| java.lang.Object
 org.apache.xerces.util.SymbolTable
All known Subclasses: org.apache.xerces.util.ShadowedSymbolTable, org.apache.xerces.util.SynchronizedSymbolTable, org.apache.xerces.util.SoftReferenceSymbolTable,
| SymbolTable | | public class SymbolTable (Code) | | 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: SymbolHash
author:
Andy Clark
author:
John Kim, IBM
version:
$Id: SymbolTable.java 496157 2007-01-14 21:24:28Z mrglavas $ |
| Inner Class :final protected static class Entry | |
| Field Summary | |
| final protected static int | TABLE_SIZE
Default table size. | | protected Entry[] | fBuckets
Buckets. | | protected transient int | fCount
The total number of entries in the hash table. | | protected float | fLoadFactor
The load factor for the SymbolTable. | | protected int | fTableSize
| | protected int | fThreshold
The table is rehashed when its size exceeds this threshold. |
| Constructor Summary | |
| public | SymbolTable(int initialCapacity, float loadFactor)
Constructs a new, empty SymbolTable with the specified initial
capacity and the specified load factor. | | public | SymbolTable(int initialCapacity)
Constructs a new, empty SymbolTable with the specified initial capacity
and default load factor, which is 0.75. | | public | SymbolTable()
Constructs a new, empty SymbolTable with a default initial capacity (101)
and load factor, which is 0.75. |
| Method Summary | |
| public String | addSymbol(String symbol)
Adds the specified symbol to the symbol table and returns a
reference to the unique symbol. | | 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. | | public boolean | containsSymbol(String symbol)
Returns true if the symbol table already contains the specified
symbol. | | public boolean | containsSymbol(char[] buffer, int offset, int length)
Returns true if the symbol table already contains the specified
symbol. | | public int | hash(String symbol)
Returns a hashcode value for the specified symbol. | | public int | hash(char[] buffer, int offset, int length)
Returns a hashcode value for the specified symbol information. | | protected void | rehash()
Increases the capacity of and internally reorganizes this
SymbolTable, in order to accommodate and access its entries more
efficiently. |
| TABLE_SIZE | | final protected static int TABLE_SIZE(Code) | | Default table size.
|
| fBuckets | | protected Entry[] fBuckets(Code) | | Buckets.
|
| fCount | | protected transient int fCount(Code) | | The total number of entries in the hash table.
|
| fLoadFactor | | protected float fLoadFactor(Code) | | The load factor for the SymbolTable.
|
| fTableSize | | protected int fTableSize(Code) | | actual table size *
|
| fThreshold | | protected int fThreshold(Code) | | The table is rehashed when its size exceeds this threshold. (The
value of this field is (int)(capacity * loadFactor).)
|
| SymbolTable | | public SymbolTable(int initialCapacity, float loadFactor)(Code) | | Constructs a new, empty SymbolTable with the specified initial
capacity and the specified load factor.
Parameters:
initialCapacity - the initial capacity of the SymbolTable.
Parameters:
loadFactor - the load factor of the SymbolTable.
throws:
IllegalArgumentException - if the initial capacity is lessthan zero, or if the load factor is nonpositive. |
| SymbolTable | | public SymbolTable(int initialCapacity)(Code) | | 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 lessthan zero. |
| SymbolTable | | public SymbolTable()(Code) | | Constructs a new, empty SymbolTable with a default initial capacity (101)
and load factor, which is 0.75.
|
| addSymbol | | public String addSymbol(String symbol)(Code) | | 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)(Code) | | 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.
Parameters:
offset - The offset into the buffer of the new symbol.
Parameters:
length - The length of the new symbol in the buffer. |
| containsSymbol | | public boolean containsSymbol(String symbol)(Code) | | 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)(Code) | | Returns true if the symbol table already contains the specified
symbol.
Parameters:
buffer - The buffer containing the symbol to look for.
Parameters:
offset - The offset into the buffer.
Parameters:
length - The length of the symbol in the buffer. |
| hash | | public int hash(String symbol)(Code) | | 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)(Code) | | 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.
Parameters:
offset - The offset into the character buffer of the startof the symbol.
Parameters:
length - The length of the symbol. |
| rehash | | protected void rehash()(Code) | | 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.
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