On Tue, 07 Jun 2005 09:13:10 +0200, Bernhard Messer wrote:
> exactly, that's what i had in mind. I know that we have to allocate a
> new string object for every call, but this must be much cheaper than the
> current implementation which has to walk thru the whole array every time
> the method is called.
If you're still concerned about performance you can use a "trie" data
structure, which takes a little effort to build but can match very quickly
(with no allocations). I've attached an example implementation that works
for filename extensions which only use the letters 'a'-'z' (minimally
tested):
private static final ExtensionMatcher MATCHER =
new ExtensionMatcher(new String[]{
"cfs", "fnm", "fdx", "fdt", "tii",
"tis", "frq", "prx", "del", "tvx",
"tvd", "tvf", "tvp",
});
MATCHER.match("foo.cfs"); // returns true
BTW, I think it is a bad idea to have FILENAME_EXTENSIONS as a public
array. It being final does not prevent code from changing the contents of
the array. If the extensions must be public I would recommend either an
accessor that returns a copy of the array, or an unmodifiable
collection/set/list.
Chris
////////////////////////////////////////////////////////////////////////
import java.util.*;
public class ExtensionMatcher
{
private Object[] tree;
public ExtensionMatcher(String[] exts)
{
tree = create(Arrays.asList(exts), 0);
}
private static Object[] create(List exts, int index)
{
Object[] array = new Object[27];
Map byLetter = new HashMap();
for (Iterator it = exts.iterator(); it.hasNext();) {
String ext = (String)it.next();
int length = ext.length();
if (length > index) {
Character c = new Character(ext.charAt(length - 1 - index));
List list = (List)byLetter.get(c);
if (list == null)
byLetter.put(c, list = new ArrayList());
list.add(ext);
} else if (length == index) {
array[26] = Boolean.TRUE;
}
}
for (Iterator it = byLetter.keySet().iterator(); it.hasNext();) {
Character c = (Character)it.next();
char val = c.charValue();
if (val < 'a' || val > 'z')
throw new IllegalArgumentException("Extension must be between
'a' and 'z'");
array[val - 'a'] = create((List)byLetter.get(c), index + 1);
}
return array;
}
public boolean match(String file)
{
int index = 0;
int length = file.length();
Object[] array = tree;
for (;;) {
if (index >= length)
return false;
char val = file.charAt(length - 1 - index);
if (val == '.' && array[26] != null)
return true;
if (val < 'a' || val > 'z')
return false;
array = (Object[])array[val - 'a'];
if (array == null)
return false;
index++;
}
}
}
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