream (i.e., deserialize it). */ private void readObject(java.io.ObjectInputStream s) throws IOException, ClassNotFoundException { // Read in the threshold (ignored), loadfactor, and any hidden stuff s.defaultReadObject(); if (loadFactor <= 0 || Float.isNaN(loadFactor)) throw new InvalidObjectException("Illegal load factor: " + loadFactor); // set hashSeed (can only happen after VM boot) Holder.UNSAFE.putIntVolatile( this, Holder.HASHSEED_OFFSET, sun.misc.Hashing.randomHashSeed(this)); // Read in number of buckets and allocate the bucket array; s.readInt(); // ignored // Read number of mappings int mappings = s.readInt(); if (mappings < 0) throw new InvalidObjectException("Illegal mappings count: " + mappings); int initialCapacity = (int) Math.min( // capacity chosen by number of mappings // and desired load (if >= 0.25) mappings * Math.min(1 / loadFactor, 4.0f), // we have limits... HashMap.MAXIMUM_CAPACITY); int capacity = 1; // find smallest power of two which holds all mappings while (capacity < initialCapacity) { capacity <<= 1; } table = new Entry[capacity]; threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1); useAltHashing = sun.misc.VM.isBooted() && (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD); init(); // Give subclass a chance to do its thing. // Read the keys and values, and put the mappings in the HashMap for (int i = 0; i < mappings; i++) { K key = (K) s.readObject(); V value = (V) s.readObject(); putForCreate(key, value); } }
/** * Hash table based implementation of the <tt>Map</tt> interface. This implementation provides all * of the optional map operations, and permits <tt>null</tt> values and the <tt>null</tt> key. (The * <tt>HashMap</tt> class is roughly equivalent to <tt>Hashtable</tt>, except that it is * unsynchronized and permits nulls.) This class makes no guarantees as to the order of the map; in * particular, it does not guarantee that the order will remain constant over time. * * <p>This implementation provides constant-time performance for the basic operations (<tt>get</tt> * and <tt>put</tt>), assuming the hash function disperses the elements properly among the buckets. * Iteration over collection views requires time proportional to the "capacity" of the * <tt>HashMap</tt> instance (the number of buckets) plus its size (the number of key-value * mappings). Thus, it's very important not to set the initial capacity too high (or the load factor * too low) if iteration performance is important. * * <p>An instance of <tt>HashMap</tt> has two parameters that affect its performance: <i>initial * capacity</i> and <i>load factor</i>. The <i>capacity</i> is the number of buckets in the hash * table, and the initial capacity is simply the capacity at the time the hash table is created. The * <i>load factor</i> is a measure of how full the hash table is allowed to get before its capacity * is automatically increased. When the number of entries in the hash table exceeds the product of * the load factor and the current capacity, the hash table is <i>rehashed</i> (that is, internal * data structures are rebuilt) so that the hash table has approximately twice the number of * buckets. * * <p>As a general rule, the default load factor (.75) offers a good tradeoff between time and space * costs. Higher values decrease the space overhead but increase the lookup cost (reflected in most * of the operations of the <tt>HashMap</tt> class, including <tt>get</tt> and <tt>put</tt>). The * expected number of entries in the map and its load factor should be taken into account when * setting its initial capacity, so as to minimize the number of rehash operations. If the initial * capacity is greater than the maximum number of entries divided by the load factor, no rehash * operations will ever occur. * * <p>If many mappings are to be stored in a <tt>HashMap</tt> instance, creating it with a * sufficiently large capacity will allow the mappings to be stored more efficiently than letting it * perform automatic rehashing as needed to grow the table. * * <p><strong>Note that this implementation is not synchronized.</strong> If multiple threads access * a hash map concurrently, and at least one of the threads modifies the map structurally, it * <i>must</i> be synchronized externally. (A structural modification is any operation that adds or * deletes one or more mappings; merely changing the value associated with a key that an instance * already contains is not a structural modification.) This is typically accomplished by * synchronizing on some object that naturally encapsulates the map. * * <p>If no such object exists, the map should be "wrapped" using the {@link * Collections#synchronizedMap Collections.synchronizedMap} method. This is best done at creation * time, to prevent accidental unsynchronized access to the map: * * <pre> * Map m = Collections.synchronizedMap(new HashMap(...));</pre> * * <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> method, the iterator will throw a {@link * ConcurrentModificationException}. 