OnJava8-Examples/collectiontopics/CanonicalMapping.java

// collectiontopics/CanonicalMapping.java
// (c)2020 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
// Demonstrates WeakHashMap
import java.util.*;

class Element {
  private String ident;
  Element(String id) { ident = id; }
  @Override
  public String toString() { return ident; }
  @Override
  public int hashCode() {
    return Objects.hashCode(ident);
  }
  @Override
  public boolean equals(Object r) {
    return r instanceof Element &&
      Objects.equals(ident, ((Element)r).ident);
  }
  @SuppressWarnings("deprecation")
  @Override
  protected void finalize() {
    System.out.println("Finalizing " +
      getClass().getSimpleName() + " " + ident);
  }
}

class Key extends Element {
  Key(String id) { super(id); }
}

class Value extends Element {
  Value(String id) { super(id); }
}

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.valueOf(args[0]);
    Key[] keys = new Key[size];
    WeakHashMap<Key,Value> 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();
  }
}