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.
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.
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.
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.
containsSymbol
public boolean containsSymbol(String symbol)
Returns true if the symbol table already contains the specified
symbol.
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.
buffer
- The buffer containing the symbol to look for.offset
- The offset into the buffer.length
- The length of the 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.
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.
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.