org.apache.commons.collections4.map

Class StaticBucketMap<K,V>

  • All Implemented Interfaces:
    Map<K,V>, Get<K,V>, IterableGet<K,V>, IterableMap<K,V>, Put<K,V> 

    public final class StaticBucketMap<K,V>
extends AbstractIterableMap<K,V>
    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    Also, much like an encyclopedia, the results of size() and isEmpty() are out-of-date as soon as they are produced.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:
    3.0 (previously in main package v2.1)

    • Constructor Detail

      • 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 Detail

      • 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
        See Also:
        Map.size()

      • isEmpty

        public boolean isEmpty()
        Checks if the size is currently zero.
        Returns:
        true if empty
        See Also:
        Map.isEmpty()

      • get

        public V get(Object key)
        Gets the value associated with the key.
        Parameters:
        key - the key to retrieve
        Returns:
        the associated value
        See Also:
        Map.get(Object)

      • containsKey

        public boolean containsKey(Object key)
        Checks if the map contains the specified key.
        Parameters:
        key - the key to check
        Returns:
        true if found
        See Also:
        Map.containsKey(Object)

      • containsValue

        public boolean containsValue(Object value)
        Checks if the map contains the specified value.
        Parameters:
        value - the value to check
        Returns:
        true if found
        See Also:
        Map.containsValue(Object)

      • put

        public V put(K key,
             V 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
        See Also:
        Map.put(Object, Object)

      • remove

        public V remove(Object key)
        Removes the specified key from the map.
        Parameters:
        key - the key to remove
        Returns:
        the previous value at this key
        See Also:
        Map.remove(Object)

      • keySet

        public Set<K> keySet()
        Gets the key set.
        Returns:
        the key set
        See Also:
        Map.keySet()

      • putAll

        public void putAll(Map<? extends K,? extends V> 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
        See Also:
        Map.putAll(Map)

      • clear

        public void clear()
        Clears the map of all entries.
        See Also:
        Map.clear()

      • equals

        public boolean equals(Object obj)
        Compares this map to another, as per the Map specification.
        Specified by:
        equals in interface  Map<K,V>
        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<K,V>
        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