org.apache.commons.collections.map
Class AbstractReferenceMap

java.lang.Object
  java.util.AbstractMap
      org.apache.commons.collections.map.AbstractHashedMap
          org.apache.commons.collections.map.AbstractReferenceMap

All Implemented Interfaces:

java.util.Map, IterableMap

Direct Known Subclasses:

ReferenceIdentityMap, ReferenceMap

public abstract class AbstractReferenceMap
extends AbstractHashedMap

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: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $

Author:

Paul Jack, Stephen Colebourne

See Also:

Reference

Nested Class Summary

protected static class

AbstractReferenceMap.ReferenceEntry A MapEntry implementation for the map.

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

AbstractHashedMap.EntrySet, AbstractHashedMap.EntrySetIterator, AbstractHashedMap.HashEntry, AbstractHashedMap.HashIterator, AbstractHashedMap.HashMapIterator, AbstractHashedMap.KeySet, AbstractHashedMap.KeySetIterator, AbstractHashedMap.Values, AbstractHashedMap.ValuesIterator

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

java.util.Map.Entry

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(java.lang.Object key) Checks whether the map contains the specified key.
boolean	containsValue(java.lang.Object value) Checks whether the map contains the specified value.
protected AbstractHashedMap.HashEntry	createEntry(AbstractHashedMap.HashEntry next, int hashCode, java.lang.Object key, java.lang.Object value) Creates a ReferenceEntry instead of a HashEntry.
protected java.util.Iterator	createEntrySetIterator() Creates an entry set iterator.
protected java.util.Iterator	createKeySetIterator() Creates an key set iterator.
protected java.util.Iterator	createValuesIterator() Creates an values iterator.
protected void	doReadObject(java.io.ObjectInputStream in) Replaces the superclassm method to read the state of this class.
protected void	doWriteObject(java.io.ObjectOutputStream out) Replaces the superclass method to store the state of this class.
java.util.Set	entrySet() Returns a set view of this map's entries.
java.lang.Object	get(java.lang.Object key) Gets the value mapped to the key specified.
protected AbstractHashedMap.HashEntry	getEntry(java.lang.Object key) Gets the entry mapped to the key specified.
protected int	hashEntry(java.lang.Object key, java.lang.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(java.lang.Object key1, java.lang.Object key2) Compares two keys, in internal converted form, to see if they are equal.
java.util.Set	keySet() Returns a set view of this map's keys.
MapIterator	mapIterator() Gets a MapIterator over the reference map.
protected void	purge() Purges stale mappings from this map.
protected void	purge(java.lang.ref.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.
java.lang.Object	put(java.lang.Object key, java.lang.Object value) Puts a key-value mapping into this map.
java.lang.Object	remove(java.lang.Object key) Removes the specified mapping from this map.
int	size() Gets the size of the map.
java.util.Collection	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, convertKey, 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 HARD, SOFT, WEAK

valueType - the type of reference to use for values; must be HARD, SOFT, 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

size

public int size()

Gets the size of the map.

Specified by:

size in interface java.util.Map

Overrides:

size in class AbstractHashedMap

Returns:

the size

isEmpty

public boolean isEmpty()

Checks whether the map is currently empty.

Specified by:

isEmpty in interface java.util.Map

Overrides:

isEmpty in class AbstractHashedMap

Returns:

true if the map is currently size zero

containsKey

public boolean containsKey(java.lang.Object key)

Checks whether the map contains the specified key.

Specified by:

containsKey in interface java.util.Map

Overrides:

containsKey in class AbstractHashedMap

Parameters:

key - the key to search for

Returns:

true if the map contains the key

containsValue

public boolean containsValue(java.lang.Object value)

Checks whether the map contains the specified value.

Specified by:

containsValue in interface java.util.Map

Overrides:

containsValue in class AbstractHashedMap

Parameters:

value - the value to search for

Returns:

true if the map contains the value

get

public java.lang.Object get(java.lang.Object key)

Gets the value mapped to the key specified.

Specified by:

get in interface java.util.Map

Overrides:

get in class AbstractHashedMap

Parameters:

key - the key

Returns:

the mapped value, null if no match

put

public java.lang.Object put(java.lang.Object key,
                            java.lang.Object value)

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

Specified by:

put in interface java.util.Map

Overrides:

put in class AbstractHashedMap

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:

java.lang.NullPointerException - if either the key or value is null

remove

public java.lang.Object remove(java.lang.Object key)

Removes the specified mapping from this map.

Specified by:

remove in interface java.util.Map

Overrides:

remove in class AbstractHashedMap

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 java.util.Map

Overrides:

clear in class AbstractHashedMap

mapIterator

public MapIterator mapIterator()

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

Specified by:

mapIterator in interface IterableMap

Overrides:

mapIterator in class AbstractHashedMap

Returns:

a map iterator

entrySet

public java.util.Set 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 java.util.Map

Overrides:

entrySet in class AbstractHashedMap

Returns:

a set view of this map's entries

keySet

public java.util.Set keySet()

Returns a set view of this map's keys.

Specified by:

keySet in interface java.util.Map

Overrides:

keySet in class AbstractHashedMap

Returns:

a set view of this map's keys

values

public java.util.Collection values()

Returns a collection view of this map's values.

Specified by:

values in interface java.util.Map

Overrides:

values in class AbstractHashedMap

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(java.lang.ref.Reference ref)

Purges the specified reference.

Parameters:

ref - the reference to purge

getEntry

protected AbstractHashedMap.HashEntry getEntry(java.lang.Object key)

Gets the entry mapped to the key specified.

Overrides:

getEntry in class AbstractHashedMap

Parameters:

key - the key

Returns:

the entry, null if no match

hashEntry

protected int hashEntry(java.lang.Object key,
                        java.lang.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(java.lang.Object key1,
                             java.lang.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

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

protected AbstractHashedMap.HashEntry createEntry(AbstractHashedMap.HashEntry next,
                                                  int hashCode,
                                                  java.lang.Object key,
                                                  java.lang.Object value)

Creates a ReferenceEntry instead of a HashEntry.

Overrides:

createEntry in class AbstractHashedMap

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 java.util.Iterator createEntrySetIterator()

Creates an entry set iterator.

Overrides:

createEntrySetIterator in class AbstractHashedMap

Returns:

the entrySet iterator

createKeySetIterator

protected java.util.Iterator createKeySetIterator()

Creates an key set iterator.

Overrides:

createKeySetIterator in class AbstractHashedMap

Returns:

the keySet iterator

createValuesIterator

protected java.util.Iterator createValuesIterator()

Creates an values iterator.

Overrides:

createValuesIterator in class AbstractHashedMap

Returns:

the values iterator

doWriteObject

protected void doWriteObject(java.io.ObjectOutputStream out)
                      throws java.io.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

Parameters:

out - the output stream

Throws:

java.io.IOException

doReadObject

protected void doReadObject(java.io.ObjectInputStream in)
                     throws java.io.IOException,
                            java.lang.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

Parameters:

in - the input stream

Throws:

java.io.IOException

java.lang.ClassNotFoundException