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Class com.sun.java.util.collections.HashMap
java.lang.Object
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+----com.sun.java.util.collections.AbstractMap
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+----com.sun.java.util.collections.HashMap
- public class HashMap
- extends AbstractMap
- implements Map, Cloneable, Serializable
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. (HashMap 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 the hash function disperses the
elements properly among the buckets. Iteration over Collection views
requires time proportional to the "capacity" of the HashMap (the number of
buckets) plus its size (the number of key-value mappings). Thus, it's very
important not to set the intial 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 hashtable 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 HashMap
operations, including get and put). The expected number
of entries in the HashMap 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, 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 HashMap concurrently, and at least one of the
threads modifies the HashMap 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 is already contained in the Table is not a structural
modification.) This is typically accomplished by synchronizing on some
object that naturally encapsulates the HashMap. If no such object exists,
the HashMap should be "wrapped" using the Collections.synchronizedMap
method. This is best done at creation time, to prevent accidental
unsynchronized access to the HashMap:
Map m = Collections.synchronizedMap(new HashMap(...));
The Iterators returned by the iterator methods of the Collections returned
by all of HashMap's "collection view methods" are fail-fast: if the
HashMap 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.
- See Also:
- hashCode, Collection, Map, TreeMap, Hashtable
-
HashMap()
- Constructs a new, empty HashMap with a default capacity and load
factor, which is 0.75.
-
HashMap(int)
- Constructs a new, empty HashMap with the specified initial capacity
and default load factor, which is 0.75.
-
HashMap(int, float)
- Constructs a new, empty HashMap with the specified initial
capacity and the specified load factor.
-
HashMap(Map)
- Constructs a new HashMap with the same mappings as the given
Map.
-
capacity()
-
-
clear()
- Removes all mappings from this HashMap.
-
clone()
- Returns a shallow copy of this HashMap.
-
containsKey(Object)
- Returns true if this HashMap contains a mapping for the specified key.
-
containsValue(Object)
- Returns true if this HashMap maps one or more keys to the specified
value.
-
entrySet()
- Returns a Collection view of the mappings contained in this HashMap.
-
get(Object)
- Returns the value to which this HashMap maps the specified key.
-
isEmpty()
- Returns true if this Map contains no key-value mappings.
-
keySet()
- Returns a Set view of the keys contained in this HashMap.
-
loadFactor()
-
-
put(Object, Object)
- Associates the specified value with the specified key in this HashMap.
-
putAll(Map)
- Copies all of the mappings from the specified Map to this HashMap
These mappings will replace any mappings that this HashMap had for any
of the keys currently in the specified Map.
-
remove(Object)
- Removes the mapping for this key from this HashMap if present.
-
size()
- Returns the number of key-value mappings in this Map.
-
values()
- Returns a Collection view of the values contained in this HashMap.
HashMap
public HashMap(int initialCapacity,
float loadFactor)
- Constructs a new, empty HashMap with the specified initial
capacity and the specified load factor.
- Parameters:
- initialCapacity - the initial capacity of the HashMap.
- loadFactor - the load factor of the HashMap
- Throws: IllegalArgumentException
- if the initial capacity is less
than zero, or if the load factor is nonpositive.
HashMap
public HashMap(int initialCapacity)
- Constructs a new, empty HashMap with the specified initial capacity
and default load factor, which is 0.75.
- Parameters:
- initialCapacity - the initial capacity of the HashMap.
- Throws: IllegalArgumentException
- if the initial capacity is less
than zero.
HashMap
public HashMap()
- Constructs a new, empty HashMap with a default capacity and load
factor, which is 0.75.
HashMap
public HashMap(Map t)
- Constructs a new HashMap with the same mappings as the given
Map. The HashMap is created with a capacity of twice the number
of mappings in the given Map or 11 (whichever is greater), and a
default load factor, which is 0.75.
size
public int size()
- Returns the number of key-value mappings in this Map.
- Returns:
- the number of key-value mappings in this Map.
- Overrides:
- size in class AbstractMap
isEmpty
public boolean isEmpty()
- Returns true if this Map contains no key-value mappings.
- Returns:
- true if this Map contains no key-value mappings.
- Overrides:
- isEmpty in class AbstractMap
containsValue
public boolean containsValue(Object value)
- Returns true if this HashMap 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 HashMap maps one or more keys to the specified
value.
- Overrides:
- containsValue in class AbstractMap
containsKey
public boolean containsKey(Object key)
- Returns true if this HashMap contains a mapping for the specified key.
- Parameters:
- key - key whose presence in this Map is to be tested.
- Returns:
- true if this HashMap contains a mapping for the specified key.
- Overrides:
- containsKey in class AbstractMap
get
public Object get(Object key)
- Returns the value to which this HashMap maps the specified key.
Returns null if the HashMap contains no mapping for this key. A return
value of null does not necessarily indicate that the HashMap
contains no mapping for the key; it's also possible that the HashMap
explicitly maps the key to null. The containsKey operation may be
used to distinguish these two cases.
- Parameters:
- key - key whose associated value is to be returned.
- Returns:
- the value to which this HashMap maps the specified key.
- Overrides:
- get in class AbstractMap
put
public Object put(Object key,
Object value)
- Associates the specified value with the specified key in this HashMap.
If the HashMap previously contained a mapping for this key, the old
value is replaced.
- Parameters:
- key - key with which the specified value is to be associated.
- value - value to be associated with the specified key.
- Returns:
- 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.
- Overrides:
- put in class AbstractMap
remove
public Object remove(Object key)
- Removes the mapping for this key from this HashMap if present.
- Parameters:
- key - key whose mapping is to be removed from the Map.
- Returns:
- 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.
- Overrides:
- remove in class AbstractMap
putAll
public void putAll(Map t)
- Copies all of the mappings from the specified Map to this HashMap
These mappings will replace any mappings that this HashMap had for any
of the keys currently in the specified Map.
- Parameters:
- t - Mappings to be stored in this Map.
- Overrides:
- putAll in class AbstractMap
clear
public void clear()
- Removes all mappings from this HashMap.
- Overrides:
- clear in class AbstractMap
clone
public Object clone()
- Returns a shallow copy of this HashMap. The keys and values
themselves are not cloned.
- Returns:
- a shallow copy of this HashMap.
- Overrides:
- clone in class Object
keySet
public Set keySet()
- Returns a Set view of the keys contained in this HashMap. The Set is
backed by the HashMap, so changes to the HashMap are reflected in the
Set, and vice-versa. The Set supports element removal, which removes
the corresponding mapping from the HashMap, 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 keys contained in this HashMap.
- Overrides:
- keySet in class AbstractMap
values
public Collection values()
- Returns a Collection view of the values contained in this HashMap.
The Collection is backed by the HashMap, so changes to the HashMap are
reflected in the Collection, and vice-versa. The Collection supports
element removal, which removes the corresponding mapping from the
HashMap, via the Iterator.remove, Collection.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 HashMap.
- Overrides:
- values in class AbstractMap
entrySet
public Set entrySet()
- Returns a Collection view of the mappings contained in this HashMap.
Each element in the returned collection is a Map.Entry. The Collection
is backed by the HashMap, so changes to the HashMap are reflected in the
Collection, and vice-versa. The Collection supports element removal,
which removes the corresponding mapping from the HashMap, via the
Iterator.remove, Collection.remove, removeAll, retainAll and clear
operations. It does not support the add or addAll operations.
- Returns:
- a Collection view of the mappings contained in this HashMap.
- Overrides:
- entrySet in class AbstractMap
- See Also:
- Entry
capacity
int capacity()
loadFactor
float loadFactor()
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