content: package org.apache.lucene.util;
  
  import java.util.concurrent.atomic.AtomicBoolean;
  
  /**
   * Licensed to the Apache Software Foundation (ASF) under one or more
   * contributor license agreements.  See the NOTICE file distributed with
   * this work for additional information regarding copyright ownership.
   * The ASF licenses this file to You 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.
   */
  
  /**
   * A convenient class which offers a semi-immutable object wrapper
   * implementation which allows one to set the value of an object exactly once,
   * and retrieve it many times. If {@link #set(Object)} is called more than once,
   * {@link AlreadySetException} is thrown and the operation
   * will fail.
   *
   * @lucene.experimental
   */
  public final class SetOnce<T> {
  
    /** Thrown when {@link SetOnce#set(Object)} is called more than once. */
    public static final class AlreadySetException extends RuntimeException {
      public AlreadySetException() {
        super("The object cannot be set twice!");
      }
    }
    
    private volatile T obj = null;
    private final AtomicBoolean set;
    
    /**
     * A default constructor which does not set the internal object, and allows
     * setting it by calling {@link #set(Object)}.
     */
    public SetOnce() {
      set = new AtomicBoolean(false);
    }
  
    /**
     * Creates a new instance with the internal object set to the given object.
     * Note that any calls to {@link #set(Object)} afterwards will result in
     * {@link AlreadySetException}
     *
     * @throws AlreadySetException if called more than once
     * @see #set(Object)
     */
    public SetOnce(T obj) {
      this.obj = obj;
      set = new AtomicBoolean(true);
    }
    
    /** Sets the given object. If the object has already been set, an exception is thrown. */
    public final void set(T obj) {
      if (set.compareAndSet(false, true)) {
        this.obj = obj;
      } else {
        throw new AlreadySetException();
      }
    }
    
    /** Returns the object set by {@link #set(Object)}. */
    public final T get() {
      return obj;
    }
  }
  