This implementation provides constant-time performance for the basic * operations (get and put), assuming the hash function * disperses the elements properly among the buckets. Iteration over * collection views requires time proportional to the "capacity" of the * HashMap 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. * *
An instance of HashMap has two parameters that affect its * performance: initial capacity and load factor. The * capacity 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 * load factor 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 capacity is roughly doubled by calling the * rehash method. * *
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 * HashMap class, including get and put). 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. * *
If many mappings are to be stored in a HashMap 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. * *
Note that this implementation is not synchronized. If multiple * threads access this map concurrently, and at least one of the threads * modifies the map structurally, it must 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. If no such object exists, the map should be * "wrapped" using the Collections.synchronizedMap method. This is * best done at creation time, to prevent accidental unsynchronized access to * the map:
Map m = Collections.synchronizedMap(new HashMap(...)); ** *
The iterators returned by all of this class's "collection view methods" * are fail-fast: if the map is structurally modified at any time after * the iterator is created, in any way except through the iterator's own * remove or add methods, the iterator will throw a * 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. * *
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 ConcurrentModificationException on a best-effort basis. * Therefore, it would be wrong to write a program that depended on this * exception for its correctness: the fail-fast behavior of iterators * should be used only to detect bugs. * * @author Doug Lea * @author Josh Bloch * @author Arthur van Hoff * @version 1.51, 01/24/02 * @see Object#hashCode() * @see Collection * @see Map * @see TreeMap * @see Hashtable * @since 1.2 */ public class HashMap extends AbstractMap implements Map, 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 fast 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[] table; /** * The number of key-value mappings contained in this identity hash 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 volatile int modCount; /** * 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. */ 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)(capacity * loadFactor); table = new Entry[capacity]; init(); } /** * 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. */ public HashMap(int initialCapacity) { this(initialCapacity, DEFAULT_LOAD_FACTOR); } /** * Constructs an empty HashMap with the default initial capacity * (16) and the default load factor (0.75). */ public HashMap() { this.loadFactor = DEFAULT_LOAD_FACTOR; threshold = (int)(DEFAULT_INITIAL_CAPACITY); table = new Entry[DEFAULT_INITIAL_CAPACITY]; init(); } /** * 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. */ public HashMap(Map 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() { } /** * Value representing null keys inside tables. */ static final Object NULL_KEY = new Object(); /** * Returns internal representation for key. Use NULL_KEY if key is null. */ static Object maskNull(Object key) { return (key == null ? NULL_KEY : key); } /** * Returns key represented by specified internal representation. */ static Object unmaskNull(Object key) { return (key == NULL_KEY ? null : key); } /** * Returns a hash code for non-null Object x. */ int hash(Object x) { int h = x.hashCode(); return h - (h << 7); // i.e., -127 * h } /** * Check for equality of non-null reference x and possibly-null y. */ boolean eq(Object x, Object y) { return x == y || x.equals(y); } /** * 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 true if this map contains no key-value mappings. * * @return true if this map contains no key-value mappings. */ public boolean isEmpty() { return size == 0; } /** * Returns the value to which the specified key is mapped in this identity * hash map, or null if the map contains no mapping for this key. * A return value of null does not necessarily indicate * that the map contains no mapping for the key; it is also possible that * the map explicitly maps the key to null. The * containsKey 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 * null if the map contains no mapping for this key. * @see #put(Object, Object) */ public Object get(Object key) { Object k = maskNull(key); int hash = hash(k); int i = indexFor(hash, table.length); Entry e = table[i]; while (true) { if (e == null) return e; if (e.hash == hash && eq(k, e.key)) return e.value; e = e.next; } } /** * 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. */ public boolean containsKey(Object key) { Object k = maskNull(key); int hash = hash(k); int i = indexFor(hash, table.length); Entry e = table[i]; while (e != null) { if (e.hash == hash && eq(k, e.key)) return true; e = e.next; } return false; } /** * 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 hash = hash(k); int i = indexFor(hash, table.length); Entry e = table[i]; while (e != null && !(e.hash == hash && eq(k, e.key))) 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 null * if there was no mapping for key. A null return can * also indicate that the HashMap previously associated * null with the specified key. */ public Object put(Object key, Object value) { Object k = maskNull(key); int hash = hash(k); int i = indexFor(hash, table.length); for (Entry e = table[i]; e != null; e = e.next) { if (e.hash == hash && eq(k, e.key)) { Object oldValue = e.value; e.value = value; e.recordAccess(this); return oldValue; } } modCount++; addEntry(hash, k, value, i); 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(Object key, Object value) { Object k = maskNull(key); int hash = hash(k); 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 e = table[i]; e != null; e = e.next) { if (e.hash == hash && eq(k, e.key)) { e.value = value; return; } } createEntry(hash, k, value, i); } void putAllForCreate(Map m) { for (Iterator i = m.entrySet().iterator(); i.hasNext(); ) { Map.Entry e = (Map.Entry) i.next(); putForCreate(e.getKey(), e.getValue()); } } /** * Rehashes the contents of this map into a new HashMap instance * with a larger capacity. This method is called automatically when the * number of keys in this map exceeds its capacity and load factor. * * @param newCapacity the new capacity, MUST be a power of two. */ void resize(int