/*
 * 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.
 */
package org.apache.lucene.util;

import java.util.concurrent.atomic.AtomicBoolean;


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 set(Object) is called more than once, AlreadySetException is thrown and the operation will fail. 
@lucene.experimental
/**
 * 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> implements Cloneable {

  
Thrown when SetOnce.set(Object) is called more than once. /** Thrown when {@link SetOnce#set(Object)} is called more than once. */
  public static final class AlreadySetException extends IllegalStateException {
    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 set(Object). /**
   * 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 set(Object) afterwards will result in AlreadySetException 
Throws:
AlreadySetException – if called more than once
See Also:
set(Object)/**
   * 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. /** 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 set(Object). /** Returns the object set by {@link #set(Object)}. */
  public final T get() {
    return obj;
  }
}