org.apache.xerces.util

Class SymbolTable

java.lang.Object

org.apache.xerces.util.SymbolTable

Direct Known Subclasses:

ShadowedSymbolTable, SoftReferenceSymbolTable, SynchronizedSymbolTable

public class SymbolTable
extends Object

This class is a symbol table implementation that guarantees that strings used as identifiers are unique references. Multiple calls to addSymbol will always return the same string reference.

The symbol table performs the same task as String.intern() with the following differences:

• A new string object does not need to be created in order to retrieve a unique reference. Symbols can be added by using a series of characters in a character array.
• Users of the symbol table can provide their own symbol hashing implementation. For example, a simple string hashing algorithm may fail to produce a balanced set of hashcodes for symbols that are mostly unique. Strings with similar leading characters are especially prone to this poor hashing behavior.

An instance of SymbolTable has two parameters that affect its performance: initial capacity and load factor. The capacity is the number of buckets in the SymbolTable, and the initial capacity is simply the capacity at the time the SymbolTable is created. Note that the SymbolTable is open: in the case of a "hash collision", a single bucket stores multiple entries, which must be searched sequentially. The load factor is a measure of how full the SymbolTable is allowed to get before its capacity is automatically increased. When the number of entries in the SymbolTable exceeds the product of the load factor and the current capacity, the capacity is increased by calling the rehash method.

Generally, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the time cost to look up an entry (which is reflected in most SymbolTable operations, including addSymbol and containsSymbol).

The initial capacity controls a tradeoff between wasted space and the need for rehash operations, which are time-consuming. No rehash operations will ever occur if the initial capacity is greater than the maximum number of entries the Hashtable will contain divided by its load factor. However, setting the initial capacity too high can waste space.

If many entries are to be made into a SymbolTable, creating it with a sufficiently large capacity may allow the entries to be inserted more efficiently than letting it perform automatic rehashing as needed to grow the table.

Version:

$Id$

Author:

Andy Clark, John Kim, IBM

See Also:

SymbolHash

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
protected static class	SymbolTable.Entry This class is a symbol table entry.

Field Summary

Fields

Modifier and Type	Field and Description
protected SymbolTable.Entry[]	fBuckets Buckets.
protected int	fCollisionThreshold A new hash function is selected and the table is rehashed when the number of keys in the bucket exceeds this threshold.
protected int	fCount The total number of entries in the hash table.
protected int[]	fHashMultipliers Array of randomly selected hash function multipliers or null if the default String.hashCode() function should be used.
protected float	fLoadFactor The load factor for the SymbolTable.
protected int	fTableSize actual table size
protected int	fThreshold The table is rehashed when its size exceeds this threshold.
protected static int	MAX_HASH_COLLISIONS Maximum hash collisions per bucket for a table with load factor == 1.
protected static int	MULTIPLIERS_MASK
protected static int	MULTIPLIERS_SIZE
protected static int	TABLE_SIZE Default table size.

Constructor Summary

Constructors

Constructor and Description
SymbolTable() Constructs a new, empty SymbolTable with a default initial capacity (101) and load factor, which is 0.75.
SymbolTable(int initialCapacity) Constructs a new, empty SymbolTable with the specified initial capacity and default load factor, which is 0.75.
SymbolTable(int initialCapacity, float loadFactor) Constructs a new, empty SymbolTable with the specified initial capacity and the specified load factor.

Method Summary

Methods

Modifier and Type	Method and Description
String	addSymbol(char[] buffer, int offset, int length) Adds the specified symbol to the symbol table and returns a reference to the unique symbol.
String	addSymbol(String symbol) Adds the specified symbol to the symbol table and returns a reference to the unique symbol.
boolean	containsSymbol(char[] buffer, int offset, int length) Returns true if the symbol table already contains the specified symbol.
boolean	containsSymbol(String symbol) Returns true if the symbol table already contains the specified symbol.
int	hash(char[] buffer, int offset, int length) Returns a hashcode value for the specified symbol information.
int	hash(String symbol) Returns a hashcode value for the specified symbol.
protected void	rebalance() Randomly selects a new hash function and reorganizes this SymbolTable in order to more evenly distribute its entries across the table.
protected void	rehash() Increases the capacity of and internally reorganizes this SymbolTable, in order to accommodate and access its entries more efficiently.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

TABLE_SIZE

protected static final int TABLE_SIZE

Default table size.

