Module org.apache.jena.core
Package org.apache.jena.ext.xerces.util

Class SymbolTable

java.lang.Object
org.apache.jena.ext.xerces.util.SymbolTable
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: SymbolTable.java 985518 2010-08-14 16:02:52Z mrglavas $
Author:
Andy Clark, John Kim, IBM
See Also:

  • Constructor Summary

    Constructors
    Constructor
    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

    Modifier and Type
    Method
    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.

    Methods inherited from class java.lang.Object

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

  • Constructor Details

    • 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 Details

    • 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.

    • 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.