ptolemy.backtrack.util.java.util

Class WeakHashMap

java.lang.Object

ptolemy.backtrack.util.java.util.AbstractMap

ptolemy.backtrack.util.java.util.WeakHashMap

All Implemented Interfaces:

Rollbackable, Map

public class WeakHashMap
extends AbstractMap
implements Map, Rollbackable

A weak hash map has only weak references to the key. This means that it allows the key to be garbage collected if it is not used otherwise. If this happens, the entry will eventually disappear from the map, asynchronously.

A weak hash map makes most sense when the keys doesn't override the equals method: If there is no other reference to the key nobody can ever look up the key in this table and so the entry can be removed. This table also works when the equals method is overloaded, such as String keys, but you should be prepared to deal with some entries disappearing spontaneously.

Other strange behaviors to be aware of: The size of this map may spontaneously shrink (even if you use a synchronized map and synchronize it); it behaves as if another thread removes entries from this table without synchronization. The entry set returned by entrySet has similar phenomenons: The size may spontaneously shrink, or an entry, that was in the set before, suddenly disappears.

A weak hash map is not meant for caches; use a normal map, with soft references as values instead, or try {

Since:

1.2

Author:

Jochen Hoenicke, Eric Blake (ebb9@email.byu.edu)

See Also:

HashMap, WeakReference, LinkedHashMap

Status

updated to 1.4

Nested Class Summary

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

Map.Entry

Field Summary

Fields inherited from class ptolemy.backtrack.util.java.util.AbstractMap

$CHECKPOINT, $RECORD$$CHECKPOINT

Constructor Summary

Constructors

Constructor and Description
WeakHashMap() Creates a new weak hash map with default load factor and default capacity.
WeakHashMap(int initialCapacity) Creates a new weak hash map with default load factor and the given capacity.
WeakHashMap(int initialCapacity, float loadFactor) Creates a new weak hash map with the given initial capacity and load factor.
WeakHashMap(Map m) Construct a new WeakHashMap with the same mappings as the given map.

Method Summary

Modifier and Type	Method and Description
void	$COMMIT(long timestamp) Commit changes up to the given timestamp, but not including changes made at timestamp and afterward.
void	$RESTORE(long timestamp, boolean trim) Restore a previous state to all the private fields of this rollbackable object.
void	clear() Clears all entries from this map.
boolean	containsKey(java.lang.Object key) Tells if the map contains the given key.
boolean	containsValue(java.lang.Object value) Returns true if the map contains at least one key which points to the specified object as a value.
Set	entrySet() Returns a set representation of the entries in this map.
java.lang.Object	get(java.lang.Object key) Gets the value the key is mapped to.
boolean	isEmpty() Tells if the map is empty.
Set	keySet() Returns a set representation of the keys in this map.
java.lang.Object	put(java.lang.Object key, java.lang.Object value) Adds a new key/value mapping to this map.
void	putAll(Map m) Puts all of the mappings from the given map into this one.
java.lang.Object	remove(java.lang.Object key) Removes the key and the corresponding value from this map.
int	size() Returns the size of this hash map.
Collection	values() Returns a collection representation of the values in this map.

Methods inherited from class ptolemy.backtrack.util.java.util.AbstractMap

$GET$CHECKPOINT, $SET$CHECKPOINT, clone, equals, hashCode, toString

Methods inherited from class java.lang.Object

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

Methods inherited from interface ptolemy.backtrack.util.java.util.Map

$GET$CHECKPOINT, $SET$CHECKPOINT, equals, hashCode

Constructor Detail

WeakHashMap

public WeakHashMap()

Creates a new weak hash map with default load factor and default capacity.

WeakHashMap

public WeakHashMap(int initialCapacity)

Creates a new weak hash map with default load factor and the given capacity.

Parameters:

initialCapacity - the initial capacity

Throws:

java.lang.IllegalArgumentException - if initialCapacity is negative

WeakHashMap

public WeakHashMap(int initialCapacity,
                   float loadFactor)

Creates a new weak hash map with the given initial capacity and load factor.

Parameters:

initialCapacity - the initial capacity.

loadFactor - the load factor (see class description of HashMap).

Throws:

java.lang.IllegalArgumentException - if initialCapacity is negative, or loadFactor is non-positive

WeakHashMap

public WeakHashMap(Map m)

Construct a new WeakHashMap with the same mappings as the given map. The WeakHashMap has a default load factor of 0.75.

Parameters:

m - the map to copy

Throws:

java.lang.NullPointerException - if m is null

Since:

1.3

Method Detail

size

public int size()

Returns the size of this hash map. Note that the size() may shrink spontaneously, if the some of the keys were only weakly reachable.

Specified by:

size in interface Map

Overrides:

size in class AbstractMap

Returns:

the number of entries in this hash map.