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> * * <p>This class is a member of the <a href="{@docRoot}/../technotes/guides/collections/index.html"> * Java Collections Framework</a>. * * @param <K> the type of keys maintained by this map * @param <V> the type of mapped values * @author Doug Lea * @author Josh Bloch * @author Arthur van Hoff * @author Neal Gafter * @see Object#hashCode() * @see Collection * @see Map * @see TreeMap * @see Hashtable * @since 1.2 */ public class HashMap<K, V> extends AbstractMap<K, V> implements Map<K, V>, Cloneable, Serializable { /** The default initial capacity - MUST be a power of two. */ 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. */ static final int MAXIMUM_CAPACITY = 1 << 30; /** The load factor used when none specified in constructor. */ static final float DEFAULT_LOAD_FACTOR = 0.75f; /** The table, resized as necessary. Length MUST Always be a power of two. */ transient Entry<K, V>[] table; /** The number of key-value mappings contained in this map. */ transient int size; /** * The next size value at which to resize (capacity * load factor). * * @serial */ int threshold; /** * The load factor for the hash table. * * @serial */ final float loadFactor; /** * 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). */ transient int modCount; /** * The default threshold of map capacity above which alternative hashing is used for String keys. * Alternative hashing reduces the incidence of collisions due to weak hash code calculation for * String keys. * * <p>This value may be overridden by defining the system property {@code * jdk.map.althashing.threshold}. A property value of {@code 1} forces alternative hashing to be * used at all times whereas {@code -1} value ensures that alternative hashing is never used. */ static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE; /** holds values which can't be initialized until after VM is booted. */ private static class Holder { // Unsafe mechanics /** Unsafe utilities */ static final sun.misc.Unsafe UNSAFE; /** Offset of "final" hashSeed field we must set in readObject() method. */ static final long HASHSEED_OFFSET; /** Table capacity above which to switch to use alternative hashing. */ static final int ALTERNATIVE_HASHING_THRESHOLD; static { String altThreshold = java.security.AccessController.doPrivileged( new sun.security.action.GetPropertyAction("jdk.map.althashing.threshold")); int threshold; try { threshold = (null != altThreshold) ? Integer.parseInt(altThreshold) : ALTERNATIVE_HASHING_THRESHOLD_DEFAULT; // disable alternative hashing if -1 if (threshold == -1) { threshold = Integer.MAX_VALUE; } if (threshold < 0) { throw new IllegalArgumentException("value must be positive integer."); } } catch (IllegalArgumentException failed) { throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed); } ALTERNATIVE_HASHING_THRESHOLD = threshold; try { UNSAFE = sun.misc.Unsafe.getUnsafe(); HASHSEED_OFFSET = UNSAFE.objectFieldOffset(HashMap.class.getDeclaredField("hashSeed")); } catch (NoSuchFieldException | SecurityException e) { throw new Error("Failed to record hashSeed offset", e); } } } /** * If {@code true} then perform alternative hashing of String keys to reduce the incidence of * collisions due to weak hash code calculation. */ transient boolean useAltHashing; /** * A randomizing value associated with this instance that is applied to hash code of keys to make * hash collisions harder to find. */ final transient int hashSeed = sun.misc.Hashing.randomHashSeed(this); /** * Constructs an empty <tt>HashMap</tt> with the specified initial capacity and load factor. * * @param initialCapacity the initial capacity * @param loadFactor the load factor * @throws IllegalArgumentException if the initial capacity is negative or the load factor is * nonpositive */ public HashMap(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); // Find a power of 2 >= initialCapacity int capacity = 1; while (capacity < initialCapacity) capacity <<= 1; this.loadFactor = loadFactor; threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1); table = new Entry[capacity]; useAltHashing = sun.misc.VM.isBooted() && (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD); init(); } /** * Constructs an empty <tt>HashMap</tt> with the specified initial capacity and the default load * factor (0.75). * * @param initialCapacity the initial capacity. * @throws IllegalArgumentException if the initial capacity is negative. */ public HashMap(int