Package it.unimi.dsi.fastutil.floats

Class Float2ReferenceOpenCustomHashMap<V>

java.lang.Object
it.unimi.dsi.fastutil.floats.AbstractFloat2ReferenceFunction<V>
it.unimi.dsi.fastutil.floats.AbstractFloat2ReferenceMap<V>
it.unimi.dsi.fastutil.floats.Float2ReferenceOpenCustomHashMap<V>
All Implemented Interfaces:
it.unimi.dsi.fastutil.floats.Float2ReferenceFunction<V>, Float2ReferenceMap<V>, it.unimi.dsi.fastutil.Function<java.lang.Float,​V>, it.unimi.dsi.fastutil.Hash, java.io.Serializable, java.lang.Cloneable, java.util.function.DoubleFunction<V>, java.util.function.Function<java.lang.Float,​V>, java.util.Map<java.lang.Float,​V>
public class Float2ReferenceOpenCustomHashMap<V>
extends AbstractFloat2ReferenceMap<V>
implements java.io.Serializable, java.lang.Cloneable, it.unimi.dsi.fastutil.Hash
A type-specific hash map with a fast, small-footprint implementation whose hashing strategy is specified at creation time.

Instances of this class use a hash table to represent a map. The table is filled up to a specified load factor, and then doubled in size to accommodate new entries. If the table is emptied below one fourth of the load factor, it is halved in size; however, the table is never reduced to a size smaller than that at creation time: this approach makes it possible to create maps with a large capacity in which insertions and deletions do not cause immediately rehashing. Moreover, halving is not performed when deleting entries from an iterator, as it would interfere with the iteration process.

Note that clear() does not modify the hash table size. Rather, a family of trimming methods lets you control the size of the table; this is particularly useful if you reuse instances of this class.

Entries returned by the type-specific Float2ReferenceMap.entrySet() method implement the suitable type-specific Pair interface; only values are mutable.

See Also:
Hash, HashCommon, Serialized Form

  • Nested Class Summary

    Nested classes/interfaces inherited from class it.unimi.dsi.fastutil.floats.AbstractFloat2ReferenceMap

    AbstractFloat2ReferenceMap.BasicEntry<V>, AbstractFloat2ReferenceMap.BasicEntrySet<V>

    Nested classes/interfaces inherited from interface it.unimi.dsi.fastutil.floats.Float2ReferenceMap

    Float2ReferenceMap.Entry<V>, Float2ReferenceMap.FastEntrySet<V>

    Nested classes/interfaces inherited from interface it.unimi.dsi.fastutil.Hash

    it.unimi.dsi.fastutil.Hash.Strategy<K extends java.lang.Object>

  • Field Summary

    Fields inherited from interface it.unimi.dsi.fastutil.Hash

    DEFAULT_GROWTH_FACTOR, DEFAULT_INITIAL_SIZE, DEFAULT_LOAD_FACTOR, FAST_LOAD_FACTOR, FREE, OCCUPIED, PRIMES, REMOVED, VERY_FAST_LOAD_FACTOR

  • Constructor Summary

    Constructors
    Constructor Description
    Float2ReferenceOpenCustomHashMap​(float[] k, V[] v, float f, FloatHash.Strategy strategy)
    Creates a new hash map using the elements of two parallel arrays.
    Float2ReferenceOpenCustomHashMap​(float[] k, V[] v, FloatHash.Strategy strategy)
    Creates a new hash map with Hash.DEFAULT_LOAD_FACTOR as load factor using the elements of two parallel arrays.
    Float2ReferenceOpenCustomHashMap​(int expected, float f, FloatHash.Strategy strategy)
    Creates a new hash map.
    Float2ReferenceOpenCustomHashMap​(int expected, FloatHash.Strategy strategy)
    Creates a new hash map with Hash.DEFAULT_LOAD_FACTOR as load factor.
    Float2ReferenceOpenCustomHashMap​(Float2ReferenceMap<V> m, float f, FloatHash.Strategy strategy)
    Creates a new hash map copying a given type-specific one.
    Float2ReferenceOpenCustomHashMap​(Float2ReferenceMap<V> m, FloatHash.Strategy strategy)
    Creates a new hash map with Hash.DEFAULT_LOAD_FACTOR as load factor copying a given type-specific one.
    Float2ReferenceOpenCustomHashMap​(FloatHash.Strategy strategy)
    Creates a new hash map with initial expected Hash.DEFAULT_INITIAL_SIZE entries and Hash.DEFAULT_LOAD_FACTOR as load factor.
    Float2ReferenceOpenCustomHashMap​(java.util.Map<? extends java.lang.Float,​? extends V> m, float f, FloatHash.Strategy strategy)
    Creates a new hash map copying a given one.
    Float2ReferenceOpenCustomHashMap​(java.util.Map<? extends java.lang.Float,​? extends V> m, FloatHash.Strategy strategy)
    Creates a new hash map with Hash.DEFAULT_LOAD_FACTOR as load factor copying a given one.

