Are keySet entries of a WeakHashMap never null?

I'm not familiar with WeakHashMap, but you might have one null object. see this example:

public static void main(String[] args)
{
    WeakHashMap<Object, WeakReference<Object>> hm
    = new WeakHashMap<Object, WeakReference<Object>>();
    hm.put(null, null);
    for ( Object item : hm.keySet() ) {
        if ( item == null ) { 
          System.out.println("null object exists");  
        } 
    }
}

Again from WeakHashMap javadoc:

A hashtable-based Map implementation with weak keys. An entry in a WeakHashMap will automatically be removed when its key is no longer in ordinary use. More precisely, the presence of a mapping for a given key will not prevent the key from being discarded by the garbage collector, that is, made finalizable, finalized, and then reclaimed. When a key has been discarded its entry is effectively removed from the map, so this class behaves somewhat differently from other Map implementations.

Which I read as: Yep... When there are no remaining external references to a Key in WeakHaskMap, then that Key maybe GC'd, making the associated Value unreachable, so it to (presuming there are no external references directly to it) is elligible for GC.

I'm going to test this theory. It's only my interpretation of the doco... I don't have any experience with WeakHashMap... but I immediately see it's potential as "memory-safe" object-cache.

Cheers. Keith.

EDIT: Exploring WeakHashMap... specifically testing my theory that an external-references to the particular key would cause that key to be retained... which is pure bunkum ;-)

My test harness:

package forums;

import java.util.Set;
import java.util.Map;
import java.util.WeakHashMap;
import krc.utilz.Random;

public class WeakCache<K,V> extends WeakHashMap<K,V>
{
  private static final int NUM_ITEMS = 2000;
  private static final Random RANDOM = new Random();

  private static void runTest() {
    Map<String, String> cache = new WeakCache<String, String>();
    String key; // Let's retain a reference to the last key object
    for (int i=0; i<NUM_ITEMS; ++i ) {
      /*String*/ key = RANDOM.nextString();
      cache.put(key, RANDOM.nextString());
    }

    System.out.println("There are " + cache.size() + " items of " + NUM_ITEMS + " in the cache before GC.");

    // try holding a reference to the keys
    Set<String> keys = cache.keySet();
    System.out.println("There are " + keys.size() + " keys");

    // a hint that now would be a good time to run the GC. Note that this
    // does NOT guarantee that the Garbage Collector has actually run, or
    // that it's done anything if it did run!
    System.gc();

    System.out.println("There are " + cache.size() + " items of " + NUM_ITEMS + " remaining after GC");
    System.out.println("There are " + keys.size() + " keys");
  }

  public static void main(String[] args) {
    try {
      for (int i=0; i<20; ++i ) {
        runTest();
        System.out.println();
      }
    } catch (Exception e) {
      e.printStackTrace();
    }
  }
}

The (rathering perplexing, I think) results of one test-run:

There are 1912 items of 2000 in the cache before GC.
There are 1378 keys
There are 1378 items of 2000 remaining after GC
There are 909 keys

There are 2000 items of 2000 in the cache before GC.
There are 2000 keys
There are 1961 items of 2000 remaining after GC
There are 1588 keys

There are 2000 items of 2000 in the cache before GC.
There are 2000 keys
There are 1936 items of 2000 remaining after GC
There are 1471 keys

There are 2000 items of 2000 in the cache before GC.
There are 2000 keys
There are 2000 items of 2000 remaining after GC
There are 1669 keys

There are 2000 items of 2000 in the cache before GC.
There are 2000 keys
There are 2000 items of 2000 remaining after GC
There are 1264 keys

There are 2000 items of 2000 in the cache before GC.
There are 2000 keys
There are 2000 items of 2000 remaining after GC
There are 1770 keys

There are 2000 items of 2000 in the cache before GC.
There are 2000 keys
There are 2000 items of 2000 remaining after GC
There are 1679 keys

There are 2000 items of 2000 in the cache before GC.
There are 2000 keys
There are 2000 items of 2000 remaining after GC
There are 1774 keys

