org.apache.commons.collections.map
Class AbstractReferenceMap<K,V>

java.lang.Object
  java.util.AbstractMap<K,V>
      org.apache.commons.collections.map.AbstractHashedMap<K,V>
          org.apache.commons.collections.map.AbstractReferenceMap<K,V>

All Implemented Interfaces:

Map<K,V>, IterableMap<K,V>

Direct Known Subclasses:

ReferenceIdentityMap, ReferenceMap

public abstract class AbstractReferenceMap<K,V>
extends AbstractHashedMap<K,V>

An abstract implementation of a hash-based map that allows the entries to be removed by the garbage collector.

This class implements all the features necessary for a subclass reference hash-based map. Key-value entries are stored in instances of the ReferenceEntry 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.

When you construct an AbstractReferenceMap, you can specify what kind of references are used to store the map's keys and values. If non-hard references are used, then the garbage collector can remove mappings if a key or value becomes unreachable, or if the JVM's memory is running low. For information on how the different reference types behave, see Reference.

Different types of references can be specified for keys and values. The keys can be configured to be weak but the values hard, in which case this class will behave like a WeakHashMap. However, you can also specify hard keys and weak values, or any other combination. The default constructor uses hard keys and soft values, providing a memory-sensitive cache.

This Map implementation does not allow null elements. Attempting to add a null key or value to the map will raise a NullPointerException.

All the available iterators can be reset back to the start by casting to ResettableIterator and calling reset().

This implementation is not synchronized. You can use Collections.synchronizedMap(java.util.Map) to provide synchronized access to a ReferenceMap.

Since:

Commons Collections 3.1 (extracted from ReferenceMap in 3.0)

Version:

$Revision: 1.1.1.1 $ $Date: 2005/05/23 04:35:44 $

Author:

Paul Jack, Matt Hall, John Watkinson, Stephen Colebourne

See Also:

Reference

Nested Class Summary

protected static class

AbstractReferenceMap.ReferenceEntry<K,V> A MapEntry implementation for the map.

Nested classes/interfaces inherited from class org.apache.commons.collections.map.AbstractHashedMap

AbstractHashedMap.EntrySet<K,V>, AbstractHashedMap.EntrySetIterator<K,V>, AbstractHashedMap.HashEntry<K,V>, AbstractHashedMap.HashIterator<K,V>, AbstractHashedMap.HashMapIterator<K,V>, AbstractHashedMap.KeySet<K,V>, AbstractHashedMap.KeySetIterator<K,V>, AbstractHashedMap.Values<K,V>, AbstractHashedMap.ValuesIterator<K,V>

Nested classes/interfaces inherited from interface java.util.Map

Map.Entry<K,V>

Field Summary

static int	HARD Constant indicating that hard references should be used
protected int	keyType The reference type for keys.
protected boolean	purgeValues Should the value be automatically purged when the associated key has been collected?
static int	SOFT Constant indicating that soft references should be used
protected int	valueType The reference type for values.
static int	WEAK Constant indicating that weak references should be used

Fields inherited from class org.apache.commons.collections.map.AbstractHashedMap

data, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_THRESHOLD, entrySet, GETKEY_INVALID, GETVALUE_INVALID, keySet, loadFactor, MAXIMUM_CAPACITY, modCount, NO_NEXT_ENTRY, NO_PREVIOUS_ENTRY, NULL, REMOVE_INVALID, SETVALUE_INVALID, size, threshold, values

Constructor Summary

protected	AbstractReferenceMap() Constructor used during deserialization.
protected	AbstractReferenceMap(int keyType, int valueType, int capacity, float loadFactor, boolean purgeValues) Constructs a new empty map with the specified reference types, load factor and initial capacity.

