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DETAIL: FIELD | CONSTR | METHOD |
edu.emory.mathcs.util
Class VolatileHashMap

java.lang.Objectedu.emory.mathcs.util.VolatileHashMap
- All Implemented Interfaces:
- java.util.Map
- public class VolatileHashMap
- extends java.lang.Object
- implements java.util.Map
- extends java.lang.Object
A hashtable-based Map implementation with weak values. An entry in a VolatileHashMap will automatically be removed when its value is no longer in ordinary use. More precisely, the presence of a mapping to a given value will not prevent the value from being discarded by the garbage collector, that is, made finalizable, finalized, and then reclaimed. When a value has been discarded, all entries referring to it are effectively removed from the map. This class is a companion to java.util.WeakHashMap.
Null values and the null key are supported. This class has performance characteristics similar to those of the HashMap class, and has the same efficiency parameters of initial capacity and load factor.
Like most collection classes, this class is not synchronized. A synchronized VolatileHashMap may be constructed using the Collections.synchronizedMap method.
The behavior of the VolatileHashMap class depends in part upon the actions of the garbage collector, so several familiar (though not required) Map invariants do not hold for this class. Because the garbage collector may discard entries at any time, a VolatileHashMap may behave as though an unknown thread is silently removing entries. In particular, even if you synchronize on a VolatileHashMap instance and invoke none of its mutator methods, it is possible for the size method to return smaller values over time, for the isEmpty method to return false and then true, for the containsKey method to return true and later false for a given key, for the get method to return a value for a given key but later return null, for the put method to return null and the remove method to return false for a key that previously appeared to be in the map, and for successive examinations of the key set, the value set, and the entry set to yield successively smaller numbers of elements.
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-determixnistic 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.
Nested Class Summary | |
private static class |
VolatileHashMap.Entry
The entries in this hash table |
private class |
VolatileHashMap.EntryIterator
|
private class |
VolatileHashMap.EntrySet
|
private class |
VolatileHashMap.HashIterator
|
private class |
VolatileHashMap.KeyIterator
|
private class |
VolatileHashMap.KeySet
|
(package private) static class |
VolatileHashMap.SimpleEntry
|
private class |
VolatileHashMap.ValueIterator
|
private class |
VolatileHashMap.Values
|
Field Summary | |
private static int |
DEFAULT_INITIAL_CAPACITY
The default initial capacity -- MUST be a power of two. |
private static float |
DEFAULT_LOAD_FACTOR
The load fast used when none specified in constructor. |
private java.util.Set |
entrySet
|
(package private) java.util.Set |
keySet
Each of these fields are initialized to contain an instance of the appropriate view the first time this view is requested. |
private float |
loadFactor
The load factor for the hash table. |
private static int |
MAXIMUM_CAPACITY
The maximum capacity, used if a higher value is implicitly specified by either of the constructors with arguments. |
private 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). |
private static java.lang.Object |
NULL_KEY
Value representing null keys inside tables. |
private java.lang.ref.ReferenceQueue |
queue
Reference queue for cleared WeakEntries |
private int |
size
The number of key-value mappings contained in this weak hash map. |
private VolatileHashMap.Entry[] |
table
The table, resized as necessary. |
private int |
threshold
The next size value at which to resize (capacity * load factor). |
(package private) java.util.Collection |
values
|
Constructor Summary | |
VolatileHashMap()
Constructs a new, empty VolatileHashMap with the default initial capacity (16) and the default load factor (0.75). |
|
VolatileHashMap(int initialCapacity)
Constructs a new, empty VolatileHashMap with the given initial capacity and the default load factor, which is 0.75. |
|
VolatileHashMap(int initialCapacity,
float loadFactor)
Constructs a new, empty VolatileHashMap with the given initial capacity and the given load factor. |
|
VolatileHashMap(java.util.Map t)
Constructs a new VolatileHashMap with the same mappings as the specified Map. |
Method Summary | |
void |
clear()