There are 2000 items of 2000 in the cache before GC.
There are 2000 keys
There are 2000 items of 2000 remaining after GC
There are 1668 keys

There are 2000 items of 2000 in the cache before GC.
There are 2000 keys
There are 2000 items of 2000 remaining after GC
There are 0 keys

There are 2000 items of 2000 in the cache before GC.
There are 2000 keys
There are 2000 items of 2000 remaining after GC
There are 1834 keys

There are 2000 items of 2000 in the cache before GC.
There are 2000 keys
There are 2000 items of 2000 remaining after GC
There are 0 keys

There are 2000 items of 2000 in the cache before GC.
There are 2000 keys
There are 2000 items of 2000 remaining after GC
There are 0 keys

There are 2000 items of 2000 in the cache before GC.
There are 2000 keys
There are 2000 items of 2000 remaining after GC
There are 0 keys

There are 2000 items of 2000 in the cache before GC.
There are 2000 keys
There are 2000 items of 2000 remaining after GC
There are 0 keys

There are 2000 items of 2000 in the cache before GC.
There are 2000 keys
There are 2000 items of 2000 remaining after GC
There are 0 keys

There are 2000 items of 2000 in the cache before GC.
There are 2000 keys
There are 2000 items of 2000 remaining after GC
There are 0 keys

There are 2000 items of 2000 in the cache before GC.
There are 2000 keys
There are 429 items of 2000 remaining after GC
There are 0 keys

There are 2000 items of 2000 in the cache before GC.
There are 2000 keys
There are 0 items of 2000 remaining after GC
There are 0 keys

There are 2000 items of 2000 in the cache before GC.
There are 2000 keys
There are 0 items of 2000 remaining after GC
There are 0 keys

It would appear that keys are still disappearing WHILE my code is executing... possibly a micro-sleep is required after the GC-hint... to give the GC time to do it's stuff. Anyway, this "volatility" is interesting behaviour.

EDIT 2: Yup, adding the line try{Thread.sleep(10);}catch(Exception e){} directly after the System.gc(); makes the results "more predictable".

There are 1571 items of 2000 in the cache before GC.
There are 1359 keys
There are 0 items of 2000 remaining after GC
There are 0 keys

There are 2000 items of 2000 in the cache before GC.
There are 2000 keys
There are 0 items of 2000 remaining after GC
There are 0 keys

There are 2000 items of 2000 in the cache before GC.
There are 2000 keys
There are 0 items of 2000 remaining after GC
There are 0 keys

There are 2000 items of 2000 in the cache before GC.
There are 2000 keys
There are 0 items of 2000 remaining after GC
There are 0 keys

.... and so on for 20 runs ...

Hmmm... A cache that just completely disappears when the GC kicks in... at arbitrary times in a real app... not much use... Hmmm... What is WeakHashMap for I wonder? ;-)

Last EDIT, I promise

Here's my krc/utilz/Random (used in the above test)

package krc.utilz;

import java.io.Serializable;
import java.nio.charset.Charset;

/**
 * Generates random values. Extends java.util.Random to do all that plus:<ul>
 * <li>generate random values in a given range, and
 * <li>generate Strings of random characters and random length.
 * </ul>
 * <p>
 * Motivation: I wanted to generate random Strings of random length for test 
 *  data in some jUnit tests, and was suprised to find no such ability in the
 *  standard libraries... so I googled it, and came up with Glen McCluskey's
 *  randomstring function at http://www.glenmccl.com/tip_010.htm. Then I thought
 *  aha, that's pretty cool, but if we just extended it a bit, and packaged it
 *  properly then it'd be useful, and reusable. Cool!
 * See: http://www.glenmccl.com/tip_010.htm
 * See: http://forum.java.sun.com/thread.jspa?threadID=5117756&messageID=9406164
 */
public class Random extends java.util.Random  implements Serializable
{

  private static final long serialVersionUID = 34324;
  public static final int DEFAULT_MIN_STRING_LENGTH = 5;
  public static final int DEFAULT_MAX_STRING_LENGTH = 25;

  public Random() {
    super();
  }

  public Random(long seed) {
    super(seed);
  }

  public double nextDouble(double lo, double hi) {
    double n = hi - lo;
    double i = super.nextDouble() % n;
    if (i < 0) i*=-1.0;
    return lo + i;
  }

