Cache Map : TreeMap « Collections « Java Tutorial

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/*******************************************************************************
 * Copyright (c) 2007, 2008 IBM Corporation and Others
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 *    Hisashi MIYASHITA - initial API and implementation
 *    Kentarou FUKUDA - initial API and implementation
 *******************************************************************************/


import java.util.Iterator;
import java.util.LinkedList;
import java.util.NoSuchElementException;
import java.util.SortedMap;
import java.util.TreeMap;

/**
 * Utility class for cache map.
 */
public class CacheMap extends TreeMap<String, Object> {
  private static final long serialVersionUID = 6681131647931821052L;

  private final int maxSize;
  private final int evictSize;

  private final LinkedList<Object> accessList = new LinkedList<Object>();

  /**
   * Constructor of cache map. If the map exceed the maximum size, the
   * key/value sets will be removed from map based on specified evict size.
   * 
   * @param maxSize
   *            maximum size of the map
   * @param evictSize
   *            number of evict object
   */
  public CacheMap(int maxSize, int evictSize) {
    this.maxSize = maxSize;
    this.evictSize = evictSize;
  }

  private void evict() {
    Iterator<Object> it = accessList.iterator();
    for (int i = 0; i < evictSize; i++) {
      if (!it.hasNext())
        return;
      Object key = it.next();
      this.remove(key);
      it.remove();
    }
  }

  private int searchAccessList(Object key) {
    return accessList.indexOf(key);
  }

  private void accessEntry(Object key) {
    int idx = searchAccessList(key);
    if (idx >= 0) {
      accessList.remove(idx);
    }
    accessList.add(key);
  }

  public Object put(String key, Object val) {
    if (size() >= maxSize)
      evict();
    accessEntry(key);
    return super.put(key, val);
  }

  public Object get(Object key) {
    accessEntry(key);
    return super.get(key);
  }

  /**
   * Search a key that starts with the specified prefix from the map, and
   * return the value corresponding to the key.
   * 
   * @param prefix
   *            target prefix
   * @return the value whose key starts with prefix, or null if not available
   */
  public Object matchStartsWith(String prefix) {
    SortedMap<String, Object> smap = super.tailMap(prefix);
    Object okey;
    try {
      okey = smap.firstKey();
    } catch (NoSuchElementException e) {
      return null;
    }
    if (!(okey instanceof String))
      return null;
    String key = (String) okey;
    // System.err.println("MSW:" + key + " / " + prefix);
    if (!key.startsWith(prefix))
      return null;
    return super.get(key);
  }
}