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org.apache.commons.collections.map

Class StaticBucketMap

java.lang.Object
|
+--org.apache.commons.collections.map.StaticBucketMap

All Implemented Interfaces:
Map


public final class StaticBucketMap
extends java.lang.Object
implements Map

A StaticBucketMap is an efficient, thread-safe implementation of java.util.Map that performs well in in a highly thread-contentious environment. The map supports very efficient get(Object) get, put(Object,Object) put, remove(Object) remove and containsKey(Object) 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(Map) putAll or the Collection.retainAll(Collection) 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Also, much like an encyclopedia, the results of size() and isEmpty() are out-of-date as soon as they are produced.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(Runnable) 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.

Since:
Commons Collections 3.0 (previously in main package v2.1)
Authors:
Berin Loritsch
Gerhard Froehlich
Michael A. Smith
Paul Jack
Leo Sutic
Janek Bogucki

Constructor Summary

StaticBucketMap()

Initializes the map with the default number of buckets (255).

StaticBucketMap(int numBuckets)

Initializes the map with a specified number of buckets.

Method Summary

void

atomic(Runnable r)

Prevents any operations from occurring on this map while the given Runnable executes.

void

clear()

Clears the map of all entries.

boolean

containsKey(final Object key)

Checks if the map contains the specified key.

boolean

containsValue(final Object value)

Checks if the map contains the specified value.

Set

entrySet()

Gets the entry set.

boolean

equals(Object obj)

Compares this map to another, as per the Map specification.

Object

get(final Object key)

Gets the value associated with the key.

int

hashCode()

Gets the hash code, as per the Map specification.

boolean

isEmpty()

Checks if the size is currently zero.

Set

keySet()

Gets the key set.

Object

put(final Object key, final Object value)

Puts a new key value mapping into the map.

void

putAll(Map map)

Puts all the entries from the specified map into this map.

Object

remove(Object key)

Removes the specified key from the map.

int

size()

Gets the current size of the map.

Collection

values()

Gets the values.

Constructor Details

StaticBucketMap

public StaticBucketMap()

Initializes the map with the default number of buckets (255).


StaticBucketMap

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

Method Details

atomic

public void atomic(Runnable r)

Prevents any operations from occurring on this map while the given Runnable 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

clear

public void clear()

Clears the map of all entries.


containsKey

public boolean containsKey(final Object key)

Checks if the map contains the specified key.

Parameters:
key - the key to check
Returns:
true if found

containsValue

public boolean containsValue(final Object value)

Checks if the map contains the specified value.

Parameters:
value - the value to check
Returns:
true if found

entrySet

public Set entrySet()

Gets the entry set.

Returns:
the entry set

equals

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

get

public Object get(final Object key)

Gets the value associated with the key.

Parameters:
key - the key to retrieve
Returns:
the associated value

hashCode

public int hashCode()

Gets the hash code, as per the Map specification.

Returns:
the hash code

isEmpty

public boolean isEmpty()

Checks if the size is currently zero.

Returns:
true if empty

keySet

public Set keySet()

Gets the key set.

Returns:
the key set

put

public Object put(final Object key, final Object value)

Puts a new key value mapping into the map.

Parameters:
key - the key to use
value - the value to use
Returns:
the previous mapping for the key

putAll

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

remove

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

size

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

values

public Collection values()

Gets the values.

Returns:
the values