org.trypticon.luceneupgrader.lucene4.internal.lucene.util.SetOnce Maven / Gradle / Ivy
package org.trypticon.luceneupgrader.lucene4.internal.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 implements Cloneable {
/** 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 {@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;
}
}