initialCapacity) { this(initialCapacity, DEFAULT_LOAD_FACTOR); } /** * Constructs an empty <tt>HashMap</tt> with the default initial capacity (16) and the default * load factor (0.75). */ public HashMap() { this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR); } /** * Constructs a new <tt>HashMap</tt> with the same mappings as the specified <tt>Map</tt>. The * <tt>HashMap</tt> is created with default load factor (0.75) and an initial capacity sufficient * to hold the mappings in the specified <tt>Map</tt>. * * @param m the map whose mappings are to be placed in this map * @throws NullPointerException if the specified map is null */ public HashMap(Map<? extends K, ? extends V> m) { this( Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR); putAllForCreate(m); } // internal utilities /** * Initialization hook for subclasses. This method is called in all constructors and * pseudo-constructors (clone, readObject) after HashMap has been initialized but before any * entries have been inserted. (In the absence of this method, readObject would require explicit * knowledge of subclasses.) */ void init() {} /** * Retrieve object hash code and applies a supplemental hash function to the result hash, which * defends against poor quality hash functions. This is critical because HashMap uses power-of-two * length hash tables, that otherwise encounter collisions for hashCodes that do not differ in * lower bits. Note: Null keys always map to hash 0, thus index 0. */ final int hash(Object k) { int h = 0; if (useAltHashing) { if (k instanceof String) { return sun.misc.Hashing.stringHash32((String) k); } h = hashSeed; } h ^= k.hashCode(); // This function ensures that hashCodes that differ only by // constant multiples at each bit position have a bounded // number of collisions (approximately 8 at default load factor). h ^= (h >>> 20) ^ (h >>> 12); return h ^ (h >>> 7) ^ (h >>> 4); } /** Returns index for hash code h. */ static int indexFor(int h, int length) { return h & (length - 1); } /** * Returns the number of key-value mappings in this map. * * @return the number of key-value mappings in this map */ public int size() { return size; } /** * Returns <tt>true</tt> if this map contains no key-value mappings. * * @return <tt>true</tt> if this map contains no key-value mappings */ public boolean isEmpty() { return size == 0; } /** * Returns the value to which the specified key is mapped, or {@code null} if this map contains no * mapping for the key. * * <p>More formally, if this map contains a mapping from a key {@code k} to a value {@code v} such * that {@code (key==null ? k==null : key.equals(k))}, then this method returns {@code v}; * otherwise it returns {@code null}. (There can be at most one such mapping.) * * <p>A return value of {@code null} does not <i>necessarily</i> indicate that the map contains no * mapping for the key; it's also possible that the map explicitly maps the key to {@code null}. * The {@link #containsKey containsKey} operation may be used to distinguish these two cases. * * @see #put(Object, Object) */ public V get(Object key) { if (key == null) return getForNullKey(); Entry<K, V> entry = getEntry(key); return null == entry ? null : entry.getValue(); } /** * Offloaded version of get() to look up null keys. Null keys map to index 0. This null case is * split out into separate methods for the sake of performance in the two most commonly used * operations (get and put), but incorporated with conditionals in others. */ private V getForNullKey() { for (Entry<K, V> e = table[0]; e != null; e = e.next) { if (e.key == null) return e.value; } 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 this map contains a mapping for the specified key. */ 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 the key. */ final Entry<K, V> getEntry(Object key) { int hash = (key == null) ? 0 : hash(key); for (Entry<K, V> e = table[indexFor(hash, table.length)]; e != null; e = e.next) { Object k; if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) return e; } return null; } /** * Associates the specified value with the specified key in this map. If the map previously * contained a mapping for the 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 the previous value associated with <tt>key</tt>, or <tt>null</tt> if there was no * mapping for <tt>key</tt>. (A <tt>null</tt> return can also indicate that the map previously * associated <tt>null</tt> with <tt>key</tt>.) */ public V put(K key, V value) { if (key == null) return putForNullKey(value); int hash = hash(key); int i = indexFor(hash, table.length); for (Entry<K, V> e = table[i]; e != null; e = e.next) { Object k; if (e.hash == hash && ((k = e.key) == key || key.equals(k))) { V oldValue = e.value; e.value = value; e.recordAccess(this); return oldValue; } } modCount++; addEntry(hash, key, value, i); return null; } /** Offloaded version of put for null keys */ private V putForNullKey(V value) { for (Entry<K, V> e = table[0]; e != null; e = e.next) { if (e.key == null) { V oldValue = e.value; e.value = value; e.recordAccess(this); return oldValue; } } modCount++; addEntry(0, null, value, 0); return null; } /** * This method is used instead of put by constructors and pseudoconstructors (clone, readObject). * It does not resize the table, check for comodification, etc. It calls createEntry rather than * addEntry. */ private void putForCreate(K key, V value) { int hash = null == key ? 