Method Summary

void	clear() Clears this map.
boolean	containsKey(Object key) Checks whether the map contains the specified key.
boolean	containsValue(Object value) Checks whether the map contains the specified value.
AbstractHashedMap.HashEntry<K,V>	createEntry(AbstractHashedMap.HashEntry<K,V> next, int hashCode, K key, V value) Creates a ReferenceEntry instead of a HashEntry.
protected Iterator<Map.Entry<K,V>>	createEntrySetIterator() Creates an entry set iterator.
protected Iterator<K>	createKeySetIterator() Creates an key set iterator.
protected Iterator<V>	createValuesIterator() Creates an values iterator.
protected void	doReadObject(ObjectInputStream in) Replaces the superclassm method to read the state of this class.
protected void	doWriteObject(ObjectOutputStream out) Replaces the superclass method to store the state of this class.
Set<Map.Entry<K,V>>	entrySet() Returns a set view of this map's entries.
V	get(Object key) Gets the value mapped to the key specified.
protected AbstractHashedMap.HashEntry<K,V>	getEntry(Object key) Gets the entry mapped to the key specified.
protected int	hashEntry(Object key, Object value) Gets the hash code for a MapEntry.
protected void	init() Initialise this subclass during construction, cloning or deserialization.
boolean	isEmpty() Checks whether the map is currently empty.
protected boolean	isEqualKey(Object key1, Object key2) Compares two keys, in internal converted form, to see if they are equal.
Set<K>	keySet() Returns a set view of this map's keys.
MapIterator<K,V>	mapIterator() Gets a MapIterator over the reference map.
protected void	purge() Purges stale mappings from this map.
protected void	purge(Reference ref) Purges the specified reference.
protected void	purgeBeforeRead() Purges stale mappings from this map before read operations.
protected void	purgeBeforeWrite() Purges stale mappings from this map before write operations.
V	put(K key, V value) Puts a key-value mapping into this map.
V	remove(Object key) Removes the specified mapping from this map.
int	size() Gets the size of the map.
Collection<V>	values() Returns a collection view of this map's values.

Methods inherited from class org.apache.commons.collections.map.AbstractHashedMap

addEntry, addMapping, calculateNewCapacity, calculateThreshold, checkCapacity, clone, destroyEntry, ensureCapacity, entryHashCode, entryKey, entryNext, entryValue, equals, hash, hashCode, hashIndex, isEqualValue, putAll, removeEntry, removeMapping, reuseEntry, toString, updateEntry

Methods inherited from class java.lang.Object

finalize, getClass, notify, notifyAll, wait, wait, wait

Field Detail

HARD

public static final int HARD

Constant indicating that hard references should be used

See Also:

Constant Field Values

SOFT

public static final int SOFT

Constant indicating that soft references should be used

See Also:

Constant Field Values

WEAK

public static final int WEAK

Constant indicating that weak references should be used

See Also:

Constant Field Values

keyType

protected int keyType

The reference type for keys. Must be HARD, SOFT, WEAK.

valueType

protected int valueType

The reference type for values. Must be HARD, SOFT, WEAK.

purgeValues

protected boolean purgeValues

Should the value be automatically purged when the associated key has been collected?

Constructor Detail

AbstractReferenceMap

protected AbstractReferenceMap()

Constructor used during deserialization.

AbstractReferenceMap

protected AbstractReferenceMap(int keyType,
                               int valueType,
                               int capacity,
                               float loadFactor,
                               boolean purgeValues)

Constructs a new empty map with the specified reference types, load factor and initial capacity.

Parameters:

keyType - the type of reference to use for keys; must be SOFT or WEAK

valueType - the type of reference to use for values; must be SOFT or WEAK

capacity - the initial capacity for the map

loadFactor - the load factor for the map

purgeValues - should the value be automatically purged when the key is garbage collected

Method Detail

init

protected void init()

Initialise this subclass during construction, cloning or deserialization.

Overrides:

init in class AbstractHashedMap<K,V>

size

public int size()

Gets the size of the map.

Specified by:

size in interface Map<K,V>

Overrides:

size in class AbstractHashedMap<K,V>

Returns:

the size

isEmpty

public boolean isEmpty()

Checks whether the map is currently empty.

Specified by:

isEmpty in interface Map<K,V>

Overrides:

isEmpty in class AbstractHashedMap<K,V>

Returns:

true if the map is currently size zero

containsKey

public boolean containsKey(Object key)

Checks whether the map contains the specified key.

Specified by:

containsKey in interface Map<K,V>

Overrides:

containsKey in class AbstractHashedMap<K,V>

Parameters:

key - the key to search for

Returns:

true if the map contains the key

containsValue

public boolean containsValue(Object value)

Checks whether the map contains the specified value.

Specified by:

containsValue in interface Map<K,V>

Overrides:

containsValue in class AbstractHashedMap<K,V>

Parameters:

value - the value to search for

Returns:

true if the map contains the value

get

public V get(Object key)

Gets the value mapped to the key specified.

Specified by:

get in interface Map<K,V>

Overrides:

get in class AbstractHashedMap<K,V>

Parameters:

key - the key

Returns:

the mapped value, null if no match

put

public V put(K key,
             V value)

Puts a key-value mapping into this map. Neither the key nor the value may be null.

Specified by:

put in interface Map<K,V>

Overrides:

put in class AbstractHashedMap<K,V>

Parameters:

key - the key to add, must not be null

value - the value to add, must not be null

Returns:

the value previously mapped to this key, null if none

Throws:

NullPointerException - if either the key or value is null

remove

public V remove(Object key)

Removes the specified mapping from this map.

