org.apache.commons.collections4.map

Class AbstractHashedMap<K,V>

  • All Implemented Interfaces:
    Map<K,V>, Get<K,V>, IterableGet<K,V>, IterableMap<K,V>, Put<K,V>
    Direct Known Subclasses:
    AbstractLinkedMap, AbstractReferenceMap, CaseInsensitiveMap, HashedMap 

    public class AbstractHashedMap<K,V>
extends AbstractMap<K,V>
implements IterableMap<K,V>
    An abstract implementation of a hash-based map which provides numerous points for subclasses to override.

    This class implements all the features necessary for a subclass hash-based map. Key-value entries are stored in instances of the HashEntry class, which can be overridden and replaced. The iterators can similarly be replaced, without the need to replace the KeySet, EntrySet and Values view classes.

    Overridable methods are provided to change the default hashing behaviour, and to change how entries are added to and removed from the map. Hopefully, all you need for unusual subclasses is here.

    NOTE: From Commons Collections 3.1 this class extends AbstractMap. This is to provide backwards compatibility for ReferenceMap between v3.0 and v3.1. This extends clause will be removed in v5.0.

    Since:
    3.0

    • Constructor Detail

      • AbstractHashedMap

        protected AbstractHashedMap()
        Constructor only used in deserialization, do not use otherwise.

      • AbstractHashedMap

        protected AbstractHashedMap(int initialCapacity,
                            float loadFactor,
                            int threshold)
        Constructor which performs no validation on the passed in parameters.
        Parameters:
        initialCapacity - the initial capacity, must be a power of two
        loadFactor - the load factor, must be > 0.0f and generally < 1.0f
        threshold - the threshold, must be sensible

      • AbstractHashedMap

        protected AbstractHashedMap(int initialCapacity)
        Constructs a new, empty map with the specified initial capacity and default load factor.
        Parameters:
        initialCapacity - the initial capacity
        Throws:
        IllegalArgumentException - if the initial capacity is negative

      • AbstractHashedMap

        protected AbstractHashedMap(int initialCapacity,
                            float loadFactor)
        Constructs a new, empty map with the specified initial capacity and load factor.
        Parameters:
        initialCapacity - the initial capacity
        loadFactor - the load factor
        Throws:
        IllegalArgumentException - if the initial capacity is negative
        IllegalArgumentException - if the load factor is less than or equal to zero

      • AbstractHashedMap

        protected AbstractHashedMap(Map<? extends K,? extends V> map)
        Constructor copying elements from another map.
        Parameters:
        map - the map to copy
        Throws:
        NullPointerException - if the map is null

    • Method Detail

      • init

        protected void init()
        Initialise subclasses during construction, cloning or deserialization.

      • get

        public V get(Object key)
        Gets the value mapped to the key specified.
        Specified by:
        get in interface  Map<K,V>
        Specified by:
        get in interface  Get<K,V>
        Overrides:
        get in class  AbstractMap<K,V>
        Parameters:
        key - the key
        Returns:
        the mapped value, null if no match
        See Also:
        Map.get(Object)

      • isEmpty

        public boolean isEmpty()
        Checks whether the map is currently empty.
        Specified by:
        isEmpty in interface  Map<K,V>
        Specified by:
        isEmpty in interface  Get<K,V>
        Overrides:
        isEmpty in class  AbstractMap<K,V>
        Returns:
        true if the map is currently size zero
        See Also:
        Map.isEmpty()

      • put

        public V put(K key,
             V value)
        Puts a key-value mapping into this map.
        Specified by:
        put in interface  Map<K,V>
        Specified by:
        put in interface  Put<K,V>
        Overrides:
        put in class  AbstractMap<K,V>
        Parameters:
        key - the key to add
        value - the value to add
        Returns:
        the value previously mapped to this key, null if none
        See Also:
        Map.put(Object, Object)

      • remove

        public V remove(Object key)
        Removes the specified mapping from this map.
        Specified by:
        remove in interface  Map<K,V>
        Specified by:
        remove in interface  Get<K,V>
        Overrides:
        remove in class  AbstractMap<K,V>
        Parameters:
        key - the mapping to remove
        Returns:
        the value mapped to the removed key, null if key not in map
        See Also:
        Map.remove(Object)

      • clear

        public void clear()
        Clears the map, resetting the size to zero and nullifying references to avoid garbage collection issues.
        Specified by:
        clear in interface  Map<K,V>
        Specified by:
        clear in interface  Put<K,V>
        Overrides:
        clear in class  AbstractMap<K,V>
        See Also:
        Map.clear()

      • convertKey

        protected Object convertKey(Object key)
        Converts input keys to another object for storage in the map. This implementation masks nulls. Subclasses can override this to perform alternate key conversions.

        The reverse conversion can be changed, if required, by overriding the getKey() method in the hash entry.

