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Utility libraries for Java. Complements Guava, Apache Commons, etc.

There is a newer version: 1.10.0

package org.plumelib.util;

import java.util.concurrent.atomic.AtomicLong;

/**
 * Provides a unique ID for classes that you cannot modify. The unique ID is useful because it makes
 * output deterministic. For classes that you can edit, implement the {@link UniqueId} interface.
 *
 * The IDs count up from 0, per instance of UniqueIdMap. When you look up an object in this map,
 * it is given a unique ID if it didn't already have one.
 *
 * 
Typical use:
 *
 * 

 *   Define a field:
 *       
 * /** Unique ids for trees. */
 * static UniqueIdMap<Tree> treeUids = new UniqueIdMap<>();
 * 
 *   
Wherever you would call {@code x.hashCode()}, instead call {@code treeUids.get(x)}.
 * 
 *
 * @param  the type of elements that get a unique ID
 */
public class UniqueIdMap {

  /** Create a new UniqueIdMap. */
  public UniqueIdMap() {}

  /** The unique ID for the next-created object. */
  private final AtomicLong nextUid = new AtomicLong(0);

  /** A mapping from objects to their IDs. */
  private WeakIdentityHashMap map = new WeakIdentityHashMap<>();

  /**
   * Get the unique ID for the given object. If the object's ID has not been previously requested, a
   * new one is generated for it.
   *
   * @param object the object to get a unique ID for
   * @return the unique ID for the given object
   */
  public long get(E object) {
    Long id = map.get(object);
    if (id != null) {
      return id;
    }
    long newId = nextUid.getAndIncrement();
    map.put(object, newId);
    return newId;
  }
}