Class HashMapSeparateChaining

java.lang.Object

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  +--java.util.AbstractMap

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        +--HashMapSeparateChaining

public class HashMapSeparateChaining

extends java.util.AbstractMap

implements java.util.Map, java.lang.Cloneable, java.io.Serializable

This is the Java version of HashMap, but modified so that the number of comparisons corresponds to the number of time objects are compared. The original Java code stored hash values for a initial comparison. This change makes comparing run times by number of comparisons fairer.
THIS CHANGE KEEPS THE OLD FUNCTIONALITY OF HASHMAP, BUT AT A LOWER LEVEL OF EFFICIENCY. RECOMMENDED TO USE THIS CODE ONLY FOR BENCHMARKING.

Hash table based implementation of the Map interface. This implementation provides all of the optional map operations, and permits null values and the null key. (The HashMapSeparateChaining class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.) This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.

This implementation provides constant-time performance for the basic operations (get and put), assuming the hash function disperses the elements properly among the buckets. Iteration over collection views requires time proportional to the "capacity" of the HashMapSeparateChaining instance (the number of buckets) plus its size (the number of key-value mappings). Thus, it's very important not to set the intial capacity too high (or the load factor too low) if iteration performance is important.

An instance of HashMapSeparateChaining has two parameters that affect its performance: initial capacity and load factor. The capacity is the number of buckets in the hash table, and the initial capacity is simply the capacity at the time the hash table is created. The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. When the number of entries in the hash table exceeds the product of the load factor and the current capacity, the capacity is roughly doubled by calling the rehash method.

As a general rule, te default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the lookup cost (reflected in most of the operations of the HashMapSeparateChaining class, including get and put). The expected number of entries in the map and its load factor should be taken into account when setting its initial capacity, so as to minimize the number of rehash operations. If the initial capacity is greater than the maximum number of entries divided by the load factor, no rehash operations will ever occur.

If many mappings are to be stored in a HashMapSeparateChaining instance, creating it with a sufficiently large capacity will allow the mappings to be stored more efficiently than letting it perform automatic rehashing as needed to grow the table.

Note that this implementation is not synchronized. If multiple threads access this map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more mappings; merely changing the value associated with a key that an instance already contains is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the map. If no such object exists, the map should be "wrapped" using the Collections.synchronizedMap method. This is best done at creation time, to prevent accidental unsynchronized access to the map:

 Map m = Collections.synchronizedMap(new HashMapSeparateChaining(...));
 

The iterators returned by all of this class's "collection view methods" are fail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

Since:

JDK1.2

See Also:

Object.hashCode(), Collection, Map, TreeMap, Hashtable, Serialized Form

Inner classes inherited from class java.util.Map

java.util.Map.Entry

Constructor Summary

HashMapSeparateChaining()
Constructs a new, empty map with a default capacity and load factor, which is 0.75.

HashMapSeparateChaining(int initialCapacity)
Constructs a new, empty map with the specified initial capacity and default load factor, which is 0.75.

HashMapSeparateChaining(int initialCapacity, float loadFactor)
Constructs a new, empty map with the specified initial capacity and the specified load factor.

HashMapSeparateChaining(java.util.Map t)
Constructs a new map with the same mappings as the given map.

Method Summary

void	clear() Removes all mappings from this map.
java.lang.Object	clone() Returns a shallow copy of this HashMapSeparateChaining instance: the keys and values themselves are not cloned.
boolean	containsKey(java.lang.Object key) Returns true if this map contains a mapping for the specified key.
boolean	containsValue(java.lang.Object value) Returns true if this map maps one or more keys to the specified value.
java.util.Set	entrySet() Returns a collection view of the mappings contained in this map.
java.lang.Object	get(java.lang.Object key) Returns the value to which this map maps the specified key.
boolean	isEmpty() Returns true if this map contains no key-value mappings.
java.util.Set	keySet() Returns a set view of the keys contained in this map.
java.lang.Object	put(java.lang.Object key, java.lang.Object value) Associates the specified value with the specified key in this map.
void	putAll(java.util.Map t) Copies all of the mappings from the specified map to this one.
java.lang.Object	remove(java.lang.Object key) Removes the mapping for this key from this map if present.
int	size() Returns the number of key-value mappings in this map.
java.util.Collection	values() Returns a collection view of the values contained in this map.

