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edu.emory.mathcs.util
Class VolatileHashMap  view VolatileHashMap download VolatileHashMap.java

java.lang.Object
  extended byedu.emory.mathcs.util.VolatileHashMap
All Implemented Interfaces:
java.util.Map

public class VolatileHashMap
extends java.lang.Object
implements java.util.Map

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 interface java.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 interface java.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 interface java.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 interface java.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 interface java.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 interface java.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 interface java.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 interface java.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 interface java.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 interface java.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 interface java.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 interface java.util.Map