  • Method Summary

    Modifier and Type Method Description
    void clear()
    Removes all of the mappings from this map (optional operation).
    Float2ReferenceOpenCustomHashMap<V> clone()
    Returns a deep copy of this map.
    V compute​(float k, java.util.function.BiFunction<? super java.lang.Float,​? super V,​? extends V> remappingFunction)
    Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping).
    V computeIfAbsent​(float key, it.unimi.dsi.fastutil.floats.Float2ReferenceFunction<? extends V> mappingFunction)
    If the specified key is not already associated with a value, attempts to compute its value using the given mapping function and enters it into this map, unless the key is not present in the given mapping function.
    V computeIfAbsent​(float k, java.util.function.DoubleFunction<? extends V> mappingFunction)
    If the specified key is not already associated with a value, attempts to compute its value using the given mapping function and enters it into this map.
    V computeIfPresent​(float k, java.util.function.BiFunction<? super java.lang.Float,​? super V,​? extends V> remappingFunction)
    If the value for the specified key is present, attempts to compute a new mapping given the key and its current mapped value.
    boolean containsKey​(float k)
    Returns true if this function contains a mapping for the specified key.
    boolean containsValue​(java.lang.Object v)
    Float2ReferenceMap.FastEntrySet<V> float2ReferenceEntrySet()
    Returns a type-specific set view of the mappings contained in this map.
    V get​(float k)  
    V getOrDefault​(float k, V defaultValue)
    Returns the value to which the specified key is mapped, or the defaultValue if this map contains no mapping for the key.
    int hashCode()
    Returns a hash code for this map.
    boolean isEmpty()  
    FloatSet keySet()
    Returns a type-specific-set view of the keys of this map.
    V merge​(float k, V v, java.util.function.BiFunction<? super V,​? super V,​? extends V> remappingFunction)
    If the specified key is not already associated with a value, associates it with the given value.
    V put​(float k, V v)  
    void putAll​(java.util.Map<? extends java.lang.Float,​? extends V> m)
    V putIfAbsent​(float k, V v)
    If the specified key is not already associated with a value, associates it with the given value and returns the default return value, else returns the current value.
    V remove​(float k)  
    boolean remove​(float k, java.lang.Object v)
    Removes the entry for the specified key only if it is currently mapped to the specified value.
    V replace​(float k, V v)
    Replaces the entry for the specified key only if it is currently mapped to some value.
    boolean replace​(float k, V oldValue, V v)
    Replaces the entry for the specified key only if currently mapped to the specified value.
    int size()
    Returns the number of key/value mappings in this map.
    FloatHash.Strategy strategy()
    Returns the hashing strategy.
    boolean trim()
    Rehashes the map, making the table as small as possible.
    boolean trim​(int n)
    Rehashes this map if the table is too large.
    it.unimi.dsi.fastutil.objects.ReferenceCollection<V> values()
    Returns a type-specific-set view of the values of this map.

