public class

SymbolTable

extends Object
java.lang.Object
   ↳ org.apache.xerces.util.SymbolTable
Known Direct Subclasses

Class Overview

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.

See Also

Summary

Nested Classes
class SymbolTable.Entry This class is a symbol table entry. 
Constants
int TABLE_SIZE Default table size.
Fields
protected Entry[] fBuckets Buckets.
protected int fCount The total number of entries in the hash table.
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.
Public Constructors
SymbolTable(int initialCapacity, float loadFactor)
Constructs a new, empty SymbolTable with the specified initial capacity and the specified load factor.
SymbolTable(int initialCapacity)
Constructs a new, empty SymbolTable with the specified initial capacity and default load factor, which is 0.75.
SymbolTable()
Constructs a new, empty SymbolTable with a default initial capacity (101) and load factor, which is 0.75.
Public Methods
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 Methods
void rehash()
Increases the capacity of and internally reorganizes this SymbolTable, in order to accommodate and access its entries more efficiently.
[Expand]
Inherited Methods
From class java.lang.Object

Constants

protected static final int TABLE_SIZE

Default table size.

Constant Value: 101 (0x00000065)

Fields

protected Entry[] fBuckets

Buckets.

protected int fCount

The total number of entries in the hash table.

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. (The value of this field is (int)(capacity * loadFactor).)

Public Constructors

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.

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.

public SymbolTable ()

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

Public Methods

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.

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.

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.

public boolean containsSymbol (String symbol)

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

Parameters
symbol The symbol to look for.

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.

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.

Protected Methods

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.