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
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.
The default initial capacity - MUST be a power of two.
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.
The load factor used when none specified in constructor.
The table, resized as necessary. Length MUST Always be a power of two.
The number of key-value mappings contained in this map.
The next size value at which to resize (capacity * load factor).
The load factor for the hash table.
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).
Constructs an empty
HashMap with the specified initial
capacity and load factor.
- Parameters:
initialCapacity the initial capacityloadFactor the load factor- Throws:
java.lang.IllegalArgumentException if the initial capacity is negative
or the load factor is nonpositive
public HashMap(int initialCapacity, float loadFactor) { if (loadFactor <= 0 || Float.isNaN(loadFactor))
while (capacity < initialCapacity)
Constructs an empty
HashMap with the specified initial
capacity and the default load factor (0.75).
public HashMap(int initialCapacity) { Constructs an empty
HashMap with the default initial capacity
(16) and the default load factor (0.75).
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.
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.)
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.
h ^= (h >>> 20) ^ (h >>> 12);
return h ^ (h >>> 7) ^ (h >>> 4);
Returns index for hash code h.
static int indexFor(int h, int length) { Returns the number of key-value mappings in this map.
- Returns:
- the number of key-value mappings in this map
Returns
true if this map contains no key-value mappings.
- Returns:
- true if this map contains no key-value mappings
Returns the value to which the specified key is mapped,
or
null if this map contains no mapping for the key.
More formally, if this map contains a mapping from a key
k to a value v such that (key==null ? k==null :
key.equals(k)), then this method returns v; otherwise
it returns null. (There can be at most one such mapping.)
A return value of 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 null.
The containsKey operation may be used to
distinguish these two cases.
if (e.hash == hash && ((k = e.key) == key || key.equals(k)))
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) { Returns
true if this map contains a mapping for the
specified key.
- Parameters:
key The key whose presence in this map is to be tested- Returns:
- true if this map contains a mapping for the specified
key.
Returns the entry associated with the specified key in the
HashMap. Returns null if the HashMap contains no mapping
for the key.
((k = e.key) == key || (key != null && key.equals(k))))
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.
- Parameters:
key key with which the specified value is to be associatedvalue value to be associated with the specified key- Returns:
- 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.)
public V put(K key, V value) { for (Entry<K,V> e = table[i]; e != null; e = e.next) { if (e.hash == hash && ((k = e.key) == key || key.equals(k))) { Offloaded version of put for null keys
for (Entry<K,V> e = table[0]; e != null; e = e.next) { 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.
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) { ((k = e.key) == key || (key != null && key.equals(k)))) { 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.
- Parameters:
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;
Entry[] newTable = new Entry[newCapacity];
Transfers all entries from current table to newTable.
int newCapacity = newTable.length;
for (int j = 0; j < src.length; j++) { Entry<K,V> next = 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.
public void putAll(Map<? extends K, ? extends V> m) { int numKeysToBeAdded = m.size();
if (numKeysToBeAdded == 0)
int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
while (newCapacity < targetCapacity)
Removes the mapping for the specified key from this map if present.
- Parameters:
key key whose mapping is to be removed from the map- Returns:
- 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.)
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.
Entry<K,V> next = e.next;
((k = e.key) == key || (key != null && key.equals(k)))) { Special version of remove for EntrySet.
Entry<K,V> next = e.next;
if (e.hash == hash && e.equals(entry)) { Removes all of the mappings from this map.
The map will be empty after this call returns.
for (int i = 0; i < tab.length; i++)
Returns
true if this map maps one or more keys to the
specified value.
- Parameters:
value value whose presence in this map is to be tested- Returns:
- true if this map maps one or more keys to the
specified value
for (int i = 0; i < tab.length ; i++)
for (Entry e = tab[i] ; e != null ; e = e.next)
Special-case code for containsValue with null argument
for (int i = 0; i < tab.length ; i++)
for (Entry e = tab[i] ; e != null ; e = e.next)
Returns a shallow copy of this
HashMap instance: the keys and
values themselves are not cloned.
- Returns:
- a shallow copy of this map
if (k1 == k2 || (k1 != null && k1.equals(k2))) { if (v1 == v2 || (v1 != null && v1.equals(v2)))
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.
This method is invoked whenever the entry is
removed from the table.
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.
void addEntry(int hash, K key, V value, int bucketIndex) { table[bucketIndex] = new Entry<K,V>(hash, key, value, e);
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, K key, V value, int bucketIndex) { table[bucketIndex] = new Entry<K,V>(hash, key, value, e);
if ((next = e.next) == 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. 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.
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.
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.
- Returns:
- a set view of the mappings contained in this map
return candidate != null && candidate.equals(e);
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.
Reconstitute the
HashMap instance from a stream (i.e.,
deserialize it).
for (int i=0; i<size; i++) {