        Parameters:
        key - the key convert
        Returns:
        the converted key

      • hash

        protected int hash(Object key)
        Gets the hash code for the key specified. This implementation uses the additional hashing routine from JDK1.4. Subclasses can override this to return alternate hash codes.
        Parameters:
        key - the key to get a hash code for
        Returns:
        the hash code

      • isEqualKey

        protected boolean isEqualKey(Object key1,
                             Object key2)
        Compares two keys, in internal converted form, to see if they are equal. This implementation uses the equals method and assumes neither key is null. Subclasses can override this to match differently.
        Parameters:
        key1 - the first key to compare passed in from outside
        key2 - the second key extracted from the entry via entry.key
        Returns:
        true if equal

      • isEqualValue

        protected boolean isEqualValue(Object value1,
                               Object value2)
        Compares two values, in external form, to see if they are equal. This implementation uses the equals method and assumes neither value is null. Subclasses can override this to match differently.
        Parameters:
        value1 - the first value to compare passed in from outside
        value2 - the second value extracted from the entry via getValue()
        Returns:
        true if equal

      • hashIndex

        protected int hashIndex(int hashCode,
                        int dataSize)
        Gets the index into the data storage for the hashCode specified. This implementation uses the least significant bits of the hashCode. Subclasses can override this to return alternate bucketing.
        Parameters:
        hashCode - the hash code to use
        dataSize - the size of the data to pick a bucket from
        Returns:
        the bucket index

      • getEntry

        protected AbstractHashedMap.HashEntry<K,V> getEntry(Object key)
        Gets the entry mapped to the key specified.

        This method exists for subclasses that may need to perform a multi-step process accessing the entry. The public methods in this class don't use this method to gain a small performance boost.

        Parameters:
        key - the key
        Returns:
        the entry, null if no match

      • updateEntry

        protected void updateEntry(AbstractHashedMap.HashEntry<K,V> entry,
                           V newValue)
        Updates an existing key-value mapping to change the value.

        This implementation calls setValue() on the entry. Subclasses could override to handle changes to the map.

        Parameters:
        entry - the entry to update
        newValue - the new value to store

      • reuseEntry

        protected void reuseEntry(AbstractHashedMap.HashEntry<K,V> entry,
                          int hashIndex,
                          int hashCode,
                          K key,
                          V value)
        Reuses an existing key-value mapping, storing completely new data.

        This implementation sets all the data fields on the entry. Subclasses could populate additional entry fields.

        Parameters:
        entry - the entry to update, not null
        hashIndex - the index in the data array
        hashCode - the hash code of the key to add
        key - the key to add
        value - the value to add

      • addMapping

        protected void addMapping(int hashIndex,
                          int hashCode,
                          K key,
                          V value)
        Adds a new key-value mapping into this map.

        This implementation calls createEntry(), addEntry() and checkCapacity(). It also handles changes to modCount and size. Subclasses could override to fully control adds to the map.

        Parameters:
        hashIndex - the index into the data array to store at
        hashCode - the hash code of the key to add
        key - the key to add
        value - the value to add

      • createEntry

        protected AbstractHashedMap.HashEntry<K,V> createEntry(AbstractHashedMap.HashEntry<K,V> next,
                                                       int hashCode,
                                                       K key,
                                                       V value)
        Creates an entry to store the key-value data.

        This implementation creates a new HashEntry instance. Subclasses can override this to return a different storage class, or implement caching.

        Parameters:
        next - the next entry in sequence
        hashCode - the hash code to use
        key - the key to store
        value - the value to store
        Returns:
        the newly created entry

      • addEntry

        protected void addEntry(AbstractHashedMap.HashEntry<K,V> entry,
                        int hashIndex)
        Adds an entry into this map.

        This implementation adds the entry to the data storage table. Subclasses could override to handle changes to the map.

        Parameters:
        entry - the entry to add
        hashIndex - the index into the data array to store at

      • removeMapping

        protected void removeMapping(AbstractHashedMap.HashEntry<K,V> entry,
                             int hashIndex,
                             AbstractHashedMap.HashEntry<K,V> previous)
        Removes a mapping from the map.

        This implementation calls removeEntry() and destroyEntry(). It also handles changes to modCount and size. Subclasses could override to fully control removals from the map.

        Parameters:
        entry - the entry to remove
        hashIndex - the index into the data structure
        previous - the previous entry in the chain

      • removeEntry

        protected void removeEntry(AbstractHashedMap.HashEntry<K,V> entry,
                           int hashIndex,
                           AbstractHashedMap.HashEntry<K,V> previous)
        Removes an entry from the chain stored in a particular index.

        This implementation removes the entry from the data storage table. The size is not updated. Subclasses could override to handle changes to the map.

        Parameters:
        entry - the entry to remove
        hashIndex - the index into the data structure
        previous - the previous entry in the chain

      • destroyEntry

        protected void destroyEntry(AbstractHashedMap.HashEntry<K,V> entry)
        Kills an entry ready for the garbage collector.

        This implementation prepares the HashEntry for garbage collection. Subclasses can override this to implement caching (override clear as well).

        Parameters:
        entry - the entry to destroy

      • checkCapacity

        protected void checkCapacity()
        Checks the capacity of the map and enlarges it if necessary.