0 : hash(key); int i = indexFor(hash, table.length); /** * Look for preexisting entry for key. This will never happen for clone or deserialize. It will * only happen for construction if the input Map is a sorted map whose ordering is inconsistent * w/ equals. */ for (Entry<K, V> e = table[i]; e != null; e = e.next) { Object k; if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) { e.value = value; return; } } createEntry(hash, key, value, i); } private void putAllForCreate(Map<? extends K, ? extends V> m) { for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) putForCreate(e.getKey(), e.getValue()); } /** * 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 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 = table; int oldCapacity = oldTable.length; if (oldCapacity == MAXIMUM_CAPACITY) { threshold = Integer.MAX_VALUE; return; } Entry[] newTable = new Entry[newCapacity]; boolean oldAltHashing = useAltHashing; useAltHashing |= sun.misc.VM.isBooted() && (newCapacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD); boolean rehash = oldAltHashing ^ useAltHashing; transfer(newTable, rehash); table = newTable; threshold = (int) Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1); } /** Transfers all entries from current table to newTable. */ void transfer(Entry[] newTable, boolean rehash) { int newCapacity = newTable.length; for (Entry<K, V> e : table) { while (null != e) { Entry<K, V> next = e.next; if (rehash) { e.hash = null == e.key ? 0 : hash(e.key); } int i = indexFor(e.hash, newCapacity); e.next = newTable[i]; newTable[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<? extends K, ? extends V> 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 (Map.Entry<? extends K, ? extends V> e : m.entrySet()) put(e.getKey(), e.getValue()); } /** * Removes the mapping for the specified key from this map if present. * * @param key key whose mapping is to be removed from the map * @return the previous value associated with <tt>key</tt>, or <tt>null</tt> if there was no * mapping for <tt>key</tt>. (A <tt>null</tt> return can also indicate that the map previously * associated <tt>null</tt> with <tt>key</tt>.) */ public V remove(Object key) { Entry<K, V> e = removeEntryForKey(key); return (e == null ? null : e.value); } /** * Removes and returns the entry associated with the specified key in the HashMap. Returns null if * the HashMap contains no mapping for this key. */ final Entry<K, V> removeEntryForKey(Object key) { int hash = (key == null) ? 0 : hash(key); int i = indexFor(hash, table.length); Entry<K, V> prev = table[i]; Entry<K, V> e = prev; while (e != null) { Entry<K, V> next = e.next; Object k; if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) { modCount++; size--; if (prev == e) table[i] = next; else prev.next = next; e.recordRemoval(this); return e; } prev = e; e = next; } return e; } /** Special version of remove for EntrySet using {@code Map.Entry.equals()} for matching. */ final Entry<K, V> removeMapping(Object o) { if (!(o instanceof Map.Entry)) return null; Map.Entry<K, V> entry = (Map.Entry<K, V>) o; Object key = entry.getKey(); int hash = (key == null) ? 0 : hash(key); int i = indexFor(hash, table.length); Entry<K, V> prev = table[i]; Entry<K, V> e = prev; while (e != null) { Entry<K, V> next = e.next; if (e.hash == hash && e.equals(entry)) { modCount++; size--; if (prev == e) table[i] = next; else prev.next = next; e.recordRemoval(this); return e; } prev = e; e = next; } return e; } /** Removes all of the mappings from this map. The map will be empty after this call returns. */ public void clear() { modCount++; Entry[] tab = table; for (int i = 0; i < tab.length; i++) tab[i] = null; size = 0; } /** * 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 = table; for (int i = 0; i < tab.length; i++) 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 = table; for (int i = 0; i < tab.length; i++) for (Entry e = tab[i]; e != null; e = e.next) if (e.value == null) return true; return false; } /** * Returns a shallow copy of this <tt>HashMap</tt> instance: the keys and values