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Thinking in Java
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The WeakHashMap

The containers library has a special Map to hold weak references: the WeakHashMap. This class is designed to make the creation of canonicalized mappings easier. In such a mapping, you are saving storage by making only one instance of a particular value. When the program needs that value, it looks up the existing object in the mapping and uses that (rather than creating one from scratch). The mapping may make the values as part of its initialization, but it’s more likely that the values are made on demand.

Since this is a storage-saving technique, it’s very convenient that the WeakHashMap allows the garbage collector to automatically clean up the keys and values. You don’t have to do anything special to the keys and values you want to place in the WeakHashMap; these are automatically wrapped in WeakReferences by the map. The trigger to allow cleanup is if the key is no longer in use, as demonstrated here:

//: c11:CanonicalMapping.java
// Demonstrates WeakHashMap.
import java.util.*;
import java.lang.ref.*;

class Key {
  private String ident;
  public Key(String id) { ident = id; }
  public String toString() { return ident; }
  public int hashCode() { return ident.hashCode(); }
  public boolean equals(Object r) {
    return (r instanceof Key)
      && ident.equals(((Key)r).ident);
  }
  public void finalize() {
    System.out.println("Finalizing Key "+ ident);
  }
}

class Value {
  private String ident;
  public Value(String id) { ident = id; }
  public String toString() { return ident; }
  public void finalize() {
    System.out.println("Finalizing Value " + ident);
  }
}

public class CanonicalMapping {
  public static void main(String[] args) {
    int size = 1000;
    // Or, choose size via the command line:
    if(args.length > 0)
      size = Integer.parseInt(args[0]);
    Key[] keys = new Key[size];
    WeakHashMap map = new WeakHashMap();
    for(int i = 0; i < size; i++) {
      Key k = new Key(Integer.toString(i));
      Value v = new Value(Integer.toString(i));
      if(i % 3 == 0)
        keys[i] = k; // Save as "real" references
      map.put(k, v);
    }
    System.gc();
  }
} ///:~


The Key class must have a hashCode( ) and an equals( ) since it is being used as a key in a hashed data structure, as described previously in this chapter.

When you run the program, you’ll see that the garbage collector will skip every third key, because an ordinary reference to that key has also been placed in the keys array, and thus those objects cannot be garbage collected.
Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire