com.neeve.lang
Class XLinkedHashMap<K,V>

java.lang.Object
  com.neeve.lang.XLinkedHashMap<K,V>

All Implemented Interfaces:

Map<K,V>

public class XLinkedHashMap<K,V>
extends Object
implements Map<K,V>

This class represents a hash map that avoid creation of garbage; smooth capacity increase and thread-safe without external synchronization when shared.

XLinkedHashMap has a predictable iteration order, which is the order in which keys are inserted into the map (similar to java.util.LinkedHashMap collection class). If the map is marked shared then all operations are thread-safe including iterations over the map's collections. Unlike ConcurrentHashMap, access never blocks; retrieval reflects the map state not older than the last time the accessing threads have been synchronized (for multi-processors systems synchronizing ensures that the CPU internal cache is not stale). In most application it is not a problem because thread synchronization is done at high level (e.g. scheduler) and threads run freely (and quickly) until the next synchronization point. In some cases the "happen before" guarantee is necessary (e.g. to ensure unicity) and threads have to be synchronized explicitly. Whenever possible such synchronization should be performed on the key object itself and not the whole map. For example: XLinkedHashMap sparseVector = new XLinkedHashMap().shared() ... // Put synchronized(index) { //XIndex instances are unique. sparseVector.put(index, value); } ... // Get synchronized(index) { // Blocking only when accessing the same index. value = sparseVector.get(index); // Latest value guaranteed. }

XLinkedHashMap.Entry can quickly be iterated over (forward or backward) without using iterators. For example: XLinkedHashMap map = ...; for (XLinkedHashMap.Entry e = map.head(), end = map.tail(); (e = e.getNext()) != end;) { String key = e.getKey(); // No typecast necessary. Thread value = e.getValue(); // No typecast necessary. }

Alternatively The map can be iterated over using reuseableEntryIterator(), reuseableKeyIterator(), or reuseableValueIterator(). Utilization of these iterators does not introduce garbage because they are created once. However, these iterators may not be used concurrently by multiple threads as they are single instance by nature.

Custom map implementations may override the newEntry() method in order to return their own XLinkedHashMap.Entry implementation (with additional fields for example).

Shared maps do not use internal synchronization, except in case of concurrent modifications of the map structure (entries being added/deleted). Reads and iterations are never synchronized and never blocking. With regards to the memory model, shared maps are equivalent to shared non-volatile variables (no "happen before" guarantee). They can be used as very efficient lookup tables. For example:[code] public class Unit { static XLinkedHashMap labels = new XLinkedHashMap().shared(); ... public String toString() { String label = labels.get(this); // No synchronization. if (label != null) return label; label = makeLabel(); labels.put(this, label); return label; } }[/code]

Implementation Note: To maintain time-determinism, rehash/resize is performed only when the map's size is small. For large maps (size > 512), the map is divided recursively into (64) smaller sub-maps. The cost of the dispatching (based upon hashcode value) has been measured to be at most 20% of the access time (and most often way less).

This class was based on collections code from http://javolution.org, but modified for use outside of realtime jvms.

Nested Class Summary

static class

XLinkedHashMap.Entry<K,V> This class represents a XLinkedHashMap entry.

Constructor Summary

XLinkedHashMap()
Creates a map whose capacity increment smoothly without large resize operations.

XLinkedHashMap(int capacity)
Creates a map of specified maximum size (a full resize may occur if the specififed capacity is exceeded).

XLinkedHashMap(Map<? extends K,? extends V> map)
Creates a map containing the specified entries, in the order they are returned by the map iterator.