  /**
   * @returns a random int between lo and hi, inclusive.
   */
  public int nextInt(int lo, int hi) 
    throws IllegalArgumentException
  {
    if(lo >= hi) throw new IllegalArgumentException("lo must be < hi");
    int n = hi - lo + 1;
    int i = super.nextInt() % n;
    if (i < 0) i = -i;
    return lo + i;
  }

  /**
   * @returns a random int between lo and hi (inclusive), but exluding values
   *  between xlo and xhi (inclusive).
   */
  public int nextInt(int lo, int hi, int xlo, int xhi) 
    throws IllegalArgumentException
  {
    if(xlo < lo) throw new IllegalArgumentException("xlo must be >= lo");
    if(xhi > hi) throw new IllegalArgumentException("xhi must be =< hi");
    if(xlo > xhi) throw new IllegalArgumentException("xlo must be >= xhi");
    int i;
    do {
      i = nextInt(lo, hi);
    } while(i>=xlo && i<=xhi);
    return(i);
  }

  /**
   * @returns a string (of between 5 and 25 characters, inclusive) 
   *  consisting of random alpha-characters [a-z]|[A-Z].
   */
  public String nextString()
    throws IllegalArgumentException
  {
    return(nextString(DEFAULT_MIN_STRING_LENGTH, DEFAULT_MAX_STRING_LENGTH));
  }

  /**
   * @returns a String (of between minLen and maxLen chars, inclusive) 
   *  which consists of random alpha-characters. The returned string matches
   *  the regex "[A-Za-z]{$minLen,$maxLan}". 
   * @nb: excludes the chars "[\]^_`" between 'Z' and 'a', ie chars (91..96).
   * @see: http://www.neurophys.wisc.edu/comp/docs/ascii.html
   */
  public String nextString(int minLen, int maxLen)
    throws IllegalArgumentException
  {
    if(minLen < 0) throw new IllegalArgumentException("minLen must be >= 0");
    if(minLen > maxLen) throw new IllegalArgumentException("minLen must be <= maxLen");
    return(nextString(minLen, maxLen, 'A', 'z', '[', '`'));
  }

  /**
   * @does: generates a String (of between minLen and maxLen chars, inclusive) 
   *  which consists of characters between lo and hi, inclusive.
   */
  public String nextString(int minLen, int maxLen, char lo, char hi)
    throws IllegalArgumentException
  {
    if(lo < 0) throw new IllegalArgumentException("lo must be >= 0");
    String retval = null;
    try {
      int n = minLen==maxLen ? maxLen : nextInt(minLen, maxLen);
      byte b[] = new byte[n];
      for (int i=0; i<n; i++)
        b[i] = (byte)nextInt((int)lo, (int)hi);
      retval = new String(b, Charset.defaultCharset().name());
    } catch (Exception e) {
      e.printStackTrace();
    }
    return retval;
  }

  /**
   * @does: generates a String (of between minLen and maxLen chars, inclusive) 
   *  which consists of characters between lo and hi, inclusive, but excluding
   *  character between 
   */
  public String nextString(int minLen, int maxLen, char lo, char hi, char xlo, char xhi) 
    throws IllegalArgumentException
  {
    if(lo < 0) throw new IllegalArgumentException("lo must be >= 0");
    String retval = null;
    try {
      int n = minLen==maxLen ? maxLen : nextInt(minLen, maxLen);
      byte b[] = new byte[n];
      for (int i=0; i<n; i++) {
        b[i] = (byte)nextInt((int)lo, (int)hi, (int)xlo, (int)xhi);
      }
      retval = new String(b, Charset.defaultCharset().name());
    } catch (Exception e) {
      e.printStackTrace();
    }
    return retval;
  }

}

Are keySet entries of a WeakHashMap never null?

Tags:

Java

Null

Iteration

Weakhashmap

Keyset

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