/*

  * Copyright 2012-2013 smartics, Kronseder & Reiner GmbH

  *

  * 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 de.smartics.properties.api.core.domain;

 import java.io.ObjectStreamException;

 import java.io.Serializable;

 import org.apache.commons.lang.StringUtils;

 /**

  * Provides information about the expression to evaluate a default value for a

  * property.

  */

 public final class PropertyExpression implements Serializable

 {

   // ********************************* Fields *********************************

   // --- constants ------------------------------------------------------------

   /**

    * The class version identifier.

    */

   private static final long serialVersionUID = 1L;

   /**

    * The expression instance to signal that there is no property expression.

    */

   public static final PropertyExpression NO_EXPRESSION =

       new PropertyExpression(null);

   // --- members --------------------------------------------------------------

   /**

    * The expression string. May be <code>null</code>.

    *

    * @serial

    */

   private final String expression;

   // ****************************** Initializer *******************************

   // ****************************** Constructors ******************************

   /**

    * Default constructor.

    *

    * @param expression the expression string.

    */

   private PropertyExpression(final String expression)

   {

     this.expression = expression;

   }

   // ****************************** Inner Classes *****************************

   // ********************************* Methods ********************************

   // --- init -----------------------------------------------------------------

   // --- create ---------------------------------------------------------------

   /**

    * Creates an instance of {@link PropertyExpression} with the given expression

    * string.

    *

    * @param expression the expression string.

    * @return the created instance. If the {@code expression} is blank, the

    *         singleton expression {@link #NO_EXPRESSION} is returned.

    */

   public static PropertyExpression create(final String expression)

   {

     if (StringUtils.isBlank(expression))

     {

       return NO_EXPRESSION;

     }

     return new PropertyExpression(expression);

   }

   // --- get&set --------------------------------------------------------------

   /**

    * Checks if an expression string is present.

    *

    * @return <code>true</code> if the expression string is not <code>null</code>

    *         , <code>false</code> if it is <code>null</code>.

    */

   public boolean hasExpression()

   {

     return expression != null;

   }

   /**

    * Returns the expression string.

    *

    * @return the expression string. May be <code>null</code>.

    */

   public String getExpression()

   {

     return expression;

   }

   // --- business -------------------------------------------------------------

   // --- object basics --------------------------------------------------------

   /**

    * Special treatment for serializable singletons.

    *

    * @return returns a reference to the (truly) unique singleton.

    */

   private Object readResolve() throws ObjectStreamException

   {

     if (StringUtils.isBlank(expression))

     {

       return NO_EXPRESSION;

     }

     return this;

   }

   /**

    * Returns the name of the type.

    *

    * @return the name of the type.

    */

   @Override

   public String toString()

   {

     return expression != null ? expression : StringUtils.EMPTY;

   }

 }