Method Summary

void	clear() Removes all map's entries.
boolean	containsKey(Object key) Indicates if this map contains a mapping for the specified key.
boolean	containsValue(Object value) Indicates if this map associates one or more keys to the specified value.
Set<Map.Entry<K,V>>	entrySet() Returns a XCollection view of the mappings contained in this map.
boolean	equals(Object obj) Compares the specified object with this map for equality.
V	get(Object key) Returns the value to which this map associates the specified key.
XLinkedHashMap.Entry<K,V>	getEntry(Object key) Returns the entry with the specified key.
int	hashCode() Returns the hash code value for this map.
XLinkedHashMap.Entry<K,V>	head() Returns the head entry of this map.
boolean	isEmpty() Indicates if this map contains no key-value mappings.
boolean	isShared() Indicates if this map supports concurrent operations without synchronization (default unshared).
Set<K>	keySet() Returns a XCollection view of the keys contained in this map.
static <K,V> XLinkedHashMap<K,V>	newInstance() Creates a new XLinkedHashMap.
void	printStatistics(PrintStream out) Prints the current statistics on this map.
V	put(K key, V value) Associates the specified value with the specified key in this map.
void	putAll(Map<? extends K,? extends V> map) Copies all of the mappings from the specified map to this map.
XLinkedHashMap.Entry<K,V>	putEntry(K key, V value) Associates the specified value with the specified key in this map and returns the corresponding entry.
V	putIfAbsent(K key, V value) Associates the specified value only if the specified key is not already associated.
V	remove(Object key) Removes the entry for the specified key if present.
XCollectionIterator<Map.Entry<K,V>>	reuseableEntryIterator() Returns a reusable value iterator.
XIterator<K>	reuseableKeyIterator() Returns a reusable key iterator.
XIterator<V>	reuseableValueIterator() Returns a reusable value iterator.
XLinkedHashMap<K,V>	shared() Sets the shared status of this map (whether the map is thread-safe or not).
int	size() Returns the number of key-value mappings in this XLinkedHashMap.
XLinkedHashMap.Entry<K,V>	tail() Returns the tail entry of this map.
String	toString() Returns a string representation of this map.
Map<K,V>	unmodifiable() Returns the unmodifiable view associated to this map.
XCollection<V>	values() Returns a XCollection view of the values contained in this map.

Methods inherited from class java.lang.Object

getClass, notify, notifyAll, wait, wait, wait

Constructor Detail

XLinkedHashMap

public XLinkedHashMap()

Creates a map whose capacity increment smoothly without large resize operations.

XLinkedHashMap

public XLinkedHashMap(int capacity)

Creates a map of specified maximum size (a full resize may occur if the specififed capacity is exceeded).

Parameters:

capacity - the maximum capacity.

XLinkedHashMap

public XLinkedHashMap(Map<? extends K,? extends V> map)

Creates a map containing the specified entries, in the order they are returned by the map iterator.

Parameters:

map - the map whose entries are to be placed into this map.

Method Detail

newInstance

public static <K,V> XLinkedHashMap<K,V> newInstance()

Creates a new XLinkedHashMap.

Returns:

a new, preallocated or recycled map instance.

head

public final XLinkedHashMap.Entry<K,V> head()

Returns the head entry of this map.

Returns:

the entry such as head().getNext() holds the first map entry.

tail

public final XLinkedHashMap.Entry<K,V> tail()

Returns the tail entry of this map.

Returns:

the entry such as tail().getPrevious() holds the last map entry.

size

public final int size()

Returns the number of key-value mappings in this XLinkedHashMap.

Note: If concurrent updates are performed, application should not rely upon the size during iterations.

Specified by:

size in interface Map<K,V>

Returns:

this map's size.

isEmpty

public final boolean isEmpty()

Indicates if this map contains no key-value mappings.

Specified by:

isEmpty in interface Map<K,V>

Returns:

true if this map contains no key-value mappings; false otherwise.

containsKey

public final boolean containsKey(Object key)

Indicates if this map contains a mapping for the specified key.

Specified by:

containsKey in interface Map<K,V>

Parameters:

key - the key whose presence in this map is to be tested.

Returns:

true if this map contains a mapping for the specified key; false otherwise.

Throws:

NullPointerException - if the key is null.

containsValue

public final boolean containsValue(Object value)

Indicates if this map associates one or more keys to the specified value.