    Methods inherited from class it.unimi.dsi.fastutil.floats.AbstractFloat2ReferenceMap

    equals, toString

    Methods inherited from class it.unimi.dsi.fastutil.floats.AbstractFloat2ReferenceFunction

    defaultReturnValue, defaultReturnValue

    Methods inherited from class java.lang.Object

    getClass, notify, notifyAll, wait, wait, wait

    Methods inherited from interface it.unimi.dsi.fastutil.floats.Float2ReferenceFunction

    andThenByte, andThenChar, andThenDouble, andThenFloat, andThenInt, andThenLong, andThenObject, andThenReference, andThenShort, apply, compose, composeByte, composeChar, composeDouble, composeFloat, composeInt, composeLong, composeObject, composeReference, composeShort

    Methods inherited from interface it.unimi.dsi.fastutil.floats.Float2ReferenceMap

    compute, computeIfAbsent, computeIfAbsentPartial, computeIfPresent, containsKey, defaultReturnValue, defaultReturnValue, entrySet, forEach, get, getOrDefault, merge, put, putIfAbsent, remove, remove, replace, replace

    Methods inherited from interface it.unimi.dsi.fastutil.Function

    apply

    Methods inherited from interface java.util.function.Function

    andThen

    Methods inherited from interface java.util.Map

    replaceAll

  • Constructor Details

    • Float2ReferenceOpenCustomHashMap

      public Float2ReferenceOpenCustomHashMap​(int expected, float f, FloatHash.Strategy strategy)
      Creates a new hash map.

      The actual table size will be the least power of two greater than expected/f.

      Parameters:
      expected - the expected number of elements in the hash map.
      f - the load factor.
      strategy - the strategy.

    • Float2ReferenceOpenCustomHashMap

      public Float2ReferenceOpenCustomHashMap​(int expected, FloatHash.Strategy strategy)
      Creates a new hash map with Hash.DEFAULT_LOAD_FACTOR as load factor.
      Parameters:
      expected - the expected number of elements in the hash map.
      strategy - the strategy.

    • Float2ReferenceOpenCustomHashMap

      public Float2ReferenceOpenCustomHashMap​(FloatHash.Strategy strategy)
      Creates a new hash map with initial expected Hash.DEFAULT_INITIAL_SIZE entries and Hash.DEFAULT_LOAD_FACTOR as load factor.
      Parameters:
      strategy - the strategy.

    • Float2ReferenceOpenCustomHashMap

      public Float2ReferenceOpenCustomHashMap​(java.util.Map<? extends java.lang.Float,​? extends V> m, float f, FloatHash.Strategy strategy)
      Creates a new hash map copying a given one.
      Parameters:
      m - a Map to be copied into the new hash map.
      f - the load factor.
      strategy - the strategy.

    • Float2ReferenceOpenCustomHashMap

      public Float2ReferenceOpenCustomHashMap​(java.util.Map<? extends java.lang.Float,​? extends V> m, FloatHash.Strategy strategy)
      Creates a new hash map with Hash.DEFAULT_LOAD_FACTOR as load factor copying a given one.
      Parameters:
      m - a Map to be copied into the new hash map.
      strategy - the strategy.

    • Float2ReferenceOpenCustomHashMap

      public Float2ReferenceOpenCustomHashMap​(Float2ReferenceMap<V> m, float f, FloatHash.Strategy strategy)
      Creates a new hash map copying a given type-specific one.
      Parameters:
      m - a type-specific map to be copied into the new hash map.
      f - the load factor.
      strategy - the strategy.

    • Float2ReferenceOpenCustomHashMap

      public Float2ReferenceOpenCustomHashMap​(Float2ReferenceMap<V> m, FloatHash.Strategy strategy)
      Creates a new hash map with Hash.DEFAULT_LOAD_FACTOR as load factor copying a given type-specific one.
      Parameters:
      m - a type-specific map to be copied into the new hash map.
      strategy - the strategy.

    • Float2ReferenceOpenCustomHashMap

      public Float2ReferenceOpenCustomHashMap​(float[] k, V[] v, float f, FloatHash.Strategy strategy)
      Creates a new hash map using the elements of two parallel arrays.
      Parameters:
      k - the array of keys of the new hash map.
      v - the array of corresponding values in the new hash map.
      f - the load factor.
      strategy - the strategy.
      Throws:
      java.lang.IllegalArgumentException - if k and v have different lengths.

    • Float2ReferenceOpenCustomHashMap

      public Float2ReferenceOpenCustomHashMap​(float[] k, V[] v, FloatHash.Strategy strategy)
      Creates a new hash map with Hash.DEFAULT_LOAD_FACTOR as load factor using the elements of two parallel arrays.
      Parameters:
      k - the array of keys of the new hash map.
      v - the array of corresponding values in the new hash map.
      strategy - the strategy.
      Throws:
      java.lang.IllegalArgumentException - if k and v have different lengths.

