File	Doc	Category	Size	Date	Package
HashMap.java	API Doc	Java SE 6 API	35909	Tue Jun 10 00:25:54 BST 2008	java.util

HashMap

• java.lang.Object
  • java.util.AbstractMap

Map m = Collections.synchronizedMap(new HashMap(...));

Fields Summary
static final int	DEFAULT_INITIAL_CAPACITY The default initial capacity - MUST be a power of two.
static final int	MAXIMUM_CAPACITY 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 float	DEFAULT_LOAD_FACTOR The load factor used when none specified in constructor.
transient Entry[]	table The table, resized as necessary. Length MUST Always be a power of two.
transient int	size The number of key-value mappings contained in this map.
int	threshold The next size value at which to resize (capacity * load factor).
final float	loadFactor The load factor for the hash table.
volatile transient int	modCount 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 transient Set	entrySet
private static final long	serialVersionUID

Constructors Summary
public HashMap(int initialCapacity, float loadFactor) Constructs an empty HashMap 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 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)(capacity * loadFactor); table = new Entry[capacity]; init();
public HashMap(int initialCapacity) Constructs an empty HashMap 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. this(initialCapacity, DEFAULT_LOAD_FACTOR);
public HashMap() Constructs an empty HashMap with the default initial capacity (16) and the default load factor (0.75). this.loadFactor = DEFAULT_LOAD_FACTOR; threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR); table = new Entry[DEFAULT_INITIAL_CAPACITY]; init();
public HashMap(Map m) Constructs a new HashMap with the same mappings as the specified Map. The HashMap is created with default load factor (0.75) and an initial capacity sufficient to hold the mappings in the specified Map. param m the map whose mappings are to be placed in this map throws NullPointerException if the specified map is null this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR); putAllForCreate(m);

