Package 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 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, 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 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Also, much like an encyclopedia, the results of size() and isEmpty() are out-of-date as soon as they are produced.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 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 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 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)
Version:
$Revision: 1713294 $ $Date: 2015-11-08 22:07:13 +0100 (Sun, 08 Nov 2015) $
Author:
Berin Loritsch, Gerhard Froehlich, Michael A. Smith, Paul Jack, Leo Sutic, Janek Bogucki, Kazuya Ujihara

  • 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

    • size

      public int size()
      Gets the current size of the map. The value is computed fresh each time the method is called.
      Specified by:
      size in interface Map
      Returns:
      the current size

    • isEmpty

      public boolean isEmpty()
      Checks if the size is currently zero.
      Specified by:
      isEmpty in interface Map
      Returns:
      true if empty

    • get

      public Object get(Object key)
      Gets the value associated with the key.
      Specified by:
      get in interface Map
      Parameters:
      key - the key to retrieve
      Returns:
      the associated value

    • containsKey

      public boolean containsKey(Object key)
      Checks if the map contains the specified key.
      Specified by:
      containsKey in interface Map
      Parameters:
      key - the key to check
      Returns:
      true if found

    • containsValue

      public boolean containsValue(Object value)
      Checks if the map contains the specified value.
      Specified by:
      containsValue in interface Map
      Parameters:
      value - the value to check
      Returns:
      true if found

    • put

      public Object put(Object key, Object value)
      Puts a new key value mapping into the map.
      Specified by:
      put in interface Map
      Parameters:
      key - the key to use
      value - the value to use
      Returns:
      the previous mapping for the key

    • remove

      public Object remove(Object key)
      Removes the specified key from the map.
      Specified by:
      remove in interface Map
      Parameters:
      key - the key to remove
      Returns:
      the previous value at this key

    • keySet

      public Set keySet()
      Gets the key set.
      Specified by:
      keySet in interface Map
      Returns:
      the key set

    • values

      public Collection values()
      Gets the values.
      Specified by:
      values in interface Map
      Returns:
      the values

    • entrySet

      public Set entrySet()
      Gets the entry set.
      Specified by:
      entrySet in interface Map
      Returns:
      the entry set

    • 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.
      Specified by:
      putAll in interface Map
      Parameters:
      map - the map of entries to add

    • clear

      public void clear()
      Clears the map of all entries.
      Specified by:
      clear in interface Map

    • equals

      public boolean equals(Object obj)
      Compares this map to another, as per the Map specification.
      Specified by:
      equals in interface Map
      Overrides:
      equals in class Object
      Parameters:
      obj - the object to compare to
      Returns:
      true if equal

    • hashCode

      public int hashCode()
      Gets the hash code, as per the Map specification.
      Specified by:
      hashCode in interface Map
      Overrides:
      hashCode in class Object
      Returns:
      the hash code

    • 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