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Use Google App Engine datastore and Objectify classes with GWT
There is a newer version: 1.2.1
package com.googlecode.objectify;

import java.io.Serializable;

import com.googlecode.objectify.Key;
import com.googlecode.objectify.Ref;

/**
 * We need to provide an alternate, stripped-down version of this so that we
 * can exclude the constructors that tie into non-GWT-safe code (the factories).
 *
 * @author Jeff Schnitzer 
 */
public class Key implements Serializable, Comparable>
{
	private static final long serialVersionUID = 2L;

	/** Key.create(key) is easier to type than new Key(key) */
	public static  Key create(com.google.appengine.api.datastore.Key raw) {
		if (raw == null)
			throw new NullPointerException("Cannot create a Key from a null datastore Key");

		return new Key(raw);
	}

	/** */
	protected com.google.appengine.api.datastore.Key raw;

	/** Cache the instance of the parent wrapper to avoid unnecessary garbage */
	transient protected Key parent;

	/** For GWT serialization */
	protected Key() {}

	/** Wrap a raw Key */
	private Key(com.google.appengine.api.datastore.Key raw) {
		this.raw = raw;
	}

	/**
	 * @return the raw datastore version of this key
	 */
	public com.google.appengine.api.datastore.Key getRaw() {
		return this.raw;
	}

	/**
	 * @return the id associated with this key, or 0 if this key has a name.
	 */
	public long getId() {
		return this.raw.getId();
	}

	/**
	 * @return the name associated with this key, or null if this key has an id
	 */
	public String getName() {
		return this.raw.getName();
	}

	/**
	 * @return the low-level datastore kind associated with this Key
	 */
	public String getKind() {
		return this.raw.getKind();
	}

	/**
	 * @return the parent key, or null if there is no parent.  Note that
	 *  the parent could potentially have any type.
	 */
	@SuppressWarnings("unchecked")
	public  Key getParent() {
		if (this.parent == null && this.raw.getParent() != null)
			this.parent = new Key(this.raw.getParent());

		return (Key)this.parent;
	}

	/**
	 * Gets the root of a parent graph of keys.  If a Key has no parent, it is the root.
	 *
	 * @return the topmost parent key, or this object itself if it is the root.
	 * Note that the root key could potentially have any type.
	 */
	@SuppressWarnings("unchecked")
	public  Key getRoot() {
		if (this.getParent() == null)
			return (Key)this;
		else
			return this.getParent().getRoot();
	}

	/** A type-safe equivalence comparison */
	public boolean equivalent(Key other) {
		return equals(other);
	}

	/** A type-safe equivalence comparison */
	public boolean equivalent(Ref other) {
		return (other == null) ? false : equals(other.key());
	}

	/**
	 * Compares based on comparison of the raw key
	 */
	@Override
	public int compareTo(Key other) {
		return this.raw.compareTo(other.raw);
	}

	/** */
	@Override
	public boolean equals(Object obj) {
		if (obj == null)
			return false;

		if (!(obj instanceof Key))
			return false;

		return this.compareTo((Key)obj) == 0;
	}

	/** */
	@Override
	public int hashCode() {
		return this.raw.hashCode();
	}

	/** Creates a human-readable version of this key */
	@Override
	public String toString() {
		return "Key(" + this.raw + ")";
	}

	/**
	 * Easy null-safe conversion of the raw key.
	 */
	public static  Key key(com.google.appengine.api.datastore.Key raw) {
		if (raw == null)
			return null;
		else
			return new Key(raw);
	}

	/**
	 * Easy null-safe conversion of the typed key.
	 */
	public static com.google.appengine.api.datastore.Key raw(Key typed) {
		if (typed == null)
			return null;
		else
			return typed.getRaw();
	}
}