Methods Summary
void	addEntry(int hash, K key, V value, int bucketIndex) 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. Subclass overrides this to alter the behavior of put method. Entry<K,V> e = table[bucketIndex]; table[bucketIndex] = new Entry<K,V>(hash, key, value, e); if (size++ >= threshold) resize(2 * table.length);
int	capacity() return table.length;
public void	clear() Removes all of the mappings from this map. The map will be empty after this call returns. modCount++; Entry[] tab = table; for (int i = 0; i < tab.length; i++) tab[i] = null; size = 0;
public java.lang.Object	clone() Returns a shallow copy of this HashMap instance: the keys and values themselves are not cloned. return a shallow copy of this map 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;
public boolean	containsKey(java.lang.Object key) Returns true if this map contains a mapping for the specified key. param key The key whose presence in this map is to be tested return true if this map contains a mapping for the specified key. return getEntry(key) != null;
private boolean	containsNullValue() Special-case code for containsValue with null argument 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;
public boolean	containsValue(java.lang.Object value) Returns true 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 true if this map maps one or more keys to the specified 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;
void	createEntry(int hash, K key, V value, int 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. Subclass overrides this to alter the behavior of HashMap(Map), clone, and readObject. Entry<K,V> e = table[bucketIndex]; table[bucketIndex] = new Entry<K,V>(hash, key, value, e); size++;
public java.util.Set	entrySet() 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 remove operation, or through the setValue 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 Iterator.remove, Set.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations. return a set view of the mappings contained in this map return entrySet0();
private java.util.Set	entrySet0() Set<Map.Entry<K,V>> es = entrySet; return es != null ? es : (entrySet = new EntrySet());
public V	get(java.lang.Object key) Returns the value to which the specified key is mapped, or {@code null} if this map contains no mapping for the key. 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.) A return value of {@code null} does not necessarily 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) if (key == null) return getForNullKey(); int hash = hash(key.hashCode()); 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.equals(k))) return e.value; } return null;
final java.util.HashMap$Entry	getEntry(java.lang.Object key) Returns the entry associated with the specified key in the HashMap. Returns null if the HashMap contains no mapping for the key. int hash = (key == null) ? 0 : hash(key.hashCode()); 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;
private V	getForNullKey() 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. for (Entry<K,V> e = table[0]; e != null; e = e.next) { if (e.key == null) return e.value; } return null;
static int	hash(int h) Applies a supplemental hash function to a given hashCode, 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. // 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);
static int	indexFor(int h, int length) Returns index for hash code h. return h & (length-1);
void	init() 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.)
public boolean	isEmpty() Returns true if this map contains no key-value mappings. return true if this map contains no key-value mappings return size == 0;
public java.util.Set	keySet() 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 remove operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations. Set<K> ks = keySet; return (ks != null ? ks : (keySet = new KeySet()));
float	loadFactor() return loadFactor;
java.util.Iterator	newEntryIterator() return new EntryIterator();
java.util.Iterator	newKeyIterator() return new KeyIterator();
java.util.Iterator	newValueIterator() return new ValueIterator();
public V	put(K key, V value) 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 key, or null if there was no mapping for key. (A null return can also indicate that the map previously associated null with key.) if (key == null) return putForNullKey(value); int hash = hash(key.hashCode()); 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;
public void	putAll(java.util.Map m) 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 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<? extends Map.Entry<? extends K, ? extends V>> i = m.entrySet().iterator(); i.hasNext(); ) { Map.Entry<? extends K, ? extends V> e = i.next(); put(e.getKey(), e.getValue()); }
private void	putAllForCreate(java.util.Map m) for (Iterator<? extends Map.Entry<? extends K, ? extends V>> i = m.entrySet().iterator(); i.hasNext(); ) { Map.Entry<? extends K, ? extends V> e = i.next(); putForCreate(e.getKey(), e.getValue()); }
private void	putForCreate(K key, V value) 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. int hash = (key == null) ? 0 : hash(key.hashCode()); 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 V	putForNullKey(V value) Offloaded version of put for null keys 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;
private void	readObject(java.io.ObjectInputStream s) Reconstitute the HashMap instance from a stream (i.e., deserialize it). // Read in the threshold, loadfactor, and any hidden stuff s.defaultReadObject(); // Read in number of buckets and allocate the bucket array; int numBuckets = s.readInt(); table = new Entry[numBuckets]; init(); // Give subclass a chance to do its thing. // Read in size (number of Mappings) int size = s.readInt(); // Read the keys and values, and put the mappings in the HashMap for (int i=0; i<size; i++) { K key = (K) s.readObject(); V value = (V) s.readObject(); putForCreate(key, value); }
public V	remove(java.lang.Object key) 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 key, or null if there was no mapping for key. (A null return can also indicate that the map previously associated null with key.) Entry<K,V> e = removeEntryForKey(key); return (e == null ? null : e.value);
final java.util.HashMap$Entry	removeEntryForKey(java.lang.Object key) Removes and returns the entry associated with the specified key in the HashMap. Returns null if the HashMap contains no mapping for this key. int hash = (key == null) ? 0 : hash(key.hashCode()); 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;
final java.util.HashMap$Entry	removeMapping(java.lang.Object o) Special version of remove for EntrySet. if (!(o instanceof Map.Entry)) return null; Map.Entry<K,V> entry = (Map.Entry<K,V>) o; Object key = entry.getKey(); int hash = (key == null) ? 0 : hash(key.hashCode()); int i = indexFor(hash, table.length); Entry<K,V> prev = table[i]; Entry<K,V> e = prev; while (e != null) { Entry<K,V> next = e.next; if (e.hash == hash && e.equals(entry)) { modCount++; size--; if (prev == e) table[i] = next; else prev.next = next; e.recordRemoval(this); return e; } prev = e; e = next; } return e;
void	resize(int newCapacity) Rehashes the contents of this map into a new array with a larger capacity. This method is called automatically when the number of keys in this map reaches its threshold. If current capacity is MAXIMUM_CAPACITY, this method does not resize the map, but sets threshold to Integer.MAX_VALUE. This has the effect of preventing future calls. param newCapacity the new capacity, MUST be a power of two; must be greater than current capacity unless current capacity is MAXIMUM_CAPACITY (in which case value is irrelevant). Entry[] oldTable = table; int oldCapacity = oldTable.length; if (oldCapacity == MAXIMUM_CAPACITY) { threshold = Integer.MAX_VALUE; return; } Entry[] newTable = new Entry[newCapacity]; transfer(newTable); table = newTable; threshold = (int)(newCapacity * loadFactor);
public int	size() Returns the number of key-value mappings in this map. return the number of key-value mappings in this map return size;
void	transfer(java.util.HashMap$Entry[] newTable) Transfers all entries from current table to newTable. Entry[] src = table; int newCapacity = newTable.length; for (int j = 0; j < src.length; j++) { Entry<K,V> e = src[j]; if (e != null) { src[j] = null; do { Entry<K,V> next = e.next; int i = indexFor(e.hash, newCapacity); e.next = newTable[i]; newTable[i] = e; e = next; } while (e != null); } }
public java.util.Collection	values() 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 remove operation), the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Collection.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations. Collection<V> vs = values; return (vs != null ? vs : (values = new Values()));
private void	writeObject(java.io.ObjectOutputStream s) Save the state of the HashMap instance to a stream (i.e., serialize it). serialData The capacity of the HashMap (the length of the bucket array) is emitted (int), followed by the size (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. 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 (i != null) { while (i.hasNext()) { Map.Entry<K,V> e = i.next(); s.writeObject(e.getKey()); s.writeObject(e.getValue()); } }