Specified by:

containsValue in interface Map<K,V>

Parameters:

value - the value whose presence in this map is to be tested.

Returns:

true if this map maps one or more keys to the specified value.

Throws:

NullPointerException - if the key is null.

get

public final V get(Object key)

Returns the value to which this map associates the specified key. This method is always thread-safe regardless whether or not the map is marked shared.

Specified by:

get in interface Map<K,V>

Parameters:

key - the key whose associated value is to be returned.

Returns:

the value to which this map maps the specified key, or null if there is no mapping for the key.

Throws:

NullPointerException - if key is null.

getEntry

public final XLinkedHashMap.Entry<K,V> getEntry(Object key)

Returns the entry with the specified key. This method is always thread-safe without synchronization.

Parameters:

key - the key whose associated entry is to be returned.

Returns:

the entry for the specified key or null if none.

put

public final V put(K key,
                   V value)

Associates the specified value with the specified key in this map. If this map previously contained a mapping for this key, the old value is replaced. For shared map, internal synchronization is performed only when new entries are created.

Specified by:

put in interface Map<K,V>

Parameters:

key - the key with which the specified value is to be associated.

value - the value to be associated with the specified key.

Returns:

the previous value associated with specified key, or null if there was no mapping for key. A null return can also indicate that the map previously associated null with the specified key.

Throws:

NullPointerException - if the key is null.

putEntry

public final XLinkedHashMap.Entry<K,V> putEntry(K key,
                                                V value)

Associates the specified value with the specified key in this map and returns the corresponding entry.

Parameters:

key - the key with which the specified value is to be associated.

value - the value to be associated with the specified key.

Returns:

the entry being added.

Throws:

NullPointerException - if the key is null.

putAll

public final void putAll(Map<? extends K,? extends V> map)

Copies all of the mappings from the specified map to this map.

Specified by:

putAll in interface Map<K,V>

Parameters:

map - the mappings to be stored in this map.

Throws:

NullPointerException - the specified map is null, or the specified map contains null keys.

putIfAbsent

public final V putIfAbsent(K key,
                           V value)

Associates the specified value only if the specified key is not already associated. This is equivalent to:[code] if (!map.containsKey(key)) return map.put(key, value); else return map.get(key);[/code] except that for shared maps the action is performed atomically. For shared maps, this method guarantees that if two threads try to put the same key concurrently only one of them will succeed.

Parameters:

key - the key with which the specified value is to be associated.

value - the value to be associated with the specified key.

Returns:

the previous value associated with specified key, or null if there was no mapping for key. A null return can also indicate that the map previously associated null with the specified key.

Throws:

NullPointerException - if the key is null.

remove

public final V remove(Object key)

Removes the entry for the specified key if present. The entry is recycled if the map is not marked as shared; otherwise the entry is candidate for garbage collection.

Note: Shared maps in ImmortalMemory (e.g. static) should not remove their entries as it could cause a memory leak (ImmortalMemory is never garbage collected), instead they should set their entry values to null.

Specified by:

remove in interface Map<K,V>

Parameters:

key - the key whose mapping is to be removed from the map.

Returns:

previous value associated with specified key, or null if there was no mapping for key. A null return can also indicate that the map previously associated null with the specified key.

Throws:

NullPointerException - if the key is null.

shared

public XLinkedHashMap<K,V> shared()

Sets the shared status of this map (whether the map is thread-safe or not). Shared maps are typically used for lookup table (e.g. static instances in ImmortalMemory). They support concurrent access (e.g. iterations) without synchronization, the maps updates themselves are synchronized internally.

Unlike ConcurrentHashMap access to a shared map never blocks. Retrieval reflects the map state not older than the last time the accessing thread has been synchronized (for multi-processors systems synchronizing ensures that the CPU internal cache is not stale).

Returns:

this

isShared

public boolean isShared()

Indicates if this map supports concurrent operations without synchronization (default unshared).

