AgingHash.java

package dmg.util  ;

import java.util.ArrayList;
import java.util.Hashtable;
import java.util.Iterator;

public class AgingHash {

   private int       _maxSize;
   private Node      _first;
   private Node      _last;
   private Hashtable<Object, Node> _hash    = new Hashtable<>() ;

   public synchronized void clear(){
      _first = null ;
      _last  = null ;
      _hash.clear() ;
   }
   private class Node {

      private Object _value;
      private Object _key;
      private Node   _next;
      private Node   _previous;

      private Node( Object key , Object value ){
         _value = value ;
         _key   = key ;
      }
      public String toString(){
         return "(" + _key + ':' + _value + ')';
      }
      private void link(){
          _next     = _first ;
          _previous = null ;
          if( _first != null ) {
              _first._previous = this;
          }
          _first = this ;

          if( _last == null ) {
              _last = this;
          }

      }
      private void unlink(){
         if( _next != null ) {
             _next._previous = _previous;
         } else {
             _last = _previous;
         }

         if( _previous != null ) {
             _previous._next = _next;
         } else {
             _first = _next;
         }

      }
   }
   public AgingHash( int maxSize ){
      _maxSize = maxSize ;
   }
   public synchronized Object get( Object key ){
       if( key == null ) {
           throw new
                   NullPointerException("Key == null");
       }

       Node node = _hash.get( key );
       if( node == null ) {
           return null;
       }

       node.unlink() ;
       node.link() ;

       return node._value ;
   }
   public synchronized void put( Object key , Object value ){
       if( ( key == null ) || ( value == null ) ) {
           throw new
                   NullPointerException("Key/Value == null");
       }

       Node node = _hash.get( key );
       if( node == null ){

           node = new Node( key , value ) ;
           _hash.put( key , node ) ;

       }else{

           node._value = value ;
           //
           // we have to relink to become top of stack.
           //
           node.unlink() ;

       }
       node.link() ;

       if( _hash.size() > _maxSize ){
          _hash.remove( _last._key ) ;
          _last.unlink() ;

       }
   }
   public int size(){ return _hash.size() ; }
   public synchronized Object remove( Object key ){
      Node node = _hash.remove( key );
      if( node == null ) {
          return null;
      }

      node.unlink() ;

      return node._value ;
   }
   public synchronized Iterator<Node> valuesIterator(){
       return new ArrayList<>(_hash.values()).iterator() ;
   }
   public synchronized Iterator<Object> keysIterator(){
       return new ArrayList<>(_hash.keySet()).iterator() ;
   }
   public synchronized String toString(){
      Node node;
      StringBuilder sb = new StringBuilder() ;
      for( node = _first ; node != null ; node = node._next ){
         sb.append(node).append(';') ;
      }
      sb.append('[').append(_first).append(';').append(_last).append(']');
      return sb.toString() ;
   }
   public static void main( String [] args ){
      AgingHash hash = new AgingHash(3) ;
      System.out.println(hash) ;
      hash.put("1","1");
      System.out.println(hash) ;
      hash.put("2","2");
      System.out.println(hash) ;
      hash.put("3","3");
      System.out.println(hash) ;
      hash.put("4","4");
      System.out.println(hash) ;
      hash.put("5","5");
      System.out.println(hash) ;
      hash.remove("4");
      System.out.println(hash) ;
      hash.remove("3");
      System.out.println(hash) ;
      hash.remove("5");
      System.out.println(hash) ;

   }

}