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java.util
public class: HashMap [javadoc | source]
java.lang.Object
   java.util.AbstractMap<K, V>
      java.util.HashMap

All Implemented Interfaces:
    Cloneable, Map, Serializable

Direct Known Subclasses:
    PrinterStateReasons, LinkedHashMap, NegativeCache, ProcessEnvironment

Hash table based implementation of the Map interface. This implementation provides all of the optional map operations, and permits null values and the null key. (The HashMap class is roughly equivalent to Hashtable, 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.

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 hash table is rehashed (that is, internal data structures are rebuilt) so that the hash table has approximately twice the number of buckets.

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 a hash 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 method, 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.

This class is a member of the Java Collections Framework.

Nested Class Summary:
static class  HashMap.Entry   
Field 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).
    serial:
 
final  float loadFactor    The load factor for the hash table.
    serial:
 
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). 
Fields inherited from java.util.AbstractMap:
keySet,  values
Constructor:
 public HashMap() 
 public HashMap(int initialCapacity) 
 public HashMap(Map<? extends K, ? extends V> 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.
    Parameters:
    m - the map whose mappings are to be placed in this map
    Throws:
    NullPointerException - if the specified map is null
 public HashMap(int initialCapacity,
    float loadFactor) 
Method from java.util.HashMap Summary:
addEntry,   capacity,   clear,   clone,   containsKey,   containsValue,   createEntry,   entrySet,   get,   getEntry,   hash,   indexFor,   init,   isEmpty,   keySet,   loadFactor,   newEntryIterator,   newKeyIterator,   newValueIterator,   put,   putAll,   remove,   removeEntryForKey,   removeMapping,   resize,   size,   transfer,   values
Methods from java.util.AbstractMap:
clear,   clone,   containsKey,   containsValue,   entrySet,   equals,   get,   hashCode,   isEmpty,   keySet,   put,   putAll,   remove,   size,   toString,   values
Methods from java.lang.Object:
clone,   equals,   finalize,   getClass,   hashCode,   notify,   notifyAll,   toString,   wait,   wait,   wait
Method from java.util.HashMap Detail:
  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.
 int capacity() 
 public  void clear() 
    Removes all of the mappings from this map. The map will be empty after this call returns.
 public Object clone() 
    Returns a shallow copy of this HashMap instance: the keys and values themselves are not cloned.
 public boolean containsKey(Object key) 
    Returns true if this map contains a mapping for the specified key.
 public boolean containsValue(Object value) 
    Returns true if this map maps one or more keys to the specified value.
  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.
 public Set<K, V> entrySet() 
    Returns a 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.
 public V get(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 containsKey operation may be used to distinguish these two cases.

 final Entry<K, V> getEntry(Object key) 
    Returns the entry associated with the specified key in the HashMap. Returns null if the HashMap contains no mapping for the key.
 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.
 static int indexFor(int h,
    int length) 
    Returns index for hash code h.
  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.
 public Set<K> keySet() 
    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. 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.
 float loadFactor() 
 Iterator<K, V> newEntryIterator() 
 Iterator<K> newKeyIterator() 
 Iterator<V> newValueIterator() 
 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.
 public  void putAll(Map<? extends K, ? extends V> 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.
 public V remove(Object key) 
    Removes the mapping for the specified key from this map if present.
 final Entry<K, V> removeEntryForKey(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.
 final Entry<K, V> removeMapping(Object o) 
    Special version of remove for EntrySet.
  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.
 public int size() 
    Returns the number of key-value mappings in this map.
  void transfer(Entry[] newTable) 
    Transfers all entries from current table to newTable.
 public Collection<V> values() 
    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. 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.