Returns:

true if this map is thread-safe; false otherwise.

clear

public final void clear()

Removes all map's entries. The entries are removed and recycled; unless this map is shared in which case the entries are candidate for garbage collection.

Note: Shared maps in ImmortalMemory (e.g. static) should not remove their entries as it could cause a memory leak (ImmortalMemory is never garbage collected), instead they should set their entry values to null.

Specified by:

clear in interface Map<K,V>

equals

public boolean equals(Object obj)

Compares the specified object with this map for equality. Returns true if the given object is also a map and the two maps represent the same mappings (regardless of collection iteration order).

Specified by:

equals in interface Map<K,V>

Overrides:

equals in class Object

Parameters:

obj - the object to be compared for equality with this map.

Returns:

true if the specified object is equal to this map; false otherwise.

hashCode

public int hashCode()

Returns the hash code value for this map.

Specified by:

hashCode in interface Map<K,V>

Overrides:

hashCode in class Object

Returns:

the hash code value for this map.

toString

public String toString()

Returns a string representation of this map. The string representation consists of a list of key-value mappings in the order returned by the map's entrySet view's iterator, enclosed in braces ("{}"). Adjacent mappings are separated by the characters ", " (comma and space). Each key-value mapping is rendered as the key followed by an equals sign ("=") followed by the associated value. Keys and values are converted to strings as by String.valueOf(Object).

Overrides:

toString in class Object

Returns:

a string representation of this map

printStatistics

public void printStatistics(PrintStream out)

Prints the current statistics on this map. This method may help identify poorly defined hash functions. The average distance should be less than 20% (most of the entries are in their slots or close).

Parameters:

out - the stream to use for output (e.g. System.out)

values

public final XCollection<V> values()

Returns a XCollection view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. The collection supports element removal, which removes the corresponding mapping from this map, via the Iterator.remove, Collection.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations.

Specified by:

values in interface Map<K,V>

Returns:

a collection view of the values contained in this map (instance of XCollection).

entrySet

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

Returns a XCollection view of the mappings contained in this map. Each element in the returned collection is a XLinkedHashMap.Entry. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. The collection supports element removal, which removes the corresponding mapping from this map, via the Iterator.remove, Collection.remove,removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

Specified by:

entrySet in interface Map<K,V>

Returns:

a collection view of the mappings contained in this map (instance of XCollection).

keySet

public final Set<K> keySet()

Returns a XCollection view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. The set supports element removal, which removes the corresponding mapping from this map, via the Iterator.remove, Collection.remove,removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

Specified by:

keySet in interface Map<K,V>

Returns:

a set view of the keys contained in this map (instance of XCollection).

reuseableKeyIterator

public XIterator<K> reuseableKeyIterator()

Returns a reusable key iterator. This iterator may only be used by a single thread at a time. Prior to returning the iterator XCollectionIterator.toFirst() is called by this method to reset it to the beginning.

Returns:

The shared key iterator.

reuseableValueIterator

public XIterator<V> reuseableValueIterator()

Returns a reusable value iterator. This iterator may only be used by a single thread at a time. Prior to returning the iterator XCollectionIterator.toFirst() is called by this method to reset it to the beginning.

Returns:

The shared key iterator.

reuseableEntryIterator

public XCollectionIterator<Map.Entry<K,V>> reuseableEntryIterator()

Returns a reusable value iterator. This iterator may only be used by a single thread at a time. Prior to returning the iterator XCollectionIterator.toFirst() is called by this method to reset it to the beginning.

Returns:

The shared key iterator.

unmodifiable

public final Map<K,V> unmodifiable()

Returns the unmodifiable view associated to this map. Attempts to modify the returned map or to directly access its (modifiable) map entries (e.g. unmodifiable().entrySet()) result in an UnsupportedOperationException being thrown. Unmodifiable XCollection views of this map keys and values are nonetheless obtainable (e.g. unmodifiable().keySet(), unmodifiable().values()).

Returns:

an unmodifiable view of this map.