Methods inherited from class java.util.AbstractMap

equals, hashCode, toString

Methods inherited from class java.lang.Object

getClass, notify, notifyAll, wait, wait, wait

Constructor Detail

HashMapSeparateChaining

public HashMapSeparateChaining(int initialCapacity,
                               float loadFactor)

Constructs a new, empty map with the specified initial capacity and the specified load factor.

Parameters:

initialCapacity - the initial capacity of the HashMapSeparateChaining.

loadFactor - the load factor of the HashMapSeparateChaining

Throws:

java.lang.IllegalArgumentException - if the initial capacity is less than zero, or if the load factor is nonpositive.

HashMapSeparateChaining

public HashMapSeparateChaining(int initialCapacity)

Constructs a new, empty map with the specified initial capacity and default load factor, which is 0.75.

Parameters:

initialCapacity - the initial capacity of the HashMapSeparateChaining.

Throws:

java.lang.IllegalArgumentException - if the initial capacity is less than zero.

HashMapSeparateChaining

public HashMapSeparateChaining()

Constructs a new, empty map with a default capacity and load factor, which is 0.75.

HashMapSeparateChaining

public HashMapSeparateChaining(java.util.Map t)

Constructs a new map with the same mappings as the given map. The map is created with a capacity of twice the number of mappings in the given map or 11 (whichever is greater), and a default load factor, which is 0.75.

Method Detail

size

public int size()

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

Specified by:

size in interface java.util.Map

Overrides:

size in class java.util.AbstractMap

Returns:

the number of key-value mappings in this map.

isEmpty

public boolean isEmpty()

Returns true if this map contains no key-value mappings.

Specified by:

isEmpty in interface java.util.Map

Overrides:

isEmpty in class java.util.AbstractMap

Returns:

true if this map contains no key-value mappings.

containsValue

public boolean containsValue(java.lang.Object value)

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

Specified by:

containsValue in interface java.util.Map

Overrides:

containsValue in class java.util.AbstractMap

Parameters:

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

Returns:

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

containsKey

public boolean containsKey(java.lang.Object key)

Returns true if this map contains a mapping for the specified key.

Specified by:

containsKey in interface java.util.Map

Overrides:

containsKey in class java.util.AbstractMap

Parameters:

key - key whose presence in this Map is to be tested.

Returns:

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

get

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

Returns the value to which this map maps the specified key. Returns null if the map contains no mapping for this key. A return value of null does not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key to null. The containsKey operation may be used to distinguish these two cases.

Specified by:

get in interface java.util.Map

Overrides:

get in class java.util.AbstractMap

Parameters:

key - key whose associated value is to be returned.

Returns:

the value to which this map maps the specified key.

put

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

Associates the specified value with the specified key in this map. If the map previously contained a mapping for this key, the old value is replaced.

Specified by:

put in interface java.util.Map

Overrides:

put in class java.util.AbstractMap

Parameters:

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

value - value to be associated with the specified key.

Returns:

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

remove

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

Removes the mapping for this key from this map if present.

Specified by:

remove in interface java.util.Map

Overrides:

remove in class java.util.AbstractMap

Parameters:

key - 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.

putAll

public void putAll(java.util.Map t)

Copies all of the mappings from the specified map to this one. These mappings replace any mappings that this map had for any of the keys currently in the specified Map.

Specified by:

putAll in interface java.util.Map

Overrides:

putAll in class java.util.AbstractMap

Parameters:

t - Mappings to be stored in this map.

clear

public void clear()

Removes all mappings from this map.

Specified by:

clear in interface java.util.Map

Overrides:

clear in class java.util.AbstractMap

clone

public java.lang.Object clone()

Returns a shallow copy of this HashMapSeparateChaining instance: the keys and values themselves are not cloned.

Returns:

a shallow copy of this map.

keySet

public java.util.Set keySet()

Returns a set 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, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

Specified by:

keySet in interface java.util.Map

Overrides:

keySet in class java.util.AbstractMap

Returns:

a set view of the keys contained in this map.

values

public java.util.Collection values()

Returns a collection 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 java.util.Map

Overrides:

values in class java.util.AbstractMap

Returns:

a collection view of the values contained in this map.

entrySet

public java.util.Set entrySet()

Returns a collection view of the mappings contained in this map. Each element in the returned collection is a Map.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 the 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 java.util.Map

Overrides:

entrySet in class java.util.AbstractMap

Returns:

a collection view of the mappings contained in this map.

See Also:

Map.Entry