Most Recently Used Map : Customized Map « Collections Data Structure « Java

1. Java
2. Collections Data Structure
3. Customized Map

Most Recently Used Map

    
/*
 * $Id: MostRecentlyUsedMap.java 457812 2005-10-06 21:20:34Z jdonnerstag $
 * $Revision: 457812 $ $Date: 2005-10-06 23:20:34 +0200 (Thu, 06 Oct 2005) $
 * 
 * 
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not
 * use this file except in compliance with the License. You may obtain a copy of
 * the License at
 * 
 * http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 */

import java.util.LinkedHashMap;
import java.util.Map;

/**
 * Holds a map of most recently used items of a given maximum size. Old entries
 * are expired when the map exceeds that maximum size.
 * 
 * @author Jonathan Locke
 */
public class MostRecentlyUsedMap extends LinkedHashMap
{
  private static final long serialVersionUID = 1L;

  /** Value most recently removed from map */
  Object removedValue;
  
  /** Maximum number of entries allowed in this map */
  private int maxEntries;

  /**
   * Constructor
   *  
   * @param maxEntries
   *            Maximum number of entries allowed in the map
   */
  public MostRecentlyUsedMap(final int maxEntries)
  {
    super(10, 0.75f, true);

    if (maxEntries <= 0)
    {
      throw new IllegalArgumentException("Must have at least one entry");
    }
    
    this.maxEntries = maxEntries;
  }
  
  /**
   * @return Returns the removedValue.
   */
  public Object getRemovedValue()
  {
    return removedValue;
  }
  
  /**
   * @see java.util.LinkedHashMap#removeEldestEntry(java.util.Map.Entry)
   */
  protected boolean removeEldestEntry(final Map.Entry eldest)
  {
    final boolean remove = size() > maxEntries;
    // when it should be removed remember the oldest value that will be removed
    if (remove)
    {
      this.removedValue = eldest.getValue();
    }
    else
    {
      removedValue = null;
    }
    return remove;
  }
}

   
    
    
    
  

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