I recently implemented a Hashtable which uses SoftReferences instead
of regular references. I thought this code might be useful to someone
else, since it has helped me be a little more gentle with my memory
footprint. I also though I might slyly benefit if someone notices a
bug that I overlooked. This code is simple, almost trivial, so it can
be public domain, but it is also useful, so especially if you notice a
bug please let me know.
package edu.dartmouth.lex.util;
import java.lang.ref.SoftReference;
import java.util.Enumeration;
import java.util.Hashtable;
import java.util.Vector;
/**
* A Hashtable which stores all its values as soft references. Objects
should be put
* in and gotten out as objects, not as soft references: use it like a
normal Hashtable.
* SoftRefHashtable will return null if you ask for an object which
has been gc'd.
*
* @author Nicholas R. Rinard
* @version 1.0
*/
public class SoftRefHashtable extends Hashtable {
public SoftRefHashtable() {
}
public Object get( Object key ) {
SoftReference sr = (SoftReference) super.get( key );
if( sr == null ) {
return null;
} else {
Object ret = sr.get();
if( ret == null ) {
remove( key );
}
return ret;
}
}
/**
* If the value parameter is null, the put operation is skipped
and null is returned.
*/
public Object put( Object key, Object value ) {
if( value != null ) {
return super.put( key, new SoftReference( value ) );
}
return null; // another option would be to throw an
IllegalArgumentException
}
public int size() {
int ret = 0;
Enumeration e = keys();
while( e.hasMoreElements() ) {
Object o = get( e.nextElement() );
if( o != null )
ret ++;
}
return ret;
}
public Enumeration elements() {
Vector v = new Vector();
Enumeration e = super.elements();
while( e.hasMoreElements() ) {
Object o = ((SoftReference)e.nextElement()).get();
v.addElement( o );
}
return v.elements();
}
/**
* This N*log( N ) method clears the hashtable of keys and
elements for which
* the element has been garbage collected. Returns the number of
keys/values removed.
*/
public int purge() {
int keysRemoved = 0;
Enumeration keys = this.keys();
while( keys.hasMoreElements() ) { // N
Object key = keys.nextElement();
Object value = this.get( key ); // log( N )
if( value == null ) {
this.remove( key );
keysRemoved++;
}
}
return keysRemoved;
}
}
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