AssignOnceWrapper.java

/*
** Module   : AssignOnceWrapper.java
** Abstract : A wrapper which allows assignment of the contained once, after which it becomes 'final' and
**            cannot be reassigned. 
**
** Copyright (c) 2024, Golden Code Development Corporation.
**
** -#- -I- --Date-- ---------------------------------------Description---------------------------------------
** 001 OM  20241024 Initial implementation extracted from Persistence static inner class.  
*/

/*
** This program is free software: you can redistribute it and/or modify
** it under the terms of the GNU Affero General Public License as
** published by the Free Software Foundation, either version 3 of the
** License, or (at your option) any later version.
**
** This program is distributed in the hope that it will be useful,
** but WITHOUT ANY WARRANTY; without even the implied warranty of
** MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
** GNU Affero General Public License for more details.
**
** You may find a copy of the GNU Affero GPL version 3 at the following
** location: https://www.gnu.org/licenses/agpl-3.0.en.html
** 
** Additional terms under GNU Affero GPL version 3 section 7:
** 
**   Under Section 7 of the GNU Affero GPL version 3, the following additional
**   terms apply to the works covered under the License.  These additional terms
**   are non-permissive additional terms allowed under Section 7 of the GNU
**   Affero GPL version 3 and may not be removed by you.
** 
**   0. Attribution Requirement.
** 
**     You must preserve all legal notices or author attributions in the covered
**     work or Appropriate Legal Notices displayed by works containing the covered
**     work.  You may not remove from the covered work any author or developer
**     credit already included within the covered work.
** 
**   1. No License To Use Trademarks.
** 
**     This license does not grant any license or rights to use the trademarks
**     Golden Code, FWD, any Golden Code or FWD logo, or any other trademarks
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**     name Golden Code, FWD, or the names of any author or contributor, for
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*/

package com.goldencode.p2j.util;

/**
 * A wrapper object which enforces a single assignation of an object, after which it becomes final (that is,
 * it cannot be re-assigned a second time). {@code null} is not a special value.
 *
 * @param <T>  The Type of the content.
 */
public class AssignOnceWrapper<T>
{
   /** The wrapped object, if any. */
   private T value = null;
   
   /** Flag for checking whether the object was set. */
   private boolean wasSet = false;
   
   /**
    * Get the wrapped object. If not set a {@code RuntimeException} will be thrown.
    *
    * @return  Get the wrapped object.
    */
   public T get()
   {
      if (!wasSet)
      {
         throw new RuntimeException("Value was not set.");
      }
      
      return value;
   }
   
   /**
    * Sets the wrapped object. This can be called a single time. Once this method was called, any attempt
    * to call it again will result in a {@code RuntimeException}, even if the argument of first call was
    * {@code null}.
    *
    * @param   value
    *          The value to be set. May be {@code null}.
    */
   public void set(T value)
   {
      if (wasSet)
      {
         throw new RuntimeException("Value was already set.");
      }
      
      this.value = value;
      wasSet = true;
   }
   
   /**
    * Checks if the object was set in the wrapper.
    *
    * @return  {@code true} if {@code set()} method was called.
    */
   public boolean wasSet()
   {
      return wasSet;
   }
   
   /**
    * Get a short description of this object. If the object was set, its {@code toString()} method will
    * be used, otherwise {@code N/A} is returned.
    *
    * @return  a short description of this object.
    */
   @Override
   public String toString()
   {
      return "final{" + (wasSet ? value : "N/A") +'}';
   }
}