# EntityReference.java

    /*
     * Copyright (c) 2007, Rickard Öberg. All Rights Reserved.
     *
     * Licensed under the Apache License, Version 2.0 (the "License");
     * you may not use this file except in compliance with the License.
     * You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
     * Unless required by applicable law or agreed to in writing, software
     * distributed under the License is distributed on an "AS IS" BASIS,
     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     * See the License for the specific language governing permissions and
     * limitations under the License.
     *
     */

    package org.qi4j.api.entity;

    import java.io.Serializable;
    import org.qi4j.api.util.NullArgumentException;

    /**
     * An EntityReference is identity of a specific Entity instance.
     * <p>When stringified, the identity is used as-is. Example:</p>
     * <pre>123456-abcde</pre>
     */
    public final class EntityReference
        implements Serializable
    {
        /**
         * Parse an URI to an EntityReference.
         * @param uri the URI to parse
         * @return the EntityReference represented by the given URI
         */
        public static EntityReference parseURI( String uri )
        {
            String identity = uri.substring( "urn:qi4j:entity:".length() );
            return new EntityReference( identity );
        }

        /**
         * Parse an Entity identity to an EntityReference.
         * @param identity the EntityReference identity
         * @return the EntityReference represented by the given identity
         */
        public static EntityReference parseEntityReference( String identity )
        {
            return new EntityReference( identity );
        }

        /**
         * @param object an EntityComposite
         * @return the EntityReference for the given EntityComposite
         */
        public static EntityReference entityReferenceFor( Object object )
        {
            return new EntityReference( (EntityComposite) object );
        }

        public static EntityReference create( Identity identity )
        {
            if( identity == null )
                return null;
            return new EntityReference( identity.identity().get() );
        }

        private static final long serialVersionUID = 1L;

        private String identity;

        /**
         * @param entityComposite a non-null EntityComposite
         * @throws NullPointerException if entityComposite is null
         */
        public EntityReference( EntityComposite entityComposite )
        {
            this( entityComposite.identity().get() );
        }

        /**
         * @param identity reference identity
         * @throws NullArgumentException if identity is null or empty
         */
        public EntityReference( String identity )
        {
            NullArgumentException.validateNotEmpty( "identity", identity );
            this.identity = identity;
        }

        /**
         * @return This EntityReference identity.
         */
        public final String identity()
        {
            return identity;
        }

        /**
         * @return An URI representation of this EntityReference.
         */
        public String toURI()
        {
            return "urn:qi4j:entity:" + identity;
        }

        @Override
        public boolean equals( Object o )
        {
            if( this == o )
            {
                return true;
            }
            if( o == null || getClass() != o.getClass() )
            {
                return false;
            }
            EntityReference that = (EntityReference) o;
            return identity.equals( that.identity );
        }

        @Override
        public int hashCode()
        {
            return identity.hashCode();
        }

        /**
         * @return This EntityReference identity.
         */
        @Override
        public String toString()
        {
            return identity;
        }
    }