Specified by:

remove in interface Map<K,V>

Overrides:

remove in class AbstractHashedMap<K,V>

Parameters:

key - the mapping to remove

Returns:

the value mapped to the removed key, null if key not in map

clear

public void clear()

Clears this map.

Specified by:

clear in interface Map<K,V>

Overrides:

clear in class AbstractHashedMap<K,V>

mapIterator

public MapIterator<K,V> mapIterator()

Gets a MapIterator over the reference map. The iterator only returns valid key/value pairs.

Specified by:

mapIterator in interface IterableMap<K,V>

Overrides:

mapIterator in class AbstractHashedMap<K,V>

Returns:

a map iterator

entrySet

public Set<Map.Entry<K,V>> entrySet()

Returns a set view of this map's entries. An iterator returned entry is valid until next() is called again. The setValue() method on the toArray entries has no effect.

Specified by:

entrySet in interface Map<K,V>

Overrides:

entrySet in class AbstractHashedMap<K,V>

Returns:

a set view of this map's entries

keySet

public Set<K> keySet()

Returns a set view of this map's keys.

Specified by:

keySet in interface Map<K,V>

Overrides:

keySet in class AbstractHashedMap<K,V>

Returns:

a set view of this map's keys

values

public Collection<V> values()

Returns a collection view of this map's values.

Specified by:

values in interface Map<K,V>

Overrides:

values in class AbstractHashedMap<K,V>

Returns:

a set view of this map's values

purgeBeforeRead

protected void purgeBeforeRead()

Purges stale mappings from this map before read operations.

This implementation calls purge() to maintain a consistent state.

purgeBeforeWrite

protected void purgeBeforeWrite()

Purges stale mappings from this map before write operations.

This implementation calls purge() to maintain a consistent state.

purge

protected void purge()

Purges stale mappings from this map.

Note that this method is not synchronized! Special care must be taken if, for instance, you want stale mappings to be removed on a periodic basis by some background thread.

purge

protected void purge(Reference ref)

Purges the specified reference.

Parameters:

ref - the reference to purge

getEntry

protected AbstractHashedMap.HashEntry<K,V> getEntry(Object key)

Gets the entry mapped to the key specified.

Overrides:

getEntry in class AbstractHashedMap<K,V>

Parameters:

key - the key

Returns:

the entry, null if no match

hashEntry

protected int hashEntry(Object key,
                        Object value)

Gets the hash code for a MapEntry. Subclasses can override this, for example to use the identityHashCode.

Parameters:

key - the key to get a hash code for, may be null

value - the value to get a hash code for, may be null

Returns:

the hash code, as per the MapEntry specification

isEqualKey

protected boolean isEqualKey(Object key1,
                             Object key2)

Compares two keys, in internal converted form, to see if they are equal.

This implementation converts the key from the entry to a real reference before comparison.

Overrides:

isEqualKey in class AbstractHashedMap<K,V>

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

createEntry

public AbstractHashedMap.HashEntry<K,V> createEntry(AbstractHashedMap.HashEntry<K,V> next,
                                                    int hashCode,
                                                    K key,
                                                    V value)

Creates a ReferenceEntry instead of a HashEntry.

Overrides:

createEntry in class AbstractHashedMap<K,V>

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

createEntrySetIterator

protected Iterator<Map.Entry<K,V>> createEntrySetIterator()

Creates an entry set iterator.

Overrides:

createEntrySetIterator in class AbstractHashedMap<K,V>

Returns:

the entrySet iterator

createKeySetIterator

protected Iterator<K> createKeySetIterator()

Creates an key set iterator.

Overrides:

createKeySetIterator in class AbstractHashedMap<K,V>

Returns:

the keySet iterator

createValuesIterator

protected Iterator<V> createValuesIterator()

Creates an values iterator.

Overrides:

createValuesIterator in class AbstractHashedMap<K,V>

Returns:

the values iterator

doWriteObject

protected void doWriteObject(ObjectOutputStream out)
                      throws IOException

Replaces the superclass method to store the state of this class.

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.

Overrides:

doWriteObject in class AbstractHashedMap<K,V>

Parameters:

out - the output stream

Throws:

IOException

doReadObject

protected void doReadObject(ObjectInputStream in)
                     throws IOException,
                            ClassNotFoundException

Replaces the superclassm method to read the state of this class.

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.

Overrides:

doReadObject in class AbstractHashedMap<K,V>

Parameters:

in - the input stream

Throws:

IOException

ClassNotFoundException