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 SymbolHash
@author Andy Clark
@author John Kim, IBM
@version $Id: SymbolTable.java 985518 2010-08-14 16:02:52Z mrglavas $