DTMStringPoolpublic 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(). |
Fields Summary |
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Vector | m_intToString | static final int | HASHPRIME | int[] | m_hashStart | IntVector | m_hashChain | public static final int | NULL |
Constructors Summary |
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public DTMStringPool(int chainSize)Create a DTMStringPool using the given chain size
m_intToString=new Vector();
m_hashChain=new IntVector(chainSize);
removeAllElements();
// -sb Add this to force empty strings to be index 0.
stringToIndex("");
| public DTMStringPool()
this(512);
|
Methods Summary |
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public static void | _main(java.lang.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.
String[] word={
"Zero","One","Two","Three","Four","Five",
"Six","Seven","Eight","Nine","Ten",
"Eleven","Twelve","Thirteen","Fourteen","Fifteen",
"Sixteen","Seventeen","Eighteen","Nineteen","Twenty",
"Twenty-One","Twenty-Two","Twenty-Three","Twenty-Four",
"Twenty-Five","Twenty-Six","Twenty-Seven","Twenty-Eight",
"Twenty-Nine","Thirty","Thirty-One","Thirty-Two",
"Thirty-Three","Thirty-Four","Thirty-Five","Thirty-Six",
"Thirty-Seven","Thirty-Eight","Thirty-Nine"};
DTMStringPool pool=new DTMStringPool();
System.out.println("If no complaints are printed below, we passed initial test.");
for(int pass=0;pass<=1;++pass)
{
int i;
for(i=0;i<word.length;++i)
{
int j=pool.stringToIndex(word[i]);
if(j!=i)
System.out.println("\tMismatch populating pool: assigned "+
j+" for create "+i);
}
for(i=0;i<word.length;++i)
{
int j=pool.stringToIndex(word[i]);
if(j!=i)
System.out.println("\tMismatch in stringToIndex: returned "+
j+" for lookup "+i);
}
for(i=0;i<word.length;++i)
{
String w=pool.indexToString(i);
if(!word[i].equals(w))
System.out.println("\tMismatch in indexToString: returned"+
w+" for lookup "+i);
}
pool.removeAllElements();
System.out.println("\nPass "+pass+" complete\n");
} // end pass loop
| public java.lang.String | indexToString(int i)
if(i==NULL) return null;
return (String) m_intToString.elementAt(i);
| public void | removeAllElements()
m_intToString.removeAllElements();
for(int i=0;i<HASHPRIME;++i)
m_hashStart[i]=NULL;
m_hashChain.removeAllElements();
| public int | stringToIndex(java.lang.String s)
if(s==null) return NULL;
int hashslot=s.hashCode()%HASHPRIME;
if(hashslot<0) hashslot=-hashslot;
// Is it one we already know?
int hashlast=m_hashStart[hashslot];
int hashcandidate=hashlast;
while(hashcandidate!=NULL)
{
if(m_intToString.elementAt(hashcandidate).equals(s))
return hashcandidate;
hashlast=hashcandidate;
hashcandidate=m_hashChain.elementAt(hashcandidate);
}
// New value. Add to tables.
int newIndex=m_intToString.size();
m_intToString.addElement(s);
m_hashChain.addElement(NULL); // Initialize to no-following-same-hash
if(hashlast==NULL) // First for this hash
m_hashStart[hashslot]=newIndex;
else // Link from previous with same hash
m_hashChain.setElementAt(newIndex,hashlast);
return newIndex;
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