themselves are * not cloned. * * @return a shallow copy of this map */ public Object clone() { HashMap<K, V> result = null; try { result = (HashMap<K, V>) super.clone(); } catch (CloneNotSupportedException e) { // assert false; } result.table = new Entry[table.length]; result.entrySet = null; result.modCount = 0; result.size = 0; result.init(); result.putAllForCreate(this); return result; } static class Entry<K, V> implements Map.Entry<K, V> { final K key; V value; Entry<K, V> next; int hash; /** Creates new entry. */ Entry(int h, K k, V v, Entry<K, V> n) { value = v; next = n; key = k; hash = h; } public final K getKey() { return key; } public final V getValue() { return value; } public final V setValue(V newValue) { V oldValue = value; value = newValue; return oldValue; } public final 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 final int hashCode() { return (key == null ? 0 : key.hashCode()) ^ (value == null ? 0 : value.hashCode()); } public final String toString() { return getKey() + "=" + getValue(); } /** * This method is invoked whenever the value in an entry is overwritten by an invocation of * put(k,v) for a key k that's already in the HashMap. */ void recordAccess(HashMap<K, V> m) {} /** This method is invoked whenever the entry is removed from the table. */ void recordRemoval(HashMap<K, V> m) {} } /** * Adds a new entry with the specified key, value and hash code to the specified bucket. It is the * responsibility of this method to resize the table if appropriate. * * <p>Subclass overrides this to alter the behavior of put method. */ void addEntry(int hash, K key, V value, int bucketIndex) { if ((size >= threshold) && (null != table[bucketIndex])) { resize(2 * table.length); hash = (null != key) ? hash(key) : 0; bucketIndex = indexFor(hash, table.length); } createEntry(hash, key, value, bucketIndex); } /** * Like addEntry except that this version is used when creating entries as part of Map * construction or "pseudo-construction" (cloning, deserialization). This version needn't worry * about resizing the table. * * <p>Subclass overrides this to alter the behavior of HashMap(Map), clone, and readObject. */ void createEntry(int hash, K key, V value, int bucketIndex) { Entry<K, V> e = table[bucketIndex]; table[bucketIndex] = new Entry<>(hash, key, value, e); size++; } private abstract class HashIterator<E> implements Iterator<E> { Entry<K, V> next; // next entry to return int expectedModCount; // For fast-fail int index; // current slot Entry<K, V> current; // current entry HashIterator() { expectedModCount = modCount; if (size > 0) { // advance to first entry Entry[] t = table; while (index < t.length && (next = t[index++]) == null) ; } } public final boolean hasNext() { return next != null; } final Entry<K, V> nextEntry() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); Entry<K, V> e = next; if (e == null) throw new NoSuchElementException(); if ((next = e.next) == null) { Entry[] t = table; while (index < t.length && (next = t[index++]) == null) ; } current = e; return e; } public void remove() { if (current == null) throw new IllegalStateException(); if (modCount != expectedModCount) throw new ConcurrentModificationException(); Object k = current.key; current = null; HashMap.this.removeEntryForKey(k); expectedModCount = modCount; } } private final class ValueIterator extends HashIterator<V> { public V next() { return nextEntry().value; } } private final class KeyIterator extends HashIterator<K> { public K next() { return nextEntry().getKey(); } } private final class EntryIterator extends HashIterator<Map.Entry<K, V>> { public Map.Entry<K, V> next() { return nextEntry(); } } // Subclass overrides these to alter behavior of views' iterator() method Iterator<K> newKeyIterator() { return new KeyIterator(); } Iterator<V> newValueIterator() { return new ValueIterator(); } Iterator<Map.Entry<K, V>> newEntryIterator() { return new EntryIterator(); } // Views private transient Set<Map.Entry<K, V>> entrySet = null; /** * Returns a {@link 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. If the map is modified while an * iteration over the set is in progress (except through the iterator's own <tt>remove</tt> * operation), the results of the iteration are undefined. The set supports element removal, which * removes the corresponding mapping from the 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. */ public Set<K> keySet() { Set<K> ks = keySet; return (ks != null ? ks : (keySet = new KeySet())); } private final class KeySet extends AbstractSet<K> { public Iterator<K> iterator() { return newKeyIterator(); } public int size() { return size; } public boolean contains(Object o) { return containsKey(o); } public boolean remove(Object o) { return HashMap.this.removeEntryForKey(o) != null; } public void clear() { HashMap.this.clear(); } } /** * Returns a {@link 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. If the map * is modified while an iteration over the collection is in progress (except through the * iterator's own <tt>remove</tt> operation), the results of the iteration are undefined. 