# 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
    **     of Golden Code Development Corporation. You are not authorized to use the
    **     name Golden Code, FWD, or the names of any author or contributor, for
    **     publicity purposes without written authorization.
    **
    **   2. No Misrepresentation of Affiliation.
    **
    **     You may not represent yourself as Golden Code Development Corporation or FWD.
    **
    **     You may not represent yourself for publicity purposes as associated with
    **     Golden Code Development Corporation, FWD, or any author or contributor to
    **     the covered work, without written authorization.
    **
    **   3. No Misrepresentation of Source or Origin.
    **
    **     You may not represent the covered work as solely your work.  All modified
    **     versions of the covered work must be marked in a reasonable way to make it
    **     clear that the modified work is not originating from Golden Code Development
    **     Corporation or FWD.  All modified versions must contain the notices of
    **     attribution required in this license.
    */

    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") +'}';
       }
    }