See Also:

Constant Field Values

MAX_HASH_COLLISIONS

protected static final int MAX_HASH_COLLISIONS

Maximum hash collisions per bucket for a table with load factor == 1.

See Also:

Constant Field Values

MULTIPLIERS_SIZE

protected static final int MULTIPLIERS_SIZE

See Also:

Constant Field Values

MULTIPLIERS_MASK

protected static final int MULTIPLIERS_MASK

See Also:

Constant Field Values

fBuckets

protected SymbolTable.Entry[] fBuckets

Buckets.

fTableSize

protected int fTableSize

actual table size

fCount

protected transient int fCount

The total number of entries in the hash table.

fThreshold

protected int fThreshold

The table is rehashed when its size exceeds this threshold. (The value of this field is (int)(capacity * loadFactor).)

fLoadFactor

protected float fLoadFactor

The load factor for the SymbolTable.

fCollisionThreshold

protected final int fCollisionThreshold

A new hash function is selected and the table is rehashed when the number of keys in the bucket exceeds this threshold.

fHashMultipliers

protected int[] fHashMultipliers

Array of randomly selected hash function multipliers or null if the default String.hashCode() function should be used.

Constructor Detail

SymbolTable

public SymbolTable(int initialCapacity,
           float loadFactor)

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

Parameters:

initialCapacity - the initial capacity of the SymbolTable.

loadFactor - the load factor of the SymbolTable.

Throws:

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

SymbolTable

public SymbolTable(int initialCapacity)

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

Parameters:

initialCapacity - the initial capacity of the hashtable.

Throws:

IllegalArgumentException - if the initial capacity is less than zero.

SymbolTable

public SymbolTable()

Constructs a new, empty SymbolTable with a default initial capacity (101) and load factor, which is 0.75.

Method Detail

addSymbol

public String addSymbol(String symbol)

Adds the specified symbol to the symbol table and returns a reference to the unique symbol. If the symbol already exists, the previous symbol reference is returned instead, in order guarantee that symbol references remain unique.

Parameters:

symbol - The new symbol.

addSymbol

public String addSymbol(char[] buffer,
               int offset,
               int length)

Adds the specified symbol to the symbol table and returns a reference to the unique symbol. If the symbol already exists, the previous symbol reference is returned instead, in order guarantee that symbol references remain unique.

Parameters:

buffer - The buffer containing the new symbol.

offset - The offset into the buffer of the new symbol.

length - The length of the new symbol in the buffer.

hash

public int hash(String symbol)

Returns a hashcode value for the specified symbol. The value returned by this method must be identical to the value returned by the hash(char[],int,int) method when called with the character array that comprises the symbol string.

Parameters:

symbol - The symbol to hash.

hash

public int hash(char[] buffer,
       int offset,
       int length)

Returns a hashcode value for the specified symbol information. The value returned by this method must be identical to the value returned by the hash(String) method when called with the string object created from the symbol information.

Parameters:

buffer - The character buffer containing the symbol.

offset - The offset into the character buffer of the start of the symbol.

length - The length of the symbol.

rehash

protected void rehash()

Increases the capacity of and internally reorganizes this SymbolTable, in order to accommodate and access its entries more efficiently. This method is called automatically when the number of keys in the SymbolTable exceeds this hashtable's capacity and load factor.

rebalance

protected void rebalance()

Randomly selects a new hash function and reorganizes this SymbolTable in order to more evenly distribute its entries across the table. This method is called automatically when the number keys in one of the SymbolTable's buckets exceeds the given collision threshold.

containsSymbol

public boolean containsSymbol(String symbol)

Returns true if the symbol table already contains the specified symbol.

Parameters:

symbol - The symbol to look for.

containsSymbol

public boolean containsSymbol(char[] buffer,
                     int offset,
                     int length)

Returns true if the symbol table already contains the specified symbol.

Parameters:

buffer - The buffer containing the symbol to look for.

offset - The offset into the buffer.

length - The length of the symbol in the buffer.