org.apache.commons.collections.map

Class AbstractReferenceMap

Implemented Interfaces:
IterableMap, Map
Known Direct 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 java.util.Collections.synchronizedMap to provide synchronized access to a ReferenceMap.

Version:
$Revision: 1.3 $ $Date: 2004/06/07 22:14:42 $
Authors:
Paul Jack
Stephen Colebourne
Since:
Commons Collections 3.1 (extracted from ReferenceMap in 3.0)
See Also:
java.lang.ref.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

Field Summary

static int
HARD
Constant indicating that hard references should be used
static int
SOFT
Constant indicating that soft references should be used
static int
WEAK
Constant indicating that weak 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?
protected int
valueType
The reference type for values.

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

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

Constructor Summary

AbstractReferenceMap()
Constructor used during deserialization.
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.
protected AbstractHashedMap.HashEntry
createEntry(AbstractHashedMap.HashEntry next, int hashCode, Object key, Object value)
Creates a ReferenceEntry instead of a HashEntry.
protected Iterator
createEntrySetIterator()
Creates an entry set iterator.
protected Iterator
createKeySetIterator()
Creates an key set iterator.
protected Iterator
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
entrySet()
Returns a set view of this map's entries.
Object
get(Object key)
Gets the value mapped to the key specified.
protected AbstractHashedMap.HashEntry
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
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(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.
Object
put(Object key, Object value)
Puts a key-value mapping into this map.
Object
remove(Object key)
Removes the specified mapping from this map.
int
size()
Gets the size of the map.
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, clear, clone, containsKey, containsValue, convertKey, createEntry, createEntrySetIterator, createKeySetIterator, createValuesIterator, destroyEntry, doReadObject, doWriteObject, ensureCapacity, entryHashCode, entryKey, entryNext, entrySet, entryValue, equals, get, getEntry, hash, hashCode, hashIndex, init, isEmpty, isEqualKey, isEqualValue, keySet, mapIterator, put, putAll, remove, removeEntry, removeMapping, reuseEntry, size, toString, updateEntry, values

Field Details

HARD

public static final int HARD
Constant indicating that hard references should be used
Field Value:
0

SOFT

public static final int SOFT
Constant indicating that soft references should be used
Field Value:
1

WEAK

public static final int WEAK
Constant indicating that weak references should be used
Field Value:
2

keyType

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

purgeValues

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

valueType

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

Constructor Details

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 Details

clear

public void clear()
Clears this map.
Overrides:
clear in interface AbstractHashedMap

containsKey

public boolean containsKey(Object key)
Checks whether the map contains the specified key.
Overrides:
containsKey in interface AbstractHashedMap
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.
Overrides:
containsValue in interface AbstractHashedMap
Parameters:
value - the value to search for
Returns:
true if the map contains the value

createEntry

protected AbstractHashedMap.HashEntry createEntry(AbstractHashedMap.HashEntry next,
                                                  int hashCode,
                                                  Object key,
                                                  Object value)
Creates a ReferenceEntry instead of a HashEntry.
Overrides:
createEntry in interface 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 Iterator createEntrySetIterator()
Creates an entry set iterator.
Overrides:
createEntrySetIterator in interface AbstractHashedMap
Returns:
the entrySet iterator

createKeySetIterator

protected Iterator createKeySetIterator()
Creates an key set iterator.
Overrides:
createKeySetIterator in interface AbstractHashedMap
Returns:
the keySet iterator

createValuesIterator

protected Iterator createValuesIterator()
Creates an values iterator.
Overrides:
createValuesIterator in interface AbstractHashedMap
Returns:
the values iterator

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 interface AbstractHashedMap
Parameters:
in - the input stream

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 interface AbstractHashedMap
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.
Overrides:
entrySet in interface AbstractHashedMap
Returns:
a set view of this map's entries

get

public Object get(Object key)
Gets the value mapped to the key specified.
Overrides:
get in interface AbstractHashedMap
Parameters:
key - the key
Returns:
the mapped value, null if no match

getEntry

protected AbstractHashedMap.HashEntry getEntry(Object key)
Gets the entry mapped to the key specified.
Overrides:
getEntry in interface AbstractHashedMap
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.
Overrides:
init in interface AbstractHashedMap

isEmpty

public boolean isEmpty()
Checks whether the map is currently empty.
Overrides:
isEmpty in interface AbstractHashedMap
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.

Overrides:
isEqualKey in interface 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

keySet

public Set keySet()
Returns a set view of this map's keys.
Overrides:
keySet in interface AbstractHashedMap
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.
Specified by:
mapIterator in interface IterableMap
Overrides:
mapIterator in interface AbstractHashedMap
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()

purgeBeforeWrite

protected void purgeBeforeWrite()

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.
Overrides:
put in interface 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

remove

public Object remove(Object key)
Removes the specified mapping from this map.
Overrides:
remove in interface AbstractHashedMap
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.
Overrides:
size in interface AbstractHashedMap
Returns:
the size

values

public Collection values()
Returns a collection view of this map's values.
Overrides:
values in interface AbstractHashedMap
Returns:
a set view of this map's values

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