See Also:

Set.size()

isEmpty

public boolean isEmpty()

Tells if the map is empty. Note that the result may change spontanously, if all of the keys were only weakly reachable.

Specified by:

isEmpty in interface Map

Overrides:

isEmpty in class AbstractMap

Returns:

true, iff the map is empty.

See Also:

AbstractMap.size()

containsKey

public boolean containsKey(java.lang.Object key)

Tells if the map contains the given key. Note that the result may change spontanously, if the key was only weakly reachable.

Specified by:

containsKey in interface Map

Overrides:

containsKey in class AbstractMap

Parameters:

key - the key to look for

Returns:

true, iff the map contains an entry for the given key.

See Also:

AbstractMap.containsValue(Object)

get

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

Gets the value the key is mapped to.

Specified by:

get in interface Map

Overrides:

get in class AbstractMap

Parameters:

key - the key to look up

Returns:

the value the key was mapped to. It returns null if the key wasn't in this map, or if the mapped value was explicitly set to null.

See Also:

AbstractMap.containsKey(Object)

put

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

Adds a new key/value mapping to this map.

Specified by:

put in interface Map

Overrides:

put in class AbstractMap

Parameters:

key - the key, may be null

value - the value, may be null

Returns:

the value the key was mapped to previously. It returns null if the key wasn't in this map, or if the mapped value was explicitly set to null.

See Also:

AbstractMap.containsKey(Object)

remove

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

Removes the key and the corresponding value from this map.

Specified by:

remove in interface Map

Overrides:

remove in class AbstractMap

Parameters:

key - the key. This may be null.

Returns:

the value the key was mapped to previously. It returns null if the key wasn't in this map, or if the mapped value was explicitly set to null.

See Also:

Iterator.remove()

entrySet

public Set entrySet()

Returns a set representation of the entries in this map. This set will not have strong references to the keys, so they can be silently removed. The returned set has therefore the same strange behaviour (shrinking size(), disappearing entries) as this weak hash map.

Specified by:

entrySet in interface Map

Specified by:

entrySet in class AbstractMap

Returns:

a set representation of the entries.

See Also:

Map.Entry

clear

public void clear()

Clears all entries from this map.

Specified by:

clear in interface Map

Overrides:

clear in class AbstractMap

See Also:

Set.clear()

containsValue

public boolean containsValue(java.lang.Object value)

Returns true if the map contains at least one key which points to the specified object as a value. Note that the result may change spontanously, if its key was only weakly reachable.

Specified by:

containsValue in interface Map

Overrides:

containsValue in class AbstractMap

Parameters:

value - the value to search for

Returns:

true if it is found in the set.

See Also:

AbstractMap.containsKey(Object)

keySet

public Set keySet()

Returns a set representation of the keys in this map. This set will not have strong references to the keys, so they can be silently removed. The returned set has therefore the same strange behaviour (shrinking size(), disappearing entries) as this weak hash map.

Specified by:

keySet in interface Map

Overrides:

keySet in class AbstractMap

Returns:

a set representation of the keys.

See Also:

Set.iterator(), AbstractMap.size(), AbstractMap.containsKey(Object), AbstractMap.values()

putAll

public void putAll(Map m)

Puts all of the mappings from the given map into this one. If the key already exists in this map, its value is replaced.

Specified by:

putAll in interface Map

Overrides:

putAll in class AbstractMap

Parameters:

m - the map to copy in

See Also:

AbstractMap.put(Object, Object)

values

public Collection values()

Returns a collection representation of the values in this map. This collection will not have strong references to the keys, so mappings can be silently removed. The returned collection has therefore the same strange behaviour (shrinking size(), disappearing entries) as this weak hash map.

Specified by:

values in interface Map

Overrides:

values in class AbstractMap

Returns:

a collection representation of the values.

See Also:

Collection.iterator(), AbstractMap.size(), AbstractMap.containsValue(Object), AbstractMap.keySet()

$COMMIT

public void $COMMIT(long timestamp)

Description copied from interface: Rollbackable

Commit changes up to the given timestamp, but not including changes made at timestamp and afterward.

Specified by:

$COMMIT in interface Rollbackable

Specified by:

$COMMIT in interface Map

Overrides:

$COMMIT in class AbstractMap

Parameters:

timestamp - The timestamp.

$RESTORE

public void $RESTORE(long timestamp,
                     boolean trim)

Description copied from interface: Rollbackable

Restore a previous state to all the private fields of this rollbackable object.

Specified by:

$RESTORE in interface Rollbackable

Specified by:

$RESTORE in interface Map

Overrides:

$RESTORE in class AbstractMap

Parameters:

timestamp - The timestamp taken at the time when the previous state was recorded.

trim - Whether to delete the records used for the rollback.

See Also:

Checkpoint.rollback(long, boolean)