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. */ public Collection<V> values() { Collection<V> vs = values; return (vs != null ? vs : (values = new Values())); } private final class Values extends AbstractCollection<V> { public Iterator<V> iterator() { return newValueIterator(); } public int size() { return size; } public boolean contains(Object o) { return containsValue(o); } public void clear() { HashMap.this.clear(); } } /** * Returns a {@link Set} view of the mappings contained in this map. The set is backed by the map, * so changes to the map are reflected in the set, and vice-versa. If the map is modified while an * iteration over the set is in progress (except through the iterator's own <tt>remove</tt> * operation, or through the <tt>setValue</tt> operation on a map entry returned by the iterator) * the results of the iteration are undefined. The set supports element removal, which removes the * corresponding mapping from the 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 mappings contained in this map */ public Set<Map.Entry<K, V>> entrySet() { return entrySet0(); } private Set<Map.Entry<K, V>> entrySet0() { Set<Map.Entry<K, V>> es = entrySet; return es != null ? es : (entrySet = new EntrySet()); } private final class EntrySet extends AbstractSet<Map.Entry<K, V>> { public Iterator<Map.Entry<K, V>> iterator() { return newEntryIterator(); } public boolean contains(Object o) { if (!(o instanceof Map.Entry)) return false; Map.Entry<K, V> e = (Map.Entry<K, V>) o; Entry<K, V> candidate = getEntry(e.getKey()); return candidate != null && candidate.equals(e); } public boolean remove(Object o) { return removeMapping(o) != null; } public int size() { return size; } public void clear() { HashMap.this.clear(); } } /** * Save the state of the <tt>HashMap</tt> instance to a stream (i.e., serialize it). * * @serialData The <i>capacity</i> of the HashMap (the length of the bucket array) is emitted * (int), followed by the <i>size</i> (an int, the number of key-value mappings), followed by * the key (Object) and value (Object) for each key-value mapping. The key-value mappings are * emitted in no particular order. */ private void writeObject(java.io.ObjectOutputStream s) throws IOException { Iterator<Map.Entry<K, V>> i = (size > 0) ? entrySet0().iterator() : null; // Write out the threshold, loadfactor, and any hidden stuff s.defaultWriteObject(); // Write out number of buckets s.writeInt(table.length); // Write out size (number of Mappings) s.writeInt(size); // Write out keys and values (alternating) if (size > 0) { for (Map.Entry<K, V> e : entrySet0()) { s.writeObject(e.getKey()); s.writeObject(e.getValue()); } } } private static final long serialVersionUID = 362498820763181265L; /** Reconstitute the {@code HashMap} instance from a stream (i.e., deserialize it). */ private void readObject(java.io.ObjectInputStream s) throws IOException, ClassNotFoundException { // Read in the threshold (ignored), loadfactor, and any hidden stuff s.defaultReadObject(); if (loadFactor <= 0 || Float.isNaN(loadFactor)) throw new InvalidObjectException("Illegal load factor: " + loadFactor); // set hashSeed (can only happen after VM boot) Holder.UNSAFE.putIntVolatile( this, Holder.HASHSEED_OFFSET, sun.misc.Hashing.randomHashSeed(this)); // Read in number of buckets and allocate the bucket array; s.readInt(); // ignored // Read number of mappings int mappings = s.readInt(); if (mappings < 0) throw new InvalidObjectException("Illegal mappings count: " + mappings); int initialCapacity = (int) Math.min( // capacity chosen by number of mappings // and desired load (if >= 0.25) mappings * Math.min(1 / loadFactor, 4.0f), // we have limits... HashMap.MAXIMUM_CAPACITY); int capacity = 1; // find smallest power of two which holds all mappings while (capacity < initialCapacity) { capacity <<= 1; } table = new Entry[capacity]; threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1); useAltHashing = sun.misc.VM.isBooted() && (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD); init(); // Give subclass a chance to do its thing. // Read the keys and values, and put the mappings in the HashMap for (int i = 0; i < mappings; i++) { K key = (K) s.readObject(); V value = (V) s.readObject(); putForCreate(key, value); } } // These methods are used when serializing HashSets int capacity() { return table.length; } float loadFactor() { return loadFactor; } }