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java.lang.Object
org.apache.commons.collections.map.StaticBucketMap
public final class StaticBucketMap
extends java.lang.Object
implements Map
java.util.Map
that performs well in in a highly
thread-contentious environment. The map supports very efficient
get
, put
,
remove
and containsKey
operations, assuming (approximate) uniform hashing and
that the number of entries does not exceed the number of buckets. If the
number of entries exceeds the number of buckets or if the hash codes of the
objects are not uniformly distributed, these operations have a worst case
scenario that is proportional to the number of elements in the map
(O(n)).
Each bucket in the hash table has its own monitor, so two threads can
safely operate on the map at the same time, often without incurring any
monitor contention. This means that you don't have to wrap instances
of this class with java.util.Collections.synchronizedMap(Map)
;
instances are already thread-safe. Unfortunately, however, this means
that this map implementation behaves in ways you may find disconcerting.
Bulk operations, such as putAll
or the
retainAll
operation in collection
views, are not atomic. If two threads are simultaneously
executing
staticBucketMapInstance.putAll(map);and
staticBucketMapInstance.entrySet().removeAll(map.entrySet());then the results are generally random. Those two statement could cancel each other out, leaving
staticBucketMapInstance
essentially
unchanged, or they could leave some random subset of map
in
staticBucketMapInstance
.
Also, much like an encyclopedia, the results of size()
and
isEmpty()
are out-of-date as soon as they are produced.
The iterators returned by the collection views of this class are not
fail-fast. They will never raise a
java.util.ConcurrentModificationException
. Keys and values
added to the map after the iterator is created do not necessarily appear
during iteration. Similarly, the iterator does not necessarily fail to
return keys and values that were removed after the iterator was created.
Finally, unlike java.util.HashMap
-style implementations, this
class never rehashes the map. The number of buckets is fixed
at construction time and never altered. Performance may degrade if
you do not allocate enough buckets upfront.
The atomic(Runnable)
method is provided to allow atomic iterations
and bulk operations; however, overuse of atomic
will basically result in a map that's slower than an ordinary synchronized
java.util.HashMap
.
Use this class if you do not require reliable bulk operations and
iterations, or if you can make your own guarantees about how bulk
operations will affect the map.
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public StaticBucketMap()
Initializes the map with the default number of buckets (255).
public StaticBucketMap(int numBuckets)
Initializes the map with a specified number of buckets. The number of buckets is never below 17, and is always an odd number (StaticBucketMap ensures this). The number of buckets is inversely proportional to the chances for thread contention. The fewer buckets, the more chances for thread contention. The more buckets the fewer chances for thread contention.
- Parameters:
numBuckets
- the number of buckets for this map
public void atomic(Runnable r)
Prevents any operations from occurring on this map while the givenRunnable
executes. This method can be used, for instance, to execute a bulk operation atomically:staticBucketMapInstance.atomic(new Runnable() { public void run() { staticBucketMapInstance.putAll(map); } });It can also be used if you need a reliable iterator:staticBucketMapInstance.atomic(new Runnable() { public void run() { Iterator iterator = staticBucketMapInstance.iterator(); while (iterator.hasNext()) { foo(iterator.next(); } } });Implementation note: This method requires a lot of time and a ton of stack space. Essentially a recursive algorithm is used to enter each bucket's monitor. If you have twenty thousand buckets in your map, then the recursive method will be invoked twenty thousand times. You have been warned.
- Parameters:
r
- the code to execute atomically
public void clear()
Clears the map of all entries.
public boolean containsKey(Object key)
Checks if the map contains the specified key.
- Parameters:
key
- the key to check
- Returns:
- true if found
public boolean containsValue(Object value)
Checks if the map contains the specified value.
- Parameters:
value
- the value to check
- Returns:
- true if found
public Set entrySet()
Gets the entry set.
- Returns:
- the entry set
public boolean equals(Object obj)
Compares this map to another, as per the Map specification.
- Parameters:
obj
- the object to compare to
- Returns:
- true if equal
public Object get(Object key)
Gets the value associated with the key.
- Parameters:
key
- the key to retrieve
- Returns:
- the associated value
public int hashCode()
Gets the hash code, as per the Map specification.
- Returns:
- the hash code
public boolean isEmpty()
Checks if the size is currently zero.
- Returns:
- true if empty
public Set keySet()
Gets the key set.
- Returns:
- the key set
public Object put(Object key, Object value)
Puts a new key value mapping into the map.
- Parameters:
key
- the key to usevalue
- the value to use
- Returns:
- the previous mapping for the key
public void putAll(Map map)
Puts all the entries from the specified map into this map. This operation is not atomic and may have undesired effects.
- Parameters:
map
- the map of entries to add
public Object remove(Object key)
Removes the specified key from the map.
- Parameters:
key
- the key to remove
- Returns:
- the previous value at this key
public int size()
Gets the current size of the map. The value is computed fresh each time the method is called.
- Returns:
- the current size
public Collection values()
Gets the values.
- Returns:
- the values