The SoftHashMap Itself

Without further ado, here comes the SoftHashMap:

  
  //: SoftHashMap.java
  import java.util.*;
  import java.lang.ref.*;

  public class SoftHashMap extends AbstractMap {
    /** The internal HashMap that will hold the SoftReference. */
    private final Map hash = new HashMap();
    /** The number of "hard" references to hold internally. */
    private final int HARD_SIZE;
    /** The FIFO list of hard references, order of last access. */
    private final LinkedList hardCache = new LinkedList();
    /** Reference queue for cleared SoftReference objects. */
    private final ReferenceQueue queue = new ReferenceQueue();

    public SoftHashMap() { this(100); }
    public SoftHashMap(int hardSize) { HARD_SIZE = hardSize; }

    public Object get(Object key) {
      Object result = null;
      // We get the SoftReference represented by that key
      SoftReference soft_ref = (SoftReference)hash.get(key);
      if (soft_ref != null) {
	// From the SoftReference we get the value, which can be
	// null if it was not in the map, or it was removed in
	// the processQueue() method defined below
	result = soft_ref.get();
	if (result == null) {
	  // If the value has been garbage collected, remove the
	  // entry from the HashMap.
	  hash.remove(key);
	} else {
	  // We now add this object to the beginning of the hard
	  // reference queue.  One reference can occur more than
	  // once, because lookups of the FIFO queue are slow, so
	  // we don't want to search through it each time to remove
	  // duplicates.
	  hardCache.addFirst(result);
	  if (hardCache.size() > HARD_SIZE) {
	    // Remove the last entry if list longer than HARD_SIZE
	    hardCache.removeLast();
	  }
	}
      }
      return result;
    }

    /** We define our own subclass of SoftReference which contains
     not only the value but also the key to make it easier to find
     the entry in the HashMap after it's been garbage collected. */
    private static class SoftValue extends SoftReference {
      private final Object key; // always make data member final
      /** Did you know that an outer class can access private data
       members and methods of an inner class?  I didn't know that!
       I thought it was only the inner class who could access the
       outer class's private information.  An outer class can also
       access private members of an inner class inside its inner
       class. */
      private SoftValue(Object k, Object key, ReferenceQueue q) {
	super(k, q);
	this.key = key;
      }
    }

    /** Here we go through the ReferenceQueue and remove garbage
     collected SoftValue objects from the HashMap by looking them
     up using the SoftValue.key data member. */
    private void processQueue() {
      SoftValue sv;
      while ((sv = (SoftValue)queue.poll()) != null) {
	hash.remove(sv.key); // we can access private data!
      }
    }
    /** Here we put the key, value pair into the HashMap using
     a SoftValue object. */
    public Object put(Object key, Object value) {
      processQueue(); // throw out garbage collected values first
      return hash.put(key, new SoftValue(value, key, queue));
    }
    public Object remove(Object key) {
      processQueue(); // throw out garbage collected values first
      return hash.remove(key);
    }
    public void clear() {
      hardCache.clear();
      processQueue(); // throw out garbage collected values
      hash.clear();
    }
    public int size() {
      processQueue(); // throw out garbage collected values first
      return hash.size();
    }
    public Set entrySet() {
      // no, no, you may NOT do that!!! GRRR
      throw new UnsupportedOperationException();
    }
  }



	

 

 Introduction 

 Test Code   





         

                       
		
In this Article 
		
		 

              

                	Introduction  

                	Test Code  

            
			
            

              

                	The SoftHashMap Itself