Xerces2 Implementation: Class SymbolTable

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## org.apache.xerces.util
Class SymbolTable

    java.lang.Object
      |
      +--org.apache.xerces.util.SymbolTable

Direct Known Subclasses:
  ShadowedSymbolTable, SoftReferenceSymbolTable, SynchronizedSymbolTable

public class SymbolTable
extends java.lang.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 496157 2007-01-14 21:24:28Z mrglavas $
Author:
  Andy Clark, John Kim, IBM
See Also:
  `SymbolHash`

Inner Class Summary
`protected static class` | `SymbolTable.Entry` This class is a symbol table entry.

Field Summary
`protected SymbolTable.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.
`protected static int` | `TABLE_SIZE` Default table size.

Constructor Summary
`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
`java.lang.String` | `addSymbol(char[] buffer, int offset, int length)` Adds the specified symbol to the symbol table and returns a reference to the unique symbol.
`java.lang.String` | `addSymbol(java.lang.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(java.lang.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(java.lang.String symbol)` Returns a hashcode value for the specified symbol.
`protected void` | `rehash()` Increases the capacity of and internally reorganizes this SymbolTable, in order to accommodate and access its entries more efficiently.

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Field Detail

### TABLE_SIZE

    protected static final int TABLE_SIZE

  Default table size.

### fBuckets

    protected SymbolTable.Entry[] fBuckets

  Buckets.

### fTableSize

    protected int fTableSize

  actual table size

### fCount

    protected transient int fCount

  The total number of entries in the hash table.

### fThreshold

    protected int fThreshold

  The table is rehashed when its size exceeds this threshold. (The value of this field is (int)(capacity * loadFactor).)

### fLoadFactor

    protected float fLoadFactor

  The load factor for the SymbolTable.

Constructor Detail

### 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:
    `java.lang.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:
    `java.lang.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 Detail

### addSymbol

    public java.lang.String addSymbol(java.lang.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 java.lang.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(java.lang.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.

### 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.

### containsSymbol

    public boolean containsSymbol(java.lang.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.

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