# WeakList.java

    /*
    ** Module   : WeakList.java
    ** Abstract : List of weak references that cleans up on iteration
    **
    ** Copyright (c) 2006-2020, Golden Code Development Corporation.
    **
    ** -#- -I- --Date-- --JPRM-- ------------------Description-------------------
    ** 001 ECF 20061102   @30965 Created initial version. A list which holds weak
    **                           references to its elements.
    ** 002 ECF 20061120   @31321 Added isEmpty() method.
    ** 003 ECF 20090521   @42988 Integrated generics.
    ** 004 SVL 20150821          Added remove() method.
    ** 005 VVT 20200203          WeakList now implements Iterable
    */
    /*
    ** This program is free software: you can redistribute it and/or modify
    ** it under the terms of the GNU Affero General Public License as
    ** published by the Free Software Foundation, either version 3 of the
    ** License, or (at your option) any later version.
    **
    ** This program is distributed in the hope that it will be useful,
    ** but WITHOUT ANY WARRANTY; without even the implied warranty of
    ** MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    ** GNU Affero General Public License for more details.
    **
    ** You may find a copy of the GNU Affero GPL version 3 at the following
    ** location: https://www.gnu.org/licenses/agpl-3.0.en.html
    **
    ** Additional terms under GNU Affero GPL version 3 section 7:
    **
    **   Under Section 7 of the GNU Affero GPL version 3, the following additional
    **   terms apply to the works covered under the License.  These additional terms
    **   are non-permissive additional terms allowed under Section 7 of the GNU
    **   Affero GPL version 3 and may not be removed by you.
    **
    **   0. Attribution Requirement.
    **
    **     You must preserve all legal notices or author attributions in the covered
    **     work or Appropriate Legal Notices displayed by works containing the covered
    **     work.  You may not remove from the covered work any author or developer
    **     credit already included within the covered work.
    **
    **   1. No License To Use Trademarks.
    **
    **     This license does not grant any license or rights to use the trademarks
    **     Golden Code, FWD, any Golden Code or FWD logo, or any other trademarks
    **     of Golden Code Development Corporation. You are not authorized to use the
    **     name Golden Code, FWD, or the names of any author or contributor, for
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    **
    **     You may not represent the covered work as solely your work.  All modified
    **     versions of the covered work must be marked in a reasonable way to make it
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    **     attribution required in this license.
    */

    package com.goldencode.p2j.util;

    import java.lang.ref.*;
    import java.util.*;

    /**
     * A wrapper for a linked list of weak references.  This implementation allows
     * entries to be garbage collected if they are no longer strongly referenced
     * by any other objects.  Each entry added is stored using a weak reference.
     * Upon iteration, references to objects which have been garbage collected are
     * removed from the list.
     * <p>
     * This is a very simple list implementation which does not implement the
     * standard Java collection <code>List</code> interface.  Currently, it has
     * only methods to add an entry to the end of the list, to iterate over all
     * elements from first to last, to indicate whether the list is empty and to
     * remove the specific element.
     *
     * @param   <E>
     *          Element being wrapped by the list.
     */
    public final class WeakList<E>
    implements Iterable<E>
    {
       /** The internal list being wrapped */
       private final List<Reference<E>> list;

       /**
        * Default constructor.
        */
       public WeakList()
       {
          this.list = new LinkedList<Reference<E>>();
       }

       /**
        * Add an entry to the end of the list.  It will be connected to the list
        * by a weak reference.
        *
        * @param   o
        *          Object to be added via weak reference.
        *
        * @return  <code>true</code> if the list was changed as a result of the
        *          call (always will return <code>true</code>).
        */
       public boolean add(E o)
       {
          return list.add(new WeakReference<E>(o));
       }

       /**
        * Indicate whether the list is empty.
        *
        * @return  <code>true</code> if the list contains no elements, else
        *          <code>false</code>.
        */
       public boolean isEmpty()
       {
          return !iterator().hasNext();
       }

       /**
        * Remove an entry from the list.
        *
        * @param   o
        *          Object to be removed.
        *
        * @return  <code>true</code> if the list was changed as a result of the call.
        */
       public boolean remove(E o)
       {
          boolean res = false;
          Iterator iter = iterator();
          while (iter.hasNext())
          {
             Object element = iter.next();
             if (element.equals(o))
             {
                iter.remove();
                res = true;
             }
          }
          return res;
       }

       /**
        * Return an iterator on the elements in the list, starting from the first
        * and proceeding in order to the last.  If any entry has been garbage
        * collected, such that the weak reference no longer holds an object, that
        * entry is removed from the list.
        *
        * @return  Iterator on the objects in the list.
        */
       public Iterator<E> iterator()
       {
          return new Iterator<E>()
          {
             private Iterator<Reference<E>> iter = list.iterator();

             private E next = null;

             public boolean hasNext()
             {
                while (iter.hasNext())
                {
                   Reference<E> ref = iter.next();
                   next = ref.get();
                   if (next != null)
                   {
                      return true;
                   }

                   // If referent was garbage collected, remove the reference
                   // from the list.
                   iter.remove();
                }

                return false;
             }

             public E next()
             {
                if (next == null)
                {
                   throw new NoSuchElementException();
                }

                return next;
             }

             public void remove()
             {
                iter.remove();
             }
          };
       }
    }