        This implementation uses the threshold to check if the map needs enlarging

      • ensureCapacity

        protected void ensureCapacity(int newCapacity)
        Changes the size of the data structure to the capacity proposed.
        Parameters:
        newCapacity - the new capacity of the array (a power of two, less or equal to max)

      • calculateNewCapacity

        protected int calculateNewCapacity(int proposedCapacity)
        Calculates the new capacity of the map. This implementation normalizes the capacity to a power of two.
        Parameters:
        proposedCapacity - the proposed capacity
        Returns:
        the normalized new capacity

      • calculateThreshold

        protected int calculateThreshold(int newCapacity,
                                 float factor)
        Calculates the new threshold of the map, where it will be resized. This implementation uses the load factor.
        Parameters:
        newCapacity - the new capacity
        factor - the load factor
        Returns:
        the new resize threshold

      • entryHashCode

        protected int entryHashCode(AbstractHashedMap.HashEntry<K,V> entry)
        Gets the hashCode field from a HashEntry. Used in subclasses that have no visibility of the field.
        Parameters:
        entry - the entry to query, must not be null
        Returns:
        the hashCode field of the entry
        Throws:
        NullPointerException - if the entry is null
        Since:
        3.1

      • entryKey

        protected K entryKey(AbstractHashedMap.HashEntry<K,V> entry)
        Gets the key field from a HashEntry. Used in subclasses that have no visibility of the field.
        Parameters:
        entry - the entry to query, must not be null
        Returns:
        the key field of the entry
        Throws:
        NullPointerException - if the entry is null
        Since:
        3.1

      • entryValue

        protected V entryValue(AbstractHashedMap.HashEntry<K,V> entry)
        Gets the value field from a HashEntry. Used in subclasses that have no visibility of the field.
        Parameters:
        entry - the entry to query, must not be null
        Returns:
        the value field of the entry
        Throws:
        NullPointerException - if the entry is null
        Since:
        3.1

      • mapIterator

        public MapIterator<K,V> mapIterator()
        Gets an iterator over the map. Changes made to the iterator affect this map.

        A MapIterator returns the keys in the map. It also provides convenient methods to get the key and value, and set the value. It avoids the need to create an entrySet/keySet/values object. It also avoids creating the Map.Entry object.

        Specified by:
        mapIterator in interface  IterableGet<K,V>
        Returns:
        the map iterator

      • createEntrySetIterator

        protected Iterator<Map.Entry<K,V>> createEntrySetIterator()
        Creates an entry set iterator. Subclasses can override this to return iterators with different properties.
        Returns:
        the entrySet iterator

      • createKeySetIterator

        protected Iterator<K> createKeySetIterator()
        Creates a key set iterator. Subclasses can override this to return iterators with different properties.
        Returns:
        the keySet iterator

      • createValuesIterator

        protected Iterator<V> createValuesIterator()
        Creates a values iterator. Subclasses can override this to return iterators with different properties.
        Returns:
        the values iterator

      • doWriteObject

        protected void doWriteObject(ObjectOutputStream out)
                      throws IOException
        Writes the map data to the stream. This method must be overridden if a subclass must be setup before put() is used.

        Serialization is not one of the JDK's nicest topics. Normal serialization will initialise the superclass before the subclass. Sometimes however, this isn't what you want, as in this case the put() method on read can be affected by subclass state.

        The solution adopted here is to serialize the state data of this class in this protected method. This method must be called by the writeObject() of the first serializable subclass.

        Subclasses may override if they have a specific field that must be present on read before this implementation will work. Generally, the read determines what must be serialized here, if anything.

        Parameters:
        out - the output stream
        Throws:
        IOException - if an error occurs while writing tothe stream

      • doReadObject

        protected void doReadObject(ObjectInputStream in)
                     throws IOException,
                            ClassNotFoundException
        Reads the map data from the stream. This method must be overridden if a subclass must be setup before put() is used.

        Serialization is not one of the JDK's nicest topics. Normal serialization will initialise the superclass before the subclass. Sometimes however, this isn't what you want, as in this case the put() method on read can be affected by subclass state.

        The solution adopted here is to deserialize the state data of this class in this protected method. This method must be called by the readObject() of the first serializable subclass.

        Subclasses may override if the subclass has a specific field that must be present before put() or calculateThreshold() will work correctly.

        Parameters:
        in - the input stream
        Throws:
        IOException - if an error occurs while reading from the stream
        ClassNotFoundException - if an object read from the stream can not be loaded

      • equals

        public boolean equals(Object obj)
        Compares this map with another.
        Specified by:
        equals in interface  Map<K,V>
        Overrides:
        equals in class  AbstractMap<K,V>
        Parameters:
        obj - the object to compare to
        Returns:
        true if equal

      • hashCode

        public int hashCode()
        Gets the standard Map hashCode.
        Specified by:
        hashCode in interface  Map<K,V>
        Overrides:
        hashCode in class  AbstractMap<K,V>
        Returns:
        the hash code defined in the Map interface

      • toString

        public String toString()
        Gets the map as a String.
        Overrides:
        toString in class  AbstractMap<K,V>
        Returns:
        a string version of the map