softhashmap.java

pastry的java实现的2.0b版

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"FreePastry" Peer-to-Peer Application Development Substrate 

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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;
  }
}