Removes all mappings from this map. |
boolean |
containsKey(java.lang.Object key)
Returns true if this map contains a mapping for the specified key. |
private boolean |
containsNullValue()
Special-case code for containsValue with null argument |
boolean |
containsValue(java.lang.Object value)
Returns true if this map maps one or more keys to the specified value. |
java.util.Set |
entrySet()
Returns a collection view of the mappings contained in this map. |
(package private) boolean |
eq(java.lang.Object x,
java.lang.Object y)
Check for equality of non-null reference x and possibly-null y. |
private void |
expungeStaleEntries()
Expunge stale entries from the table. |
java.lang.Object |
get(java.lang.Object key)
Returns the value to which the specified key is mapped in this volatile hash map, or null if the map contains no mapping for this key. |
(package private) VolatileHashMap.Entry |
getEntry(java.lang.Object key)
Returns the entry associated with the specified key in the HashMap. |
private VolatileHashMap.Entry[] |
getTable()
Return the table after first expunging stale entries |
(package private) int |
hash(java.lang.Object x)
Return a hash code for non-null Object x. |
(package private) static int |
indexFor(int h,
int length)
Return index for hash code h. |
boolean |
isEmpty()
Returns true if this map contains no key-value mappings. |
java.util.Set |
keySet()
Returns a set view of the keys contained in this map. |
private static java.lang.Object |
maskNull(java.lang.Object key)
Use NULL_KEY for key if it is null. |
java.lang.Object |
put(java.lang.Object key,
java.lang.Object value)
Associates the specified value with the specified key in this map. |
void |
putAll(java.util.Map t)
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. |
java.lang.Object |
remove(java.lang.Object key)
Removes the mapping for this key from this map if present. |
(package private) VolatileHashMap.Entry |
removeMapping(java.lang.Object o)
Special version of remove needed by Entry set |
(package private) void |
resize(int newCapacity)
Rehashes the contents of this map into a new HashMap instance with a larger capacity. |
int |
size()
Returns the number of key-value mappings in this map. |
private void |
transfer(VolatileHashMap.Entry[] src,
VolatileHashMap.Entry[] dest)
Transfer all entries from src to dest tables |
private static java.lang.Object |
unmaskNull(java.lang.Object key)
Return internal representation of null key back to caller as null |
java.util.Collection |
values()
Returns a collection view of the values contained in this map. |
Methods inherited from class java.lang.Object |
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
Methods inherited from interface java.util.Map |
equals, hashCode |
Field Detail |
DEFAULT_INITIAL_CAPACITY
private static final int DEFAULT_INITIAL_CAPACITY
- The default initial capacity -- MUST be a power of two.
- See Also:
- Constant Field Values
MAXIMUM_CAPACITY
private 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.
- See Also:
- Constant Field Values
DEFAULT_LOAD_FACTOR
private static final float DEFAULT_LOAD_FACTOR
- The load fast used when none specified in constructor.
- See Also:
- Constant Field Values
table
private VolatileHashMap.Entry[] table
- The table, resized as necessary. Length MUST Always be a power of two.
size
private int size
- The number of key-value mappings contained in this weak hash map.
threshold
private int threshold
- The next size value at which to resize (capacity * load factor).
loadFactor
private final float loadFactor
- The load factor for the hash table.
queue
private final java.lang.ref.ReferenceQueue queue
- Reference queue for cleared WeakEntries
modCount
private volatile 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).
keySet
transient volatile java.util.Set keySet
- Each of these fields are initialized to contain an instance of the
appropriate view the first time this view is requested. The views are
stateless, so there's no reason to create more than one of each.
values
transient volatile java.util.Collection values
NULL_KEY
private static final java.lang.Object NULL_KEY
- Value representing null keys inside tables.
entrySet
private transient java.util.Set entrySet
Constructor Detail |
VolatileHashMap
public VolatileHashMap(int initialCapacity, float loadFactor)
- Constructs a new, empty VolatileHashMap with the given initial
capacity and the given load factor.
VolatileHashMap
public VolatileHashMap(int initialCapacity)
- Constructs a new, empty VolatileHashMap with the given initial
capacity and the default load factor, which is 0.75.
