org.apache.commons.collections.map

Class AbstractReferenceMap

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 java.util.Collections#synchronizedMap to provide synchronized access to a ReferenceMap.

Since: Commons Collections 3.1 (extracted from ReferenceMap in 3.0)

Version: $Revision: 155406 $ $Date: 2005-02-26 12:55:26 +0000 (Sat, 26 Feb 2005) $

Author: Paul Jack Stephen Colebourne

See Also: java.lang.ref.Reference

Nested Class Summary
protected static classAbstractReferenceMap.ReferenceEntry
A MapEntry implementation for the map.
Field Summary
static intHARD
Constant indicating that hard references should be used
protected intkeyType
The reference type for keys.
protected booleanpurgeValues
Should the value be automatically purged when the associated key has been collected?
static intSOFT
Constant indicating that soft references should be used
protected intvalueType
The reference type for values.
static intWEAK
Constant indicating that weak references should be used
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
voidclear()
Clears this map.
booleancontainsKey(Object key)
Checks whether the map contains the specified key.
booleancontainsValue(Object value)
Checks whether the map contains the specified value.
protected HashEntrycreateEntry(HashEntry next, int hashCode, Object key, Object value)
Creates a ReferenceEntry instead of a HashEntry.
protected IteratorcreateEntrySetIterator()
Creates an entry set iterator.
protected IteratorcreateKeySetIterator()
Creates an key set iterator.
protected IteratorcreateValuesIterator()
Creates an values iterator.
protected voiddoReadObject(ObjectInputStream in)
Replaces the superclassm method to read the state of this class.
protected voiddoWriteObject(ObjectOutputStream out)
Replaces the superclass method to store the state of this class.
SetentrySet()
Returns a set view of this map's entries.
Objectget(Object key)
Gets the value mapped to the key specified.
protected HashEntrygetEntry(Object key)
Gets the entry mapped to the key specified.
protected inthashEntry(Object key, Object value)
Gets the hash code for a MapEntry.
protected voidinit()
Initialise this subclass during construction, cloning or deserialization.
booleanisEmpty()
Checks whether the map is currently empty.
protected booleanisEqualKey(Object key1, Object key2)
Compares two keys, in internal converted form, to see if they are equal.
SetkeySet()
Returns a set view of this map's keys.
MapIteratormapIterator()
Gets a MapIterator over the reference map.
protected voidpurge()
Purges stale mappings from this map.
protected voidpurge(Reference ref)
Purges the specified reference.
protected voidpurgeBeforeRead()
Purges stale mappings from this map before read operations.
protected voidpurgeBeforeWrite()
Purges stale mappings from this map before write operations.
Objectput(Object key, Object value)
Puts a key-value mapping into this map.
Objectremove(Object key)
Removes the specified mapping from this map.
intsize()
Gets the size of the map.
Collectionvalues()
Returns a collection view of this map's values.

Field Detail

HARD

public static final int HARD
Constant indicating that hard references should be used

keyType

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

Serial:

purgeValues

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

SOFT

public static final int SOFT
Constant indicating that soft references should be used

valueType

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

Serial:

WEAK

public static final int WEAK
Constant indicating that weak references should be used

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

clear

public void clear()
Clears this map.

containsKey

public boolean containsKey(Object key)
Checks whether the map contains the specified key.

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.

Parameters: value the value to search for

Returns: true if the map contains the value

createEntry

protected HashEntry createEntry(HashEntry next, int hashCode, Object key, Object value)
Creates a ReferenceEntry instead of a HashEntry.

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 createEntrySetIterator()
Creates an entry set iterator.

Returns: the entrySet iterator

createKeySetIterator

protected Iterator createKeySetIterator()
Creates an key set iterator.

Returns: the keySet iterator

createValuesIterator

protected Iterator createValuesIterator()
Creates an values iterator.

Returns: the values iterator

doReadObject

protected void doReadObject(ObjectInputStream in)
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.

Parameters: in the input stream

doWriteObject

protected void doWriteObject(ObjectOutputStream out)
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.

Parameters: out the output stream

entrySet

public 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.

Returns: a set view of this map's entries

get

public Object get(Object key)
Gets the value mapped to the key specified.

Parameters: key the key

Returns: the mapped value, null if no match

getEntry

protected HashEntry getEntry(Object key)
Gets the entry mapped to the key specified.

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

init

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

isEmpty

public boolean isEmpty()
Checks whether the map is currently empty.

Returns: true if the map is currently size zero

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.

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

keySet

public Set keySet()
Returns a set view of this map's keys.

Returns: a set view of this map's keys

mapIterator

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

Returns: a map iterator

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

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.

put

public Object put(Object key, Object value)
Puts a key-value mapping into this map. Neither the key nor the value may be null.

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 Object remove(Object key)
Removes the specified mapping from this map.

Parameters: key the mapping to remove

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

size

public int size()
Gets the size of the map.

Returns: the size

values

public Collection values()
Returns a collection view of this map's values.

Returns: a set view of this map's values

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