/*************************************************************************

"FreePastry" Peer-to-Peer Application Development Substrate 

Copyright 2002, Rice University. All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:

- Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.

- Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.

- Neither  the name  of Rice  University (RICE) nor  the names  of its
contributors may be  used to endorse or promote  products derived from
this software without specific prior written permission.

This software is provided by RICE and the contributors on an "as is"
basis, without any representations or warranties of any kind, express
or implied including, but not limited to, representations or
warranties of non-infringement, merchantability or fitness for a
particular purpose. In no event shall RICE or contributors be liable
for any direct, indirect, incidental, special, exemplary, or
consequential damages (including, but not limited to, procurement of
substitute goods or services; loss of use, data, or profits; or
business interruption) however caused and on any theory of liability,
whether in contract, strict liability, or tort (including negligence
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if advised of the possibility of such damage.

********************************************************************************/

package rice.p2p.util;

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

/**
 * Class which implements a Soft-Reference based HashMap, allowing the garbage
 * collector to collection stuff if memory pressure is tight. Should be
 * transparent to applications, except that items may disappear.
 *
 * @version $Id: pretty.settings 2305 2005-03-11 20:22:33Z jeffh $
 * @author Alan Mislove
 */
public class SoftHashMap extends HashMap {

  /**
   * Returns the object associated with the key. May return null, if the soft
   * reference has been GC'ed.
   *
   * @param key The key
   * @return The value
   */
  public Object get(Object key) {
    SoftReference value = (SoftReference) super.get(key);

    if (value == null) {
      return null;
    } else {
      Object result = value.get();

      if (result != null) {
        return result;
      } else {
        remove(key);
        return null;
      }
    }
  }

  /**
   * Returns whether or not the key is contained in this map. Only returns true
   * if the softreference has not been GC'ed.
   *
   * @param key The key to check for
   * @return The result
   */
  public boolean containsKey(Object key) {
    if (!super.containsKey(key)) {
      return false;
    }

    if (super.get(key) == null) {
      return true;
    } else {
      return (((SoftReference) super.get(key)).get() != null);
    }
  }

  /**
   * Returns whether or not the value is contained in this map. Only returns
   * true if the softreference has not been GC'ed.
   *
   * @param value The value to check for
   * @return The result
   */
  public boolean containsValue(Object value) {
    if (value == null) {
      return super.containsValue(null);
    } else {
      return super.containsValue(new SoftReference(value));
    }
  }

  /**
   * Adds an entry to the soft hash map. May not persist for very long, though.
   *
   * @param key The key
   * @param value The value
   * @return The previous value of the key
   */
  public Object put(Object key, Object value) {
    Object result = get(key);

    if (value != null) {
      super.put(key, new SoftReference(value));
    } else {
      super.put(key, null);
    }

    return result;
  }
}