newCapacity) { // assert (newCapacity & -newCapacity) == newCapacity; // power of 2 Entry[] oldTable = table; int oldCapacity = oldTable.length; // check if needed if (size < threshold || oldCapacity > newCapacity) return; Entry[] newTable = new Entry[newCapacity]; transfer(newTable); table = newTable; threshold = (int)(newCapacity * loadFactor); } /** * Transfer all entries from current table to newTable. */ void transfer(Entry[] newTable) { Entry[] src = table; int newCapacity = newTable.length; for (int j = 0; j < src.length; j++) { Entry e = src[j]; if (e != null) { src[j] = null; do { Entry next = e.next; int i = indexFor(e.hash, newCapacity); e.next = newTable[i]; newTable[i] = e; e = next; } while (e != null); } } } /** * 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 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 null * if there was no mapping for key. A null return can * also indicate that the map previously associated null * with the specified key. */ public Object remove(Object key) { Entry e = removeEntryForKey(key); return (e == null ? e : 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. */ Entry removeEntryForKey(Object key) { Object k = maskNull(key); int hash = hash(k); int i = indexFor(hash, table.length); Entry prev = table[i]; Entry e = prev; while (e != null) { Entry next = e.next; if (e.hash == hash && eq(k, e.key)) { 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. */ Entry removeMapping(Object o) { if (!(o instanceof Map.Entry)) return null; Map.Entry entry = (Map.Entry)o; Object k = maskNull(entry.getKey()); int hash = hash(k); int i = indexFor(hash, table.length); Entry prev = table[i]; Entry e = prev; while (e != null) { Entry 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 mappings from this map. */ public void clear() { modCount++; Entry tab[] = table; for (int i = 0; i < tab.length; i++) tab[i] = null; size = 0; } /** * 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. */ 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 HashMap instance: the keys and * values themselves are not cloned. * * @return a shallow copy of this map. */ public Object clone() { HashMap result = null; try { result = (HashMap)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 implements Map.Entry { final Object key; Object value; final int hash; Entry next; /** * Create new entry. */ Entry(int h, Object k, Object v, Entry n) { value = v; next = n; key = k; hash = h; } public Object getKey() { return unmaskNull(key); } 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() { return (key==NULL_KEY ? 0 : key.hashCode()) ^ (value==null ? 0 : value.hashCode()); } public 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 m) { } /** * This method is invoked whenever the entry is * removed from the table. */ void recordRemoval(HashMap m) { } } /** * Add 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. */ void addEntry(int hash, Object key, Object value, int bucketIndex) { table[bucketIndex] = new Entry(hash, key, value, table[bucketIndex]); if (size++ >= threshold) resize(2 * table.length); } /** * 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. */ void createEntry(int hash, Object key, Object value, int bucketIndex) { table[bucketIndex] = new Entry(hash, key, value, table[bucketIndex]); size++; } private abstract class HashIterator implements Iterator { Entry next; // next entry to return int expectedModCount; // For fast-fail int index; // current slot Entry current; // current entry HashIterator() { expectedModCount = modCount; Entry[] t = table; int i = t.length; Entry n = null; if (size != 0) { // advance to first entry while (i > 0 && (n = t[--i]) == null) ; } next = n; index = i; } public boolean hasNext() { return next != null; } Entry nextEntry() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); Entry e = next; if (e == null) throw new NoSuchElementException(); Entry n = e.next; Entry[] t = table; int i = index; while (n == null && i > 0) n = t[--i]; index = i; next = n; return current = 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 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(); } } // Subclass overrides these to alter behavior of views' iterator() method Iterator newKeyIterator() { return new KeyIterator(); } Iterator newValueIterator() { return new ValueIterator(); } Iterator newEntryIterator() { return new EntryIterator(); } // 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 Iterator.remove, * Set.remove, removeAll, retainAll, and * clear operations. It does not support the add or * addAll 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 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 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 * Iterator.remove, Collection.remove, * removeAll, retainAll, and clear operations. * It does not support the add or addAll 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 newValueIterator(); } public int size() { return size; } public boolean contains(Object o) { return containsValue(o); } public void clear() { HashMap.this.clear(); } } /** * Returns a collection view of the mappings contained in this map. Each * element in the returned collection is a Map.Entry. 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 * Iterator.remove, Collection.remove, * removeAll, retainAll, and clear operations. * It does not support the add or addAll operations. * * @return a collection view of the mappings contained in this map. * @see Map.Entry */ public Set entrySet() { Set es = entrySet; return (es != null ? es : (entrySet = new EntrySet())); } private class EntrySet extends AbstractSet { public Iterator iterator() { return newEntryIterator(); } 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 size; } public void clear() { HashMap.this.clear(); } } /** * 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 of the HashMap (the number of key-value * mappings), followed by the key (Object) and value (Object) * for each key-value mapping represented by the HashMap * The key-value mappings are emitted in the order that they * are returned by entrySet().iterator(). * */ private void writeObject(java.io.ObjectOutputStream s) throws IOException { // 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) for (Iterator i = entrySet().iterator(); i.hasNext(); ) { Map.Entry e = (Map.Entry) i.next(); s.writeObject(e.getKey()); s.writeObject(e.getValue()); } } private static final long serialVersionUID = 362498820763181265L; /** * Reconstitute the HashMap instance from a stream (i.e., * deserialize it). */ private void readObject(java.io.ObjectInputStream s) throws IOException, ClassNotFoundException { // 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