VolatileHashMap
public VolatileHashMap()
- Constructs a new, empty VolatileHashMap with the default initial
capacity (16) and the default load factor (0.75).
VolatileHashMap
public VolatileHashMap(java.util.Map t)
- Constructs a new VolatileHashMap with the same mappings as the
specified Map. The VolatileHashMap is created with
default load factor, which is 0.75 and an initial capacity
sufficient to hold the mappings in the specified Map.
- Since:
- 1.3
Method Detail |
maskNull
private static java.lang.Object maskNull(java.lang.Object key)
- Use NULL_KEY for key if it is null.
unmaskNull
private static java.lang.Object unmaskNull(java.lang.Object key)
- Return internal representation of null key back to caller as null
hash
int hash(java.lang.Object x)
- Return a hash code for non-null Object x.
eq
boolean eq(java.lang.Object x, java.lang.Object y)
- Check for equality of non-null reference x and possibly-null y. By
default uses Object.equals.
indexFor
static int indexFor(int h, int length)
- Return index for hash code h.
expungeStaleEntries
private void expungeStaleEntries()
- Expunge stale entries from the table.
getTable
private VolatileHashMap.Entry[] getTable()
- Return the table after first expunging stale entries
size
public int size()
- Returns the number of key-value mappings in this map.
This result is a snapshot, and may not reflect unprocessed
entries that will be removed before next attempted access
because they are no longer referenced.
- Specified by:
size
in interfacejava.util.Map
isEmpty
public boolean isEmpty()
- Returns true if this map contains no key-value mappings.
This result is a snapshot, and may not reflect unprocessed
entries that will be removed before next attempted access
because they are no longer referenced.
- Specified by:
isEmpty
in interfacejava.util.Map
get
public java.lang.Object get(java.lang.Object key)
- Returns the value to which the specified key is mapped in this volatile
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.
- Specified by:
get
in interfacejava.util.Map
containsKey
public boolean containsKey(java.lang.Object key)
- Returns true if this map contains a mapping for the
specified key.
- Specified by:
containsKey
in interfacejava.util.Map
getEntry
VolatileHashMap.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 this key.
put
public java.lang.Object put(java.lang.Object key, java.lang.Object value)
- 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.
- Specified by:
put
in interfacejava.util.Map
resize
void resize(int newCapacity)
- 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.
Note that this method is a no-op if it's called with newCapacity ==
2*MAXIMUM_CAPACITY (which is Integer.MIN_VALUE).
transfer
private void transfer(VolatileHashMap.Entry[] src, VolatileHashMap.Entry[] dest)
- Transfer all entries from src to dest tables
putAll
public void putAll(java.util.Map t)
- 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.
- Specified by:
putAll
in interfacejava.util.Map
remove
public java.lang.Object remove(java.lang.Object key)
- Removes the mapping for this key from this map if present.
- Specified by:
remove
in interfacejava.util.Map
removeMapping
VolatileHashMap.Entry removeMapping(java.lang.Object o)
- Special version of remove needed by Entry set
clear
public void clear()
- Removes all mappings from this map.
- Specified by:
clear
in interfacejava.util.Map
containsValue
public boolean containsValue(java.lang.Object value)
- Returns true if this map maps one or more keys to the
specified value.
- Specified by:
containsValue
in interfacejava.util.Map
containsNullValue
private boolean containsNullValue()
- Special-case code for containsValue with null argument
keySet
public java.util.Set 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. 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.
- Specified by:
keySet
in interfacejava.util.Map
values
public java.util.Collection 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. 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.
- Specified by:
values
in interfacejava.util.Map
entrySet
public java.util.Set entrySet()
- 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.
- Specified by:
entrySet
in interfacejava.util.Map
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Home >> All >> edu >> emory >> mathcs >> [ util overview ] | PREV CLASS NEXT CLASS | ||||||||
SUMMARY: ![]() ![]() ![]() |
DETAIL: FIELD | CONSTR | METHOD |