org.apache.xml.dtm.ref
Class DTMStringPool

java.lang.Object
  org.apache.xml.dtm.ref.DTMStringPool

Direct Known Subclasses:

CustomStringPool, DTMSafeStringPool

public class DTMStringPool

extends Object

DTMStringPool is an "interning" mechanism for strings. It will create a stable 1:1 mapping between a set of string values and a set of integer index values, so the integers can be used to reliably and uniquely identify (and when necessary retrieve) the strings.

Design Priorities:

• String-to-index lookup speed is critical.
• Index-to-String lookup speed is slightly less so.
• Threadsafety is not guaranteed at this level. Enforce that in the application if needed.
• Storage efficiency is an issue but not a huge one. It is expected that string pools won't exceed about 2000 entries.

Implementation detail: A standard Hashtable is relatively inefficient when looking up primitive int values, especially when we're already maintaining an int-to-string vector. So I'm maintaining a simple hash chain within this class.

NOTE: There is nothing in the code that has a real dependency upon String. It would work with any object type that implements reliable .hashCode() and .equals() operations. The API enforces Strings because it's safer that way, but this could trivially be turned into a general ObjectPool if one was needed.

Status: Passed basic test in main().

Field Summary

(package private) static int	HASHPRIME
(package private) IntVector	m_hashChain
(package private) int[]	m_hashStart
(package private) Vector	m_intToString
static int	NULL

Constructor Summary

DTMStringPool()

Method Summary

String	indexToString(int i)
static void	main(String[] args) Command-line unit test driver.
void	removeAllElements()
int	stringToIndex(String s)

Methods inherited from class java.lang.Object

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

Field Detail

m_intToString

Vector m_intToString

HASHPRIME

static final int HASHPRIME

See Also:

Constant Field Values

m_hashStart

int[] m_hashStart

m_hashChain

IntVector m_hashChain

NULL

public static final int NULL

See Also:

Constant Field Values

Constructor Detail

DTMStringPool

public DTMStringPool()

Method Detail

removeAllElements

public void removeAllElements()

indexToString

public String indexToString(int i)
                     throws ArrayIndexOutOfBoundsException

Returns:

string whose value is uniquely identified by this integer index.

Throws:

ArrayIndexOutOfBoundsException - if index doesn't map to a string.

stringToIndex

public int stringToIndex(String s)

Returns:

integer index uniquely identifying the value of this string.

main

public static void main(String[] args)

Command-line unit test driver. This test relies on the fact that this version of the pool assigns indices consecutively, starting from zero, as new unique strings are encountered.