  • Method Details

    • strategy

      public FloatHash.Strategy strategy()
      Returns the hashing strategy.
      Returns:
      the hashing strategy of this custom hash map.

    • putAll

      public void putAll​(java.util.Map<? extends java.lang.Float,​? extends V> m)
      Description copied from class: AbstractFloat2ReferenceMap
      Specified by:
      putAll in interface java.util.Map<java.lang.Float,​V>
      Overrides:
      putAll in class AbstractFloat2ReferenceMap<V>

    • put

      public V put​(float k, V v)
      Specified by:
      put in interface it.unimi.dsi.fastutil.floats.Float2ReferenceFunction<V>

    • remove

      public V remove​(float k)
      Specified by:
      remove in interface it.unimi.dsi.fastutil.floats.Float2ReferenceFunction<V>

    • get

      public V get​(float k)
      Specified by:
      get in interface it.unimi.dsi.fastutil.floats.Float2ReferenceFunction<V>

    • containsKey

      public boolean containsKey​(float k)
      Description copied from class: AbstractFloat2ReferenceMap
      Returns true if this function contains a mapping for the specified key.
      Specified by:
      containsKey in interface it.unimi.dsi.fastutil.floats.Float2ReferenceFunction<V>
      Specified by:
      containsKey in interface Float2ReferenceMap<V>
      Overrides:
      containsKey in class AbstractFloat2ReferenceMap<V>
      Parameters:
      k - the key.
      Returns:
      true if this function associates a value to key.
      See Also:
      Map.containsKey(Object)

    • containsValue

      public boolean containsValue​(java.lang.Object v)
      Description copied from class: AbstractFloat2ReferenceMap
      Specified by:
      containsValue in interface java.util.Map<java.lang.Float,​V>
      Overrides:
      containsValue in class AbstractFloat2ReferenceMap<V>

    • getOrDefault

      public V getOrDefault​(float k, V defaultValue)
      Returns the value to which the specified key is mapped, or the defaultValue if this map contains no mapping for the key.
      Specified by:
      getOrDefault in interface it.unimi.dsi.fastutil.floats.Float2ReferenceFunction<V>
      Specified by:
      getOrDefault in interface Float2ReferenceMap<V>
      Parameters:
      k - the key.
      defaultValue - the default mapping of the key.
      Returns:
      the value to which the specified key is mapped, or the defaultValue if this map contains no mapping for the key.
      See Also:
      Map.getOrDefault(Object, Object)

    • putIfAbsent

      public V putIfAbsent​(float k, V v)
      If the specified key is not already associated with a value, associates it with the given value and returns the default return value, else returns the current value.
      Specified by:
      putIfAbsent in interface Float2ReferenceMap<V>
      Parameters:
      k - key with which the specified value is to be associated.
      v - value to be associated with the specified key.
      Returns:
      the previous value associated with the specified key, or the default return value if there was no mapping for the key.
      See Also:
      Map.putIfAbsent(Object, Object)

    • remove

      public boolean remove​(float k, java.lang.Object v)
      Removes the entry for the specified key only if it is currently mapped to the specified value.
      Specified by:
      remove in interface Float2ReferenceMap<V>
      Parameters:
      k - key with which the specified value is associated.
      v - value expected to be associated with the specified key.
      Returns:
      true if the value was removed.
      See Also:
      Map.remove(Object, Object)

    • replace

      public boolean replace​(float k, V oldValue, V v)
      Replaces the entry for the specified key only if currently mapped to the specified value.
      Specified by:
      replace in interface Float2ReferenceMap<V>
      Parameters:
      k - key with which the specified value is associated.
      oldValue - value expected to be associated with the specified key.
      v - value to be associated with the specified key.
      Returns:
      true if the value was replaced.
      See Also:
      Map.replace(Object, Object, Object)

    • replace

      public V replace​(float k, V v)
      Replaces the entry for the specified key only if it is currently mapped to some value.
      Specified by:
      replace in interface Float2ReferenceMap<V>
      Parameters:
      k - key with which the specified value is associated.
      v - value to be associated with the specified key.
      Returns:
      the previous value associated with the specified key, or the default return value if there was no mapping for the key.
      See Also:
      Map.replace(Object, Object)

    • computeIfAbsent

      public V computeIfAbsent​(float k, java.util.function.DoubleFunction<? extends V> mappingFunction)
      If the specified key is not already associated with a value, attempts to compute its value using the given mapping function and enters it into this map.

