001/* 
002    Licensed to the Apache Software Foundation (ASF) under one
003    or more contributor license agreements.  See the NOTICE file
004    distributed with this work for additional information
005    regarding copyright ownership.  The ASF licenses this file
006    to you under the Apache License, Version 2.0 (the
007    "License"); you may not use this file except in compliance
008    with the License.  You may obtain a copy of the License at
009
010       http://www.apache.org/licenses/LICENSE-2.0
011
012    Unless required by applicable law or agreed to in writing,
013    software distributed under the License is distributed on an
014    "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
015    KIND, either express or implied.  See the License for the
016    specific language governing permissions and limitations
017    under the License.  
018 */
019package org.apache.wiki.util;
020
021import java.util.AbstractList;
022import java.util.concurrent.CopyOnWriteArrayList;
023
024/**
025 *  Builds a simple, priority-based List implementation.  The list
026 *  will be sorted according to the priority.  If two items are
027 *  inserted with the same priority, their order is the insertion order - i.e. the new one
028 *  is appended last in the insertion list.
029 *  <p>
030 *  Priority is an integer, and the list is sorted in descending order
031 *  (that is, 100 is before 10 is before 0 is before -40).
032 */
033public class PriorityList<E>
034    extends AbstractList<E>
035{
036    private final CopyOnWriteArrayList<Item<E>> m_elements = new CopyOnWriteArrayList<>();
037
038    /**
039     *  This is the default priority, which is used if no priority
040     *  is defined.  It's current value is zero.
041     */
042    public static final int DEFAULT_PRIORITY = 0;
043
044    /**
045     *  Adds an object to its correct place in the list, using the
046     *  given priority.
047     *
048     *  @param o Object to add.
049     *  @param priority Priority.
050     */
051    public void add(final E o, final int priority )
052    {
053        int i = 0;
054
055        for( ; i < m_elements.size(); i++ )
056        {
057            final Item< E > item = m_elements.get(i);
058
059            if( item.m_priority < priority )
060            {
061                break;
062            }
063        }
064
065        final Item<E> newItem = new Item<>();
066        newItem.m_priority = priority;
067        newItem.m_object   = o;
068
069        m_elements.add( i, newItem );
070    }
071
072    /**
073     *  Adds an object using the default priority to the List.
074     *
075     *  @param o Object to add.
076     *  @return true, as per the general Collections.add contract.
077     */
078    @Override
079    public boolean add(final E o )
080    {
081        add( o, DEFAULT_PRIORITY );
082
083        return true;
084    }
085
086    /**
087     *  Returns the object at index "index".
088     *
089     *  @param index The index.
090     *  @return The object at the list at the position "index".
091     */
092    @Override
093    public E get(final int index )
094    {
095        return m_elements.get( index ).m_object;
096    }
097
098    /**
099     *  Returns the current size of the list.
100     *  
101     *  @return size of the list.
102     */
103    @Override
104    public int size()
105    {
106        return m_elements.size();
107    }
108
109    /**
110     *  Provides a holder for the priority-object 2-tuple.
111     */
112    private static class Item<E>
113    {
114        public int     m_priority;
115        public E       m_object;
116    }
117}