      Note that contrarily to the default computeIfAbsent(), it is not possible to not add a value for a given key, since the mappingFunction cannot return null. If such a behavior is needed, please use the corresponding nullable version.

      Specified by:
      computeIfAbsent in interface Float2ReferenceMap<V>
      Parameters:
      k - key with which the specified value is to be associated.
      mappingFunction - the function to compute a value.
      Returns:
      the current (existing or computed) value associated with the specified key.
      See Also:
      Map.computeIfAbsent(Object, java.util.function.Function)

    • computeIfAbsent

      public V computeIfAbsent​(float key, it.unimi.dsi.fastutil.floats.Float2ReferenceFunction<? extends V> mappingFunction)
      If the specified key is not already associated with a value, attempts to compute its value using the given mapping function and enters it into this map, unless the key is not present in the given mapping function.

      This version of computeIfAbsent() uses a type-specific version of fastutil's Function. Since Function has a containsKey() method, it is possible to avoid adding a key by having containsKey() return false for that key.

      Specified by:
      computeIfAbsent in interface Float2ReferenceMap<V>
      Parameters:
      key - key with which the specified value is to be associated.
      mappingFunction - the function to compute a value.
      Returns:
      the current (existing or computed) value associated with the specified key.
      See Also:
      Map.computeIfAbsent(Object, java.util.function.Function)

    • computeIfPresent

      public V computeIfPresent​(float k, java.util.function.BiFunction<? super java.lang.Float,​? super V,​? extends V> remappingFunction)
      If the value for the specified key is present, attempts to compute a new mapping given the key and its current mapped value.
      Specified by:
      computeIfPresent in interface Float2ReferenceMap<V>
      Parameters:
      k - key with which the specified value is to be associated.
      remappingFunction - the function to compute a value.
      Returns:
      the new value associated with the specified key, or the default return value if none.
      See Also:
      Map.computeIfPresent(Object, java.util.function.BiFunction)

    • compute

      public V compute​(float k, java.util.function.BiFunction<? super java.lang.Float,​? super V,​? extends V> remappingFunction)
      Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping).

      If the function returns null, the mapping is removed (or remains absent if initially absent). If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

      Specified by:
      compute in interface Float2ReferenceMap<V>
      Parameters:
      k - key with which the specified value is to be associated.
      remappingFunction - the function to compute a value.
      Returns:
      the new value associated with the specified key, or the default return value if none.
      See Also:
      Map.compute(Object, java.util.function.BiFunction)

    • merge

      public V merge​(float k, V v, java.util.function.BiFunction<? super V,​? super V,​? extends V> remappingFunction)
      If the specified key is not already associated with a value, associates it with the given value. Otherwise, replaces the associated value with the results of the given remapping function, or removes if the result is null.
      Specified by:
      merge in interface Float2ReferenceMap<V>
      Parameters:
      k - key with which the resulting value is to be associated.
      v - the value to be merged with the existing value associated with the key or, if no existing value is associated with the key, to be associated with the key.
      remappingFunction - the function to recompute a value if present.
      Returns:
      the new value associated with the specified key, or the default return value if no value is associated with the key.
      See Also:
      Map.merge(Object, Object, java.util.function.BiFunction)

    • clear

      public void clear()
      Description copied from interface: Float2ReferenceMap
      Removes all of the mappings from this map (optional operation). The map will be empty after this call returns.
      Specified by:
      clear in interface Float2ReferenceMap<V>
      Specified by:
      clear in interface it.unimi.dsi.fastutil.Function<java.lang.Float,​V>
      Specified by:
      clear in interface java.util.Map<java.lang.Float,​V>

    • size

      public int size()
      Description copied from interface: Float2ReferenceMap
      Returns the number of key/value mappings in this map. If the map contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.
      Specified by:
      size in interface Float2ReferenceMap<V>
      Specified by:
      size in interface it.unimi.dsi.fastutil.Function<java.lang.Float,​V>
      Specified by:
      size in interface java.util.Map<java.lang.Float,​V>
      Returns:
      the number of key-value mappings in this map.
      See Also:
      Size64

    • isEmpty

      public boolean isEmpty()
      Specified by:
      isEmpty in interface java.util.Map<java.lang.Float,​V>
      Overrides:
      isEmpty in class AbstractFloat2ReferenceMap<V>

    • float2ReferenceEntrySet

      public Float2ReferenceMap.FastEntrySet<V> float2ReferenceEntrySet()
      Description copied from interface: Float2ReferenceMap
      Returns a type-specific set view of the mappings contained in this map.

      This method is necessary because there is no inheritance along type parameters: it is thus impossible to strengthen Map.entrySet() so that it returns an ObjectSet of type-specific entries (the latter makes it possible to access keys and values with type-specific methods).

      Specified by:
      float2ReferenceEntrySet in interface Float2ReferenceMap<V>
      Returns:
      a type-specific set view of the mappings contained in this map.
      See Also:
      Map.entrySet()

    • keySet

      public FloatSet keySet()
      Description copied from class: AbstractFloat2ReferenceMap
      Returns a type-specific-set view of the keys of this map.

      The view is backed by the set returned by Map.entrySet(). Note that no attempt is made at caching the result of this method, as this would require adding some attributes that lightweight implementations would not need. Subclasses may easily override this policy by calling this method and caching the result, but implementors are encouraged to write more efficient ad-hoc implementations.

      Specified by:
      keySet in interface Float2ReferenceMap<V>
      Specified by:
      keySet in interface java.util.Map<java.lang.Float,​V>
      Overrides:
      keySet in class AbstractFloat2ReferenceMap<V>
      Returns:
      a set view of the keys of this map; it may be safely cast to a type-specific interface.
      See Also:
      Map.keySet()

    • values

      public it.unimi.dsi.fastutil.objects.ReferenceCollection<V> values()
      Description copied from class: AbstractFloat2ReferenceMap
      Returns a type-specific-set view of the values of this map.

      The view is backed by the set returned by Map.entrySet(). Note that no attempt is made at caching the result of this method, as this would require adding some attributes that lightweight implementations would not need. Subclasses may easily override this policy by calling this method and caching the result, but implementors are encouraged to write more efficient ad-hoc implementations.

      Specified by:
      values in interface Float2ReferenceMap<V>
      Specified by:
      values in interface java.util.Map<java.lang.Float,​V>
      Overrides:
      values in class AbstractFloat2ReferenceMap<V>
      Returns:
      a set view of the values of this map; it may be safely cast to a type-specific interface.
      See Also:
      Map.values()

    • trim

      public boolean trim()
      Rehashes the map, making the table as small as possible.

      This method rehashes the table to the smallest size satisfying the load factor. It can be used when the set will not be changed anymore, so to optimize access speed and size.

      If the table size is already the minimum possible, this method does nothing.

      Returns:
      true if there was enough memory to trim the map.
      See Also:
      trim(int)

    • trim

      public boolean trim​(int n)
      Rehashes this map if the table is too large.

      Let N be the smallest table size that can hold max(n,size()) entries, still satisfying the load factor. If the current table size is smaller than or equal to N, this method does nothing. Otherwise, it rehashes this map in a table of size N.

      This method is useful when reusing maps. Clearing a map leaves the table size untouched. If you are reusing a map many times, you can call this method with a typical size to avoid keeping around a very large table just because of a few large transient maps.

      Parameters:
      n - the threshold for the trimming.
      Returns:
      true if there was enough memory to trim the map.
      See Also:
      trim()

    • clone

      public Float2ReferenceOpenCustomHashMap<V> clone()
      Returns a deep copy of this map.

      This method performs a deep copy of this hash map; the data stored in the map, however, is not cloned. Note that this makes a difference only for object keys.

      Returns:
      a deep copy of this map.

    • hashCode

      public int hashCode()
      Returns a hash code for this map. This method overrides the generic method provided by the superclass. Since equals() is not overriden, it is important that the value returned by this method is the same value as the one returned by the overriden method.
      Specified by:
      hashCode in interface java.util.Map<java.lang.Float,​V>
      Overrides:
      hashCode in class AbstractFloat2ReferenceMap<